Nizar Habbachi
email: habbachinizar@yahoo.fr

Hatem Boussetta
email: hatem_boussetta@yahoo.fr

Mohamed Adel Kallala
email: adel.kallala@gmail.com

Ali Boukabache

Patrick Pons
email: ppons@laas.fr

Kamel Besbes
email: kamel.besbes@fsm.rnu.tn

Design and simulation of 10 GHz VCO based on RF MEMS solenoid inductor

Keywords: MEMS, VCO, solenoid inductor, wide band, high spectral, I

This paper reports the design and simulation of 10

GHz VCO based on RF MEMS solenoid inductor. We have investigated four RF MEMS solenoid inductors using FEM software. Indeed, we have studied the effect of different dielectric substrate and metallic coil on inductors responses. Higher performances are obtained using copper coil and SU8 dielectric substrate: SRF= 20.8 GHz, Qmax= 60.9, and L = 2.6 nH at 10 GHz. Therefore, we have designed and investigated a crosscoupled CMOS VCO based on the best RF MEMS solenoid inductor. The obtained results show a wide tuning range TR = 46% comprised between 10 GHz and 14.6 GHz, and a good linearity of frequency variation in response of control voltage. Moreover, output signals present a high voltage upper than 1.2 V and a low phase-noise PN = -102.37 dBc/Hz at 1 MHz. In addition, the spectral analyze show that output peak power reaches 14.56 dBm at a center frequency of 10 GHz and the second harmonic is less than -58.9 dBm. These results prove high spectral signal ability of the proposed RF MEMS CMOS VCO at 10 GHz.

INTRODUCTION

Technological progress provides a fundamental basis for RF applications evolution and remains the essential brick of all its advances. Furthermore, MEMS technics are increasingly used in RF synthesizer for higher performances resonators embedded in oscillators' architecture.

The first oscillators use quartz resonator in order to generate a high spectral signal. The operation frequency of quartz oscillators ranges from a few kHz to tens of MHz. There are several oscillator using quartz in their architecture: Pierce oscillator [START_REF] Bahadur | Modified Pierce oscillator circuit[END_REF], Overtone oscillator [START_REF] Tsuzuki | Fast start-up crystal oscillator circuits[END_REF], Miller oscillator [START_REF] Rodriguez-Pardo | Miller Oscillators for High Sensitivity Quartz Crystal Microbalance Sensors in Damping Media[END_REF], and Colpitts oscillator [START_REF] Ou | Phase-noise Analysis for single and Dual-mode Colpitts Crystal Oscillators[END_REF]. Their operating frequency is limited by the resonant frequency of quartz and they could be integrated in PLL architecture leading to compensate the phase noise error. In this paper, we are interested by wide band VCO operating at 10 GHz using RF MEMS solenoid inductor. In the first paragraph, we have explored the effect of different materials on RF MEMS solenoid inductor. We have used two types of substrate: dielectric resin SU8 and conventional silicon. We have analyzed the performances at each case using two different metal core: gold and copper. In the second paragraph, we have integrated the most performant inductor in the VCO architecture. Moreover, we have analyzed the time domain outputs for the two extremities control voltages and we have presented the frequency responses of our VCO.

II. PERFORMANCES ANALYZES OF RF MEMS SOLENOID INDUCTORS:

In this part, we expose the results of four solenoids inductors having the same geometrical dimensions and different materials for the substrate and the core metal. The objective is to study the RF responses of these inductors for high frequency range comprised between 100 MHz and 100 GHz, and we integrate the best performances' inductor in a 10 GHz VCO architecture. The Fig. 1 presents the 3D perspective of 5 turns RF MEMS solenoid inductor with the appropriate dimensions on top and front view:

A. Dielectric SU8 substrate:

The two first inductors use a dielectric substrate "SU8 resin" and they are formed by two different metallic core: gold and copper. The SU8 resin is a low loss dielectric resin tanδ = 0.04 presenting a very low permittivity ε= 2.9, and it is appropriate for high frequency domain [START_REF] Pinon | Development of a microsystem based on a microfluidic network to tune and reconfigure RF circuits[END_REF][START_REF] El Gmati | Fabrication and evaluation of an onchip liquid micro-variable inductor[END_REF][START_REF] Habbachi | Tunable MEMS capacitor: influence of fluids[END_REF][START_REF] Habbachi | Fabrication and modeling of a capacitor microfluidically tuned by water[END_REF]. Moreover, the employed metals having high electric conductivity such: σ = 41.10 6 S/m for gold and σ = 57.10 6 S/m for copper. The performances of different RF MEMS solenoid inductors are determined by equations below: The inductance value versus frequency is presented in Fig. 2a.

(1) (2
As we can see, in two cases the inductance curves are almost identical. The inductance value is stable for a large frequency range and it is close to L = 2.6 nH until 10 GHz and the resonant frequency is higher than 20.8 GHz. The Fig. 2b illustrates the quality factor variation in response of frequency. It is clearly seen that copper core quality factor is higher than gold core. Indeed, the quality factor is limited by the serial resistance of the inductor core and when the coil conductivity rises the resistance weakens. The maximum value of the quality factor occurs at 5.7 GHz and we obtain Qmax= 60.9 for copper coil and Qmax=49.4 for gold coil.

B. Conventional silicon substrate:

The third and fourth inductors are superposed on a conventional silicon substrate having a low conductivity σ = 3 S/m and possess a high relative permittivity Ԑ = 11.2. We have also used two different metal for the inductor core: gold and copper. The inductors performances results in response of frequency are joined in Fig. 3 The Fig. 3a shows a good similarity of inductance curves in response of frequency in two cases: gold core and copper core.

In addition, the resonant frequency is upper than 12 GHz and but it is lower than the two first cases using SU8 substrate. Once more, the Fig. 3b shows that the quality factor of copper coil Qmax= 16.2 is higher than Qmax=14.4 of gold coil.

The Table 1 resume the different results of four designed RF MEMS solenoid inductors: These results demonstrate the importance of the SU8 substrate and the copper metal in the design of high performances RF MEMS solenoid inductors.

III. DESIGN AND SIMULATION OF 10 GHZ RF MEMS VCO:

A. VCO architecture:

The design and the simulation of the VCO is realized using ADS software and TSMC RF CMOS 0.13 µm standard technology. We have used the second solenoid inductor based on copper core and SU8 substrate in order to design the 10 GHz VCO. The Fig. 4 presents the architecture of complementary cross-coupled VCO and the dimensions of PMOS and NMOS transistors: The Fig. 6a show symmetrical sinusoidal oscillations with a period of 0.1 ns and a peak voltage of 1.62 V at Vctrl=0V. At the last Vctrl=0.5V (Fig. 6b), the period reaches 0.07 ns and the output peak voltage remains greater than 1.2V (the supply voltage of the VCO). These results reveal a wide variation of oscillation frequency with high voltage sinusoidal output signals.

C. Frequency response analyzes:

The phase noise presents a critical parameter for RF designer and they need to decrease its value as much as possible for high spectral frequency synthesizer. Consequently, we have extracted the phase noise value and the output spectrum at 10 GHz. Moreover, we have investigated the frequency responses of our VCO as illustrated in Fig. 7 The Fig. 7a shows a good linearity of the frequency variation in response of control voltage. In addition, the frequency band is comprised between 10 GHz and 14.6 GHz leading to a wide tuning range of TR = 46%. Moreover, as we can see in Fig. 7b, the phase noise is about -102.37dBc/Hz at 1 MHz of offset frequency. This result proves that our VCO could be efficient for low phase-noise wireless system. Moreover, the peak output power of our VCO reaches 14.56dBm at a center frequency of 10 GHz and the peak of the second harmonic is less than -58.9dBm (Fig. 7(c)). This result shows high spectral signal ability at 10 GHz.

IV. CONCLUSION:

Different RF MEMS solenoid inductors are designed using two substrates and two metals cores. The best performances' solenoid inductor are achieved with a dielectric SU8 substrate and a copper coil. The obtained results show a high quality factor that reaches Qmax= 60.9, a wide resonant frequency band upper than 20 GHz, and a stable inductor value L = 2.6 nH until 10 GHz. Furthermore, the selected RF MEMS solenoid inductor is integrated in VCO architecture for wireless communication systems operating at 10 GHz. The time domain and the frequency investigations of RF MEMS VCO show a high spectrum output signals and wide band tuning range upper than TR = 46%.

Fig. 1 RF

 1 Fig. 1 RF MEMS solenoid inductor (a) 3D perspective (b) top view (c) front view with detailed dimensions.

Fig. 2

 2 Fig. 2 Inductor performances in response of frequency variation (a) inductance value (b) quality factor.

Fig. 3

 3 Fig. 3 Inductor performances in response of frequency variation (a) inductance value (b) quality factor.

Fig. 4 Fig. 5 6 :Fig. 6

 4566 Fig. 4 Cross-coupled CMOS VCO using RF MEMS solenoid inductor.

Fig. 7

 7 Fig. 7 Frequency analyzes (a) tuning range (b) phase noise response (c) spectral output voltage harmonics.

TABLE I .

 I PERFORMANCES RESULTS OF DIFFERENT RF MEMS

SOLENOID INDUCTORS Substrate Core metal Maximum quality factor Inductance value at 10 GHz Resonant frequency Dielectric: SU8

	Gold 49.4 at 6.1 GHz Copper 60.9 at 5.7 GHz	2.6 nH	20.8 GHz
	Conventional:
	Silicon

Gold 14.4 at 0.8 GHz 3.5 nH 12.8 GHz Copper 16.2 at 0.8 GHz


Nizar Habbachi
email: habbachinizar@yahoo.fr

Hatem Boussetta
email: hatem_boussetta@yahoo.fr

Mohamed Adel Kallala
email: adel.kallala@gmail.com

Ali Boukabache

Patrick Pons
email: ppons@laas.fr

Kamel Besbes
email: kamel.besbes@fsm.rnu.tn

Modeling of microfluidically tuned capacitor for RF applications

Keywords: Microfluidic, RF MEMS, capacitor, tunable, wide band. I

this paper presents the modeling of microfluidically tuned capacitor for RF applications. The designed structure is based on performances variations following DI water displacement between capacitor's electrodes. We have modeled the electric field and the current distribution using FEM tool for different DI water position in microchannels. The obtained results at 4.5 GHz show an important variation of electric field and current distribution that impacts the capacitor performances: the capacitance value is comprised between Cmin = 0.11 pF and Cmax = 5.76 pF, the factor value decreases from Qmax = 84.27 to Qmin = 3.99, and the resonant frequency ranges from 5.67 GHz to 19.8 GHz. Indeed, the capacitance variation reaches Tr = 5136% and the broadband ability is higher than 240%.

INTRODUCTION

Tuned capacitors are used in multitude RF applications such as: satellite communication, wireless networks, radar systems, and cellular mobile. The main features involved in the design of variable RF MEMS capacitors are: high tuning range, low insertion loss, high quality factor, broadband, high frequency operation. The versatility of actuation modes of RF MEMS capacitors enables several essential functions such as impedance matching, adjustable filters, variable phase shifters, multi-band amplifiers or adjustable amplifiers. Originally, actuations modes are based on operating forces generated due to the conversion of mechanical energy into electrical energy and vice versa. Furthermore, the implantation of an actuator to the micrometer scale requires specific manufacturing techniques and materials. The most important feature in RF MEMS capacitors is variation range, which is usually related to the nature of the used mode. Since the invention of MEMS components the researchers have tried to cross the limits of variation based on different actuations modes [START_REF] Nieminem | Microelectromechanical capacitors for RF applications[END_REF][START_REF] Seonho-Seok | A novel linearly tunable MEMS variable capacitor[END_REF][START_REF] Oz | RF CMOS-MEMS capacitor having large tuning range[END_REF][START_REF] Nguyen | Angular vertical comb-driven tunable capacitor with high tuning capabilities[END_REF][START_REF] Gu | Rotational Driven RF Variable Capacitors with Post-CMOS Processes[END_REF][START_REF] Habbachi | Tunable MEMS capacitor: influence of fluids[END_REF].

In this paper, we study a highly tunable RF MEMS capacitor based on microfluidic actuation. In the first section, we present the 3D perspective of our microfluidic capacitor and our principal of variation. In the next section, we discuss the electric field and current distribution following the DI water displacement. In third section, we investigate the effect of fluidic variation on capacitance and quality factor in response of frequency. Finally, we analyze the RF performances of our capacitor: resonant frequency and insertion loss in response of DI water displacement in microchannels.

II. MICROFLUIDIC CAPACITOR DESIGN:

Our principle of variation relies on the modification of driving medium permittivity where prevailing electric field following DI water displacement between capacitor's electrodes. The change will take place gradually and continuously as a function of dielectric fluid passage between microchannels separating the armatures of capacitor. This change alters the distribution of electric field, which varies capacitor's performances. The microfluidic actuation is used in wide tunable RF MEMS components [START_REF] Habbachi | Tunable MEMS capacitor: influence of fluids[END_REF][START_REF] Habbachi | Fabrication and modeling of a capacitor microfluidically tuned by water[END_REF][START_REF] El Gmati | Fabrication and evaluation of an onchip liquid micro-variable inductor[END_REF][START_REF] Pinon | Development of a microsystem based on a microfluidic network to tune and reconfigure RF circuits[END_REF].

III. CAPACITOR MODELING:

Analytical models describing the operation of microwave capacitors are usually based on equivalent circuit's using lumped and ideals elements and they are generally insufficient to give fairly and accurate results. Indeed, the obtained results give an approximation to real behaviors and they not consider the whole RF losses that are not all taken into account. Furthermore, numerical methods based on the resolution of Maxwell's equations are more appropriate for RF structures. There are several types of numerical methods such as finite difference method (FDM), method of moment (MoM), finite element method (FEM), etc. During our analyses, we have chosen FEM software for its high precision of calculation, although it requires a higher calculation time. In this part, we expose the effect of DI water displacement on electric field distribution and current density at 4.5 GHz for four DI water positions.

1) Electric field distribution:

The As we can see in Fig. 2 (a), when the capacitor is empty, the electric field is maximal in microchannel and at excitation port 1. Indeed, the permittivity in microchannel (vacuum) is less than that of dielectric medium, which encloses it (substrate and SU-8 layers), and consequently the fluidic channel represents the confinement medium of the electric field. We also note that electric field is high at SU-8 layers that separate spirals and ground. This result shows that electrodes can be considered as a source of electric field prevailing in the structure when it is empty. On the other hand, when the deionized water enters capacitor, it changes the permittivity of the microchannel and subsequently modifies the distribution of the electric field. As we can see in Fig. 2 (b), at first position of fluid, the field decreases in microchannels and remains high in the area closest to electrodes. We also note the appearance of electric field part near the second excitation port. In second position (Fig 2 (c)); the electric field continue to dissipate from microchannels and a residual part remains focused at it. From third position (Fig. 2 (d)); we notice an excessive and continuous decrease of E field residual part in microchannels. Indeed, the increase of permittivity in microchannel following the displacement of DI water increasingly influences the depletion of E field in microchannel. The last position of fluid (Fig. 2 (e)) reveals a cancellation of electric field in microchannel and a homogeneous distribution of electric field in structure as well as in the two excitations ports.

2) Current density distribution:

The Fig. 3 As we can see in Fig. 3 (a), current density value is low at capacitor center when it is empty. With fluid penetration in microchannels (Fig. 3 (b-d)), current density value increases continuously in electrodes and remains zero at the ground surface. At fourth position (Fig. 4 (e)), current density reaches its maximum at electrodes surface and begins to increase at ground surface. We would like to clarify that current density value is mainly related to the operation frequency and reaches its maximum at resonant frequency. Knowing that fluid position in microchannels changes physical properties of the structure and consequently changes the resonant frequency. The results presented in Fig. 3 show a partial current density because the frequency is fixed at 4.5 GHz.

B. Fluidic variation effect on capacitor performances:

In this section, we analyze the effect of fluidic actuation on the electrical and RF performances of microfluidic capacitor: capacitance, quality factor, and reflection/transmission losses based on S parameters. The simulation of the structure is carried out under "HFSS-software" for a frequency range from 10 MHz to 40 GHz for four positions of DI water between capacitor's electrodes.

1) Capacitance and quality factor:

The Fig. 4 The curves of Fig. 4 (a) show that the capacitance value increases exponentially in response of frequency up to resonant frequency (Fres); from there, capacitance value decreases and becomes negative. This change of sign indicates a new mode of operation, which becomes inductive. Furthermore, we note that when the capacitor is empty, its value is minimal and increases with the progress of fluid circulation. This increase is followed by a degradation of quality factor as shown in Fig. 4 (b). Indeed, the quality factor reaches its maximum when the capacitor is empty and then it decreases rapidly with DI water displacement. In order to investigate the effect of fluid variation, we have taken the value of the capacitor and the quality factor at 4.5 GHz for each fluid position. Fig. 5 below summarizes the different results: Capacitance (pF) According to Fig. 5, the capacitance value increases continuously with the penetration of DI water and reaches its maximum when it is completely filled. The capacitance value is comprised between Cmin = 0.11 pF and Cmax = 5.76 pF thus ensuring a very wide variation that reaches Tr = 5136%. However, the quality factor decreases from Qmax = 84.27 when it is empty to Qmin = 3.99 when it is fully filled with DI water.

2) RF performances analyses: The Fig. 6 presents the variation of gain dB(S11) and dB(S21) in response of frequency for four DI water positions: Fig. 6 (a) illustrates the curves of dB(S11) as a function of frequency for the different positions of the fluid. We notice that the resonance frequency is maximum when the capacitance is empty and decreases with DI water penetration in microchannels. Furthermore, Fig. 6 (b) shows dB(S21) versus frequency curves for the different positions of the deionized water. It is clear that the frequency band is wider when the capacitor is empty and it decreases as a function of the filling with deionized water. We note that the range of the capacitive mode operation is described by dB(S21) gain. To better evaluate the resonant frequency variation, we have taken the minimum values of dB(S11) for each position of DI water (Fig. 7). The resonant frequency is comprised between 19.81 GHz when the capacitor is empty and 5.67 GHz when it is full. We obtain a broadband variation, which is greater than 240%. In addition, insertion loss is comprised between IL = -0.28 dB and IL = -0.14 dB. These results demonstrate a low insertion loss of the microfluidic capacitor for all DI water positions and could be used for high spectral microwave applications.

C. Conclusion

A wide tunable microfluidic RF MEMS capacitor is designed and analyzed. We have used FEM software in order to model the electric field and the current distribution for different DI water displacement. In addition, we have studied different performances of microfluidic capacitor in response of frequency for four DI water positions. The obtained results present an important variation of the capacitance value and a wide band of resonant frequency. Furthermore, the dB(S21) investigations exhibit a low insertion loss for all water positions.

The Fig. 1 Figure 1 :

 11 presents the 3D perspective of our microfluidic capacitor and the different positions of DI water in microchannels: Design of the squared microfluidic capacitor: (a) 3D representation of the structure; fluid positions in channel: (b) POS1 (c) POS2 (d) POS3 (e) POS4.

Figure 2 :

 2 Fig 2 shows the surface distribution of electric field at 4.5 GHz in response of dielectric fluid penetration in microchannels: Surface distribution of electric field at 4.5 GHz when the capacitor is empty and for four DI water position, (a) empty (b) POS1 (c) POS2 (d) POS3 (e) POS4.

Figure 3 :

 3 presents the current density distribution for four DI water positions at 4.5 GHz: Current density distribution at 4.5 GHz when the capacitor is empty and for four DI water position, (a) empty (b) POS1 (c) POS2 (d) POS3 (e) POS4.

Figure 4 :

 4 below shows the variation of capacitance and quality as a function of frequency for four positions of DI water: Capacitor performances in response of frequency for four DI water positions (a) capacitance value (b) quality factor.

Figure 5 :

 5 Figure 5: Capacitor performances variations at 4.5 GHz for four DI water positions.

Figure 6 :

 6 RF performances in response of four DI water positions (a) dB(S11) gain (b) dB(S21) gain.

Figure 7 :

 7 Figure 7: RF performances variations for four DI water positions.


Le Han

Sylvain Galier

Hélène Roux-De Balmann

A phenomenological model to evaluate the performances of electrodialysis for the desalination of saline water containing organic solutes

Keywords:

Electrodialysis is promising to treat saline solutions containing organic solutes. However, there is still a need for a model to describe the desalination performances according to the process parameters. In this work a phenomenological model is proposed based on the analysis of the different contributions to the mass transfer of salt, water, and organic compounds. Once the characteristic parameters are determined experimentally, it is possible to predict the salt and organic concentrations in both compartments according to the operating conditions. The methodology is illustrated for the evaluation of electrodialysis performances considering a case study, and the effect of the current and solution compositions are discussed. To remove a fixed salt quantity, a lower current gives a higher organic solute transfer due to a higher time dependent solute diffusion, the additional transfer, fixed by the quantity of charge, remaining constant. Regarding the influence of the inorganic salts, higher transfer of organics, like glucose, acetic acid or acetate, are observed with sulfate compared to chloride, while a contrary behavior is obtained with phenol. This model also permits to discuss the influence of possible pretreatments to change the pH or the salt composition in order to improve the electrodialysis performances.

Introduction

Aqueous waste streams of increasing quantities are often complex, containing salt and organic matter (OM) [START_REF] Lefebvre | Treatment of organic pollution in industrial saline wastewater: a literature review[END_REF]. Membrane processes like nanofiltration (NF) or electrodialysis (ED) are very promising for the separation of the organic compounds and the salt, leading to a feasible treatment, reuse and/or valorization of these fractions. ED can be better option due to process availability in varying salinity range and flexibility for process control e.g. on current [START_REF] Strathmann | Ion-exchange Membrane Separation Processes[END_REF][START_REF] Strathmann | Electrodialysis, a mature technology with a multitude of new applications[END_REF]. In addition, successful applications of ED for desalting complex saline solutions in varying fields of biotechnological, chemical industries and water treatment systems are reported, with solution containing both organic solute and salt [START_REF] Huang | Application of electrodialysis to the production of organic acids: state-of-the-art and recent developments[END_REF][START_REF] Abels | Membrane processes in biorefinery applications[END_REF][START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF][START_REF] Zhang | Electrodialysis on RO concentrate to improve water recovery in wastewater reclamation[END_REF]. In each case, two different streams are produced, as shown in Fig. 1.

Considering further integration of the produced streams, ED desalination can have many different objectives. Concerning the organic content, the solute decomposition or recovery could be the objective depending whether it is a pollutant or valuable fraction. Meanwhile, concerning the salt extracted in the concentrate, further utilization such as production of raw materials or exploitation salinity gradient energy is of great interest [START_REF] Logan | Membrane-based processes for sustainable power generation using water[END_REF]. Nevertheless, it is noteworthy that desalination and organic solute transfer occur simultaneously. The process control regarding the solution impurity in the diluate and concentrate should be given to great attention, as it may limit the integrated process followed.

In fact, previous ED studies mainly focus on partial factors like solute concentration and nature, membrane type selection, operating current, pH and system running mode etc. [START_REF] Zhang | Electrodialysis on RO concentrate to improve water recovery in wastewater reclamation[END_REF][START_REF] Vanoppen | Properties governing the transport of trace organic contaminants through ion-exchange membranes[END_REF][START_REF] Zhang | Separation of nutrient ions and organic compounds from salts in RO concentrates by standard and monovalent selective ion-exchange membranes used in electrodialysis[END_REF][START_REF] Zhang | Separation of small organic ions from salts by ion-exchange membrane in electrodialysis[END_REF][START_REF] Dydo | The mechanism of boric acid transport during an electrodialytic desalination process[END_REF][START_REF] Banasiak | Removal of boron, fluoride and nitrate by electrodialysis in the presence of organic matter[END_REF], while a systematic investigation concerning ED performance for the separation of the organic solute/salt is still lacked. Moreover, in such cases, the membranes, the salt and organic solute together build a complex system, making process performance hard to predict [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF]. For instance, the salt transfer (to be maximized) and the simultaneous organic solute transfer (to be minimized) are reported to be highly linked due to the water electroosmosis and the applied current in ED [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF]. Furthermore, modification on both the ion-exchange membrane (IEM) structural [START_REF] Fuoco | Correlation between macroscopic sugar transfer and nanoscale interactions in cation exchange membranes[END_REF] and the solute hydration [START_REF] Boy | Relationship between volumetric properties and mass transfer through NF membrane for saccharide/electrolyte systems[END_REF] by salt has been reported, possibly resulting to unexpected poor performance of ED. Thus, considering a complex interaction among salt-solute-membrane involved, a theoretic model in order to better predict/understand ED performance is urgently necessary. However, few studies focus on the predicted model for ED process [START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF], with only limited solution investigated such as in agroprocessing field [START_REF] Bawornruttanaboonya | Mathematical modeling of transport phenomena and quality changes of fish sauce undergoing electrodialysis desalination[END_REF][START_REF] Bailly | Production processes of fermented organic acids targeted around membrane operations: design of the concentration step by conventional electrodialysis[END_REF]. Furthermore, the model based on Nernst-Planck equation integrating many parameters is often hard to use in real practice of different aspects [START_REF] Fidaleo | Electrodialytic desalting of model concentrated NaCl brines as such or enriched with a non-electrolyte osmotic component[END_REF].

The aim of this study is to propose a simple, phenomenological model to evaluate the ED performance desalting various saline brines containing organic solute. Synthetic saline solutions consisting of different salt (NaCl, MgCl 2 , Na 2 SO 4 , respectively) mixed with varying organic compounds (namely acetic acid, phenol and glucose as neutral compounds and acetate as charged one) are used. Based on the mass transfer investigation, characteristic parameters of the proposed model are experimentally obtained. Then, according to the model, the ED process performance is discussed concerning the influence of the current and solution composition (the salt and organic solute), on the variation of solute concentration for both two streams, i.e., diluate and concentrate.

Phenomenological modelling based on mass transfer

A phenomenological model in the ED process is depicted according to the transfer of inorganic salt, water and organic compound. Then, the variation of the solute concentration in each compartment can be predicted with the proposed model, enabling to further evaluate the process performance.

Inorganic salt and water transfer

The salt flux in ED, j s (eq•m -2 •s -1 ), is reported to be dominated by migration due to the electrical potential gradient, j mig , which is proportional to the current [START_REF] Strathmann | Ion-exchange Membrane Separation Processes[END_REF][START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF][START_REF] Bailly | Production processes of fermented organic acids targeted around membrane operations: design of the concentration step by conventional electrodialysis[END_REF]. The salt flux density is expressed as in Eq. ( 1), introducing the current coefficient α (eq•m -2 •s -1 •A -1 ):

≈ = j j α I s mig (1)
In the same manner, it is reported that the water transfer, j w (m 3 •m -2 •s -1 ), is dominated by the electroosmotic flux (j eo ), i.e. the water transfer owing to the migration of ion species, corresponding to the ion hydration [START_REF] Strathmann | Ion-exchange Membrane Separation Processes[END_REF][START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF][START_REF] Jiang | Electrodialysis of concentrated brine from RO plant to produce coarse salt and freshwater[END_REF][START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF]. Due to its relation with the salt transfer, electroosmosis j eo varies linearly with the current. Then, the water flux density is given by Eq. ( 2), by introducing an electroosmotic coefficient,

β (m 3 •m -2 •s -1 •A -1 ): ≈ = j j βI w eo (2)

Organic solute transfer

It is documented that the mass transfer of the organic compound can be owing to the diffusion driven by the concentration gradient [START_REF] Gong | Effects of transport properties of ion-exchange membranes on desalination of 1,3-propanediol fermentation broth by electrodialysis[END_REF]. But an additional contribution was demonstrated to be also involved (convection for neutral solute and migration for charged solute) and that it is not negligible compared to diffusion, which is further discussed later [START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF][START_REF] Dydo | The mechanism of boric acid transport during an electrodialytic desalination process[END_REF][START_REF] Fuller | Experimental determination of the transport number of water in nafion 117 membrane[END_REF][START_REF] Bath | Electrically facilitated molecular transport. Analysis of the relative contributions of diffusion, migration, and electroosmosis to solute transport in an ion-exchange membrane[END_REF][START_REF] Gu | Electroosmotic enhancement of the binding of a neutral molecule to a transmembrane pore[END_REF]. The flux of organic solute, j OM , consisting of the diffusion flux, j diff , and the additional flux, j additional is given by Eq.

(3):

= + j j j OM diff additional (3)
Concerning the diffusion transfer, it can be further expressed as:

= j P C Δ diff OM OM (4)
where ΔC OM is the concentration gradient (Δ

C OM = C OM , D 0 -C OM , C 0 )
and P OM (m•s -1 ) is the solute permeability. ΔC OM is assumed to be constant in this study, with the value equal to C OM , D 0 , the initial organic solute concentration in diluate (feed). Indeed, comparing with the initial amount added in the diluate, this amount of transferred organic compound is negligible which will be further discussed in Section 4.2.

Concerning the additional transfer, it corresponds to two cases, considering different solute charge nature. For a neutral organic solute, the additional transfer was reported to be convection due to electroosmosis which is proportional to the current [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF]. A charged organic solute is supposed to migrate under current, same to inorganic charged species above mentioned. Thus, the migration of the charged solute is also proportional to the current, referred to Eq. ( 1).

Therefore, the additional transfer of organic solute, whatever charge nature, can be expressed as follows:

= j γI additional (5) introducing γ (mol•m -2 •s -1 •A -1 )
as the additional transfer characteristic coefficient. The total flux of solute transfer, shown as Eq. ( 3) can be rewritten as Eq. ( 6):

= + = + j j j P C γ I OM diff additional OM OM D , 0 (6)
consisting of diffusion, which is characterized by coefficient P OM (m•s -1 ), and an additional transfer, characterized by γ (mol•m -2 •s -1 •A -1 ).

Solute concentration variation

Knowing the transfer of solute and water across the ion-exchange membrane, the quantity of the solute and water in real time in the two compartments can be calculated, based on mass balance analysis for an ED experiment run in a batch mode. Then, the concentration of salt (C S , eq•m -3 ) and concentration of organic solute (C OM , mol•m -3 ) resepctively are given by: for the concentrate compartment:

= + + C C V αIS t V βIS t 2 s C s C m C m ,C 0 0 0 (7) = + + C P C γI S t V βIS t ( ) 2 2 OM C OM OM D m C m , 0 0 (8)
then based on the mass balance, for the diluate compartment:

= - - C C V αIS t V β I S t 2 s D s D m D m ,D 0 0 0 (9) = - + - C C V P C γI S t V β I S t ( ) 2 2 OM D OM D OM OM D m D m 0 0 , 0 0 (10)
The numerators and denominators describe the solute quantity and solvent quantity at each time t, respectively. The four characteristic parameters for solute and solvent transfer (α, β, P OM and γ) are obtained experimentally. α and β can be determined from the variations of flux density versus current for salt and water, respectively; P OM , i.e., the solute permeability, can be deduced according to diffusion flux, while γ, i.e. the additional transfer coefficient can be determined from the variations of organic solute flux density versus current.

Other parameters in Eqs. ( 7)- [START_REF] Zhang | Separation of nutrient ions and organic compounds from salts in RO concentrates by standard and monovalent selective ion-exchange membranes used in electrodialysis[END_REF] are from solution characteristics like initial solute concentration (C s 0 ,C OM 0 ) and solution volume (V C 0 ,V D 0 ), and from the electrodialysis apparatus such as membrane surface for one type (S m ) or operation parameters like current (I) and time (t). It is noteworthy that inorganic salt transfer concerns effective membrane surface as S m , while water and organic solute transfer concerns membrane surface as 2S m (the surface referring to the transfer of water and organics is the total membrane area, including both AEM and CEM, i.e. 2S m , S m referring to the surface of one type of membrane only).

The variation of solute concentration is then possible to be predicted in real time. The unit of the solute concentration is changeable depending on the investigated condition, e.g. for process point of view, a change of C OM unit from mole to ppm by multiplying solute M w .

Materials and methods

Salt and organic solutes

Three inorganic salts were selected: NaCl, Na 2 SO 4 , MgCl 2 containing ions of various hydrations. The sequence of ion hydration for anion and cation respectively is reported as [START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF]:

< < - - + + Anions:Cl SO cations: Na Mg 4 2 2
Four organic solutes of varying characteristics used in this study are reported in Table 1, including three neutral and one charged solutes. They were chosen since they typically represent the ones found in different saline fluids, characterized with varying molecular weights (M w ), hydrophobicity and charge.

Notably, Table 1 reports the pH value of each synthetic solution when organic compound was dissolved, without adjustment, regardless of the presence of salt. Particularly, the pH value of solution containing acetic acid is ca. 3.0. This indicates that < 5% of the acetic acid is dissociated (i.e. as acetate), considering the pKa value. In the pH conditions investigated here, one can further consider that acetic acid, phenol and glucose are neutral. Phenol is the most hydrophobic solute (indicated by the highest value of log P), with acetic acid second and glucose followed (lowest log P). This sequence can be mainly attributed to the different solute functional groups of benzene carboxyl and hydroxyl (Table 1). Acetate is negatively charged, with its M w close to acetic acid.

The concentration for organic solute was 0.1 mol•L -1 , and the concentration of inorganic salt (compared to sodium acetate, i.e. an organic salt) was 0.8 eq•L -1 .

Membrane and electrodialysis set-up

The ED stack used in this study (EUR 2B-10, Eurodia, France) consisting of 10 cells of anion-exchange membrane (AEM) and cationexchange membrane (CEM), i.e., Neosepta AMX/CMX, from Tokuyama Corp, Japan [START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF][START_REF] Bailly | Production processes of fermented organic acids targeted around membrane operations: design of the concentration step by conventional electrodialysis[END_REF][START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF]. The effective membrane surface for each type (AEM or CEM) is 0.02 m 2 per cell, giving a total area for each type of membrane (S m ) 0.2 m 2 .

The electrodialysis set-up has three loops, for diluate and concentrate as well as electrode solution respectively. Batch mode operation was used to carry out the experiments, with constant flowrates of 180 L•h -1 for concentrate and diluate (corresponding to a linear flow velocity of 10 cm•s -1 ), 360 L•h -1 for electrode solution. Constant operation temperature of 25 ± 1 °C was fixed by a circulating thermostatic bath for each experiment [START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF].

The electrode solution was 10 g•L -1 Na 2 SO 4 of 3 L. 2 L of the salt solution with the organic solute was fed into diluate compartment, while another 2 L of the same salt solution but without organic solute was fed into the concentrate compartment.

Protocol

For each experiment, the ion-exchange membranes were soaked firstly by saline water with same composition as in ED experiments, following a previously stated procedure [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF][START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF][START_REF] Galier | Influence of the ionic composition on the diffusion mass transfer of saccharides through cation-exchange membrane[END_REF]. The IEMs were consequently regarded in their counter-ion form, in order to highlight the impact of salt on membrane structure [START_REF] Galier | Influence of the ionic composition on the diffusion mass transfer of saccharides through cation-exchange membrane[END_REF].

Experiments were then performed without current (I = 0), corresponding to a diffusion system, and with current (I ≠ 0). Without current, owing to the concentration difference, the solute diffusion from diluate to concentrate was noticed. Under normal ED conditions, four currents (2, 3, 4, 6 A) corresponding to a current density ranging 0-300 A•m -2 were used. All the ED experiments were stopped before the final conductivity of diluate reached 5 mS•cm -1 to avoid exceeding the limiting current. As a consequence, experiment durations varied from case to case. The fluxes for solvent and solute (inorganic salt and organic compound) were determined according to the solution volume and the transferred solute quantity variation with time, respectively.

Analytical methods

Solution pH, conductivities, temperature for both diluate and concentrate as well as the system voltage were monitored in real time.

The ionic species concentrations were measured by ion chromatography (IEC, Dionex, France) [START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF]. The concentrations of organic solutes except phenol were determined by HPLC (Jasco, France) [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF], while UV spectroscopy determines the concentration of phenol [START_REF] Borges | Investigation of the mass transfer processes during the desalination of water containing phenol and sodium chloride by electrodialysis[END_REF]. Samples dilution was carried out before each analysis and more details are referred to [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF].

The maximum deviation of mass balance is no > 2% for water, 5% for ions, 10% for organic solute, respectively.

Results and discussions

Firstly, the transfer for inorganic salt (α) and water (β) are investigated, and then the transfer of organic solute across the membranes is studied without current (P OM ) and with current (γ), respectively. After determining these four parameters, the model is used to evaluate the ED performances with a regard to the effect of the current, the solute and the salt.

Inorganic salt and water transfer

The inorganic salt and water flux is deduced from the linear variation of the transferred quantity versus time (R 2 > 99%) respectively, according to the experiment results for each current (not shown here), in agreement with previous results [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF]. Fig. 2 depicts the variations of these fluxes versus current.

Fluxes for the inorganic salt and water vary linearly with the current (R 2 > 99%), respectively. This finding confirms that migration dominates the salt transfer and electroosmosis dominates the water transfer under current, as stated in Eqs. ( 1) and ( 2) respectively. Such results are observed under any investigated condition.

The coefficients α and β are determined by Eqs. ( 1) and ( 2). For different solutions investigated here, Table 2 reports all the values of current coefficients α, showing one corresponding α value for each neutral solute, two corresponding values for the charged solute (with the total α value considering the transfer of both the inorganic salt and sodium acetate), in parallel with the case without any organic solute.

Values excluding the inorganic α for sodium acetate are firstly dicussed. Results for the three neutral solutes are already discussed in [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF], showing the current coefficient α is mainly fixed by the current, without significant variations with the composition of solution. The same finding is observed for the value of the total α for acetate, as well as in the case without any organic solute.

For acetate, as expected, lower value for the inorganic salt, compared with the total one is observed for each salt composition. Concerning different salt compositions, the values for the inorganic salt varies, showing the transfer of SO 4 2for Na 2 SO 4 system (4.0) is slightly lower than those of Cl -for NaCl and MgCl 2 systems (ca. 4.5).

Considering comparable values of total α for different systems, this indicates a possible different transfer of acetate with the presence of different inorganic anions, which is further disscussed in Section 4.2.

Regarding the water transfer, the determined electroosmotic coefficients are reported in Table 3 for the different systems investigated, in parallel with the case without any organic solute.

For a fixed solute, it is observed that β varies with the composition of salt (maximum variation ca. 30%). For each solute, one can find the same trend for β as MgCl 2 > NaCl > Na 2 SO 4 . Then, for varying organic compounds, Table 3 shows that the values of glucose and acetic acid as well as acetate are close, and very comparable to those obtained in case without any organic solute, slightly higher than the values obtained with phenol (difference below 12%). This agrees with the previous report on neutral organic solute [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF].

Since the water transfer consists of a total contribution of each ion's hydration [START_REF] Han | Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution[END_REF], the reported β is a global parameter. Particularly in the case of acetate, the value of β consists of both the contribution of the inorganic salt and sodium acetate, whereas for the other cases only the inorganic salt's contribution is involved. Nevertheless, no significant variation of β is observed comparing the case of acetate with the others. This is because water transfer due to acetate is negligible, due to a very low concentration comparing to the inorganic salts (1:8).

Organic solute transfer

Transfer of organic solute without current (in diffusion system) and then with current are studied. In each system, considering the solutemembrane interaction, the results for three neutral solutes are firstly discussed together. Then for the charged solute, i.e. acetate, its transfer is discussed in comparison with acetic acid.

Transfer of organic solute without current

Diffusion flux of organic solute is deduced from the linear variation for transferred solute quantity with time (R 2 > 98%) observed in the experiment (results not shown here), agreeing with literatures [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF]. The obtained solute permeability according to Eq. ( 4), characterizing the diffusion transfer for varying systems are shown in Table 4.

The results for the neutral solutes highlight the steric effect governing the transfer of neutral solute transfer across the IEM and influence of salt (ion hydration) referred as membrane swelling effect, which are already discussed in our previous work [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF].

The transfer of acetate is compared with that of acetic acid, of similar M w but different charge nature to acetate. Acetate permeability is always lower than that of acetic acid in each case, by a factor of ca. 2. Regarding the effect of salt, different behavior of acetate transfer (NaCl < MgCl 2 ≈ Na 2 SO 4 ) is also observed, compared to the fixed sequence found for the neutral solute (NaCl > MgCl 2 > Na 2 SO 4 ). This can be attributed to the additional electrostatic effects for acetate which influences its transfer, compared to the steric effect for acetic acid. The transfer mechanism is out of the scope of this work.

Transfer of organic solute with current

Under each current, the flux of organic compound is deduced from a linear variation of solute transfer versus time (R 2 > 98%) according to the experiment results for each current (not shown here), in line with literatures [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF]. Fig. 3 depicts variations of organic solute flux with current including diffusion (I = 0), using systems containing NaCl as an example.

Regarding the neutral solutes, the sequence for the flux density of organic compound under each current is reported as: glucose <

Table 2

Current coefficient (α, eq•m -2 •s -1 •A -1 ) for varying systems according to Eq. ( 1); [inorganic salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 . Total*: this value includes contribution of both the inorganic and acetate

α × 10 -4 eq•m -2 •s -1 •A -1 Glucose
Table 3 Electroosmotic coefficient (β, m 3 •m -2 •s -1 •A -1
) for varying systems according to Eq. ( 2); [inorganic salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 . phenol < acetic acid, in agreement with that the steric effect governing the neutral compound transfer across IEM [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF]. This result for the charged solute shows that acetate is more sensitive to the current.

β × 10 -8 m 3 •m -2 •s -1 •A -1 Glucose
Comparing with acetic acid, acetate has a lower diffusion flux, while higher flux under the investigated current since 2A. The above sequence for NaCl system is found in other cases. Fig. 3 shows that the flux density linearly increases with the current, beginning with the value for diffusion (I = 0). These findings agree with Eq. ( 3), clearly showing two contributions i.e. diffusion and additional one for the solute transfer, with the latter proportional to the current (R 2 > 98%), which is found in each system investigated here.

As aforementioned, the additional transfer for neutral solute is convection due to the electroosmosis flux and for charged solute it is migration. Then, in each case, the coefficient γ determined as the slope in Fig. 3 (the variation of the flux density versus current), characterizes this additional transfer. Table 5 reports all the γ values from varying systems investigated.

One can find that γ values vary with organic solute. Given a fixed salt, the following order of sequence as acetate > acetic acid > phenol > glucose is always found. This result suggests migration (acetate) can be more important than convection (neutral solute), in agreement with the aforementioned solute transfer.

On the other hand, the influence of salt composition is discussed, starting with the neutral solute [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF]. Except the surprisingly low γ value for phenol -Na 2 SO 4 system, which is probably due to specific interaction, other results are in line with the solute dehydration effect, where a higher convection flux was reported with an increasing ion hydration [START_REF] Han | Transfer of neutral organic solutes during desalination by electrodialysis: influence of the salt composition[END_REF][START_REF] Galier | Demineralization of glucose solutions by electrodialysis: influence of the ionic composition on the mass transfer and process performances[END_REF].

For acetate, regarding the nature of salt, similar values with NaCl and MgCl 2 are observed, higher than those for acetic acid by a factor of ca. 2. However, the acetate -Na 2 SO 4 system gives the highest γ value. In fact, for both acetic acid and acetate, an identical trend as Cl -< SO [START_REF] Huang | Application of electrodialysis to the production of organic acids: state-of-the-art and recent developments[END_REF] 2is shown, with the trend more pronounced on acetate.

For acetic acid, this trend is mainly attributed to the fact that SO 4 2has a stronger solute dehydration than Cl -, which can facilitate the solute transfer [START_REF] Boy | Relationship between volumetric properties and mass transfer through NF membrane for saccharide/electrolyte systems[END_REF]. For acetate, this result suggests an additional effect arising from the acetate migration with presence of inorganic anion, i.e. membrane selectivity, which deserves future investigation.

ED desalination process performance

Once obtaining the characteristic parameters of mass transfer, one can evaluate ED process performance according to the proposed phenomenological model. As aforementioned, there are many different objectives for ED desalination, regarding varying water characteristic in different field. Then a case study in waste water treatment field, i.e. to biologically oxidize the pollutant organic content, was selected in order to illustrate the factors influencing the ED performance.

It is reported that higher salinity than a critical salt concentration inhibits the activity of the microbial, resulting into a limited biodegradation of the target organic compounds [START_REF] Ludzack | Tolerance of high salinities by conventional wastewater treatment processes[END_REF][START_REF] Maciel | Phenol removal from high salinity effluents using Fenton's reagent and photo-Fenton reactions[END_REF]. Hence, ED desalination as pretreatment before biological process is to decrease salt concentration until its critical concentration, e.g. 0.3 eq•L -1 for sodium chloride (ca.18 g•L -1 ) [START_REF] Singlande | Improvement of the treatment of salted liquid waste by integrated electrodialysis upstream biological treatment[END_REF]. Such a concentration for diluate is thus a controlling factor of ED desalination in the following discussion of the ED performances.

Influence of the current

Fig. 4 depicts how the concentration of salt and organic solute in diluate and concentrate respectively vary with time, with results for acetic acid/NaCl system under 2A and 6A as an example. Desalting the diluate NaCl to 0.3 eq•L -1 under different currents means extracting a given amount of salt, or more precisely identical quantity of electrical charge, as production of current and time (I.t).

Clearly, to reach the objective (critical salt concentration in diluate), a higher current gives less time required. Meanwhile, different concentration of organic solute is achieved, with a higher current giving a lower organic solute concentration.

Table 6 further compares the organic solute transfer under different current, given the same amount of salt removed, where organic solute concentration in concentrate are calculated by Eq. ( 8). More precisely, three concentration values are reported, referred as the total value Table 4 Organic solute permeability in different diffusion systems according to Eq. ( 4

Table 5

Additional transfer coefficients γ for varying organic solutes according to Eq. ( 6); [inorganic salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 . C OM total considering both the diffusion and convection, the value owing to diffusion only (C OM diff , assuming γ = 0), and the one owing to convection only (C OM conv , assuming P OM = 0), respectively.

γ × 10 -7 mol•m -2 •s -1 •A -1 Glucose
Expectedly, the fixed initial and final salt concentrations give the constant electrical charge. Regarding the total concentrations C OM total , the value under 2A is about 2 times of that under 6A, showing that higher current leads to lower organic concentration. Furthermore, the value C OM diff is time dependent under these two currents, while the values C OM conv are constant, fixed by electrical charge (I.t). The fact that under the two currents less difference between the organic solute concentrations (C OM total ) is found, comparing with the gap between the two values of C OM diff , can be attributed to the contribution of convection, This finding confirms that in ED organic solute convection is important to be considered. Similar finding is also observed for other systems. Specifically, the migration of acetate, as the convection for neutral solute, is also fixed by electrical charge, referred to Eq. ( 8).

Influence of the solute

The impact of the present organic solute nature is discussed regarding ED performance. Fig. 5 depicts how the concentration of organic solute in concentrate varies with the concentration of salt in diluate, for system of NaCl mixed with varying organic compounds respectively, under 2A. Notably, X and Y axis represents the desalination process and the simultaneous organic solute transfer with time, respectively.

Expectedly, the concentration of organic solute increases in concentrate with decreasing concentration of salt in diluate. The influence of solute nature is observed on such a concentration increase, the order of the sequence is glucose < acetic acid < acetate < phenol, for the concentration values in ppm.

Different sequence for organic solute transfer as glucose < phenol < acetic acid < acetate is reported (Section 4.2), for the concentration values in unit of mole. Higher M w of phenol (94 g•mol -1 ) explains the displacement of phenol in sequence, comparing with that of acetate (59 g•mol -1 ). Nevertheless, this result in Fig. 5 suggests that it is more difficult to treat phenol-contained solution than others, owing to its higher phenol transfer.

Influence of the salt

Fig. 6 illustrates the effect of salt on ED performance, comparing the transfer of acetic acid and acetate due to the present NaCl and Na 2 SO 4 , respectively.

Regardless of the solute nature of charge, the organic compound transfer with Cl -is always lower than that with SO 4 2 -, under given salt concentration in diluate. Such a difference owing to the present anion on the transfer of organic compound is more significant for acetate than for acetic acid.

Table 7 further compares the impact of the salt composition on organic transfer, when the final diluate concentration reached 0.3 eq•L -1 .

Clearly, the difference of organic transfer with anion SO 4 2and Cl - is shown with each compound. The same tendency for transfer of glucose, acetic acid and acetate is observed: SO 4 2-> Cl -. Such an impact of anion is more pronounced for acetate, acetate transfer with SO 4 2being higher compared to that with Cl -by a factor of 3-4.

Contrarily, in the case of phenol, Cl -gives a higher transfer than that with SO 4 2by a factor of ca. 1.5. The reasons are already discussed in Section 4.2, thus not detailed here.

In ED process point of view, improved desalination performances are suggested by the solution composition modification. Using the reported highest transfer for system of acetate -SO 4 2for example, the following two approaches can be recommended to reduce the acetate transfer. One is to transform acetate, from a weak acid salt to acetic acid (its neutral form), by pH adjustment. Indeed, Table 7 clearly shows that the concentration for acetate, ca. 3200 ppm drastically declined to ca. 800 ppm for acetic acid in the investigated condition. The other approach to decrease the acetate transfer depends on co-existing salt or ion species, e.g. choosing a membrane with a stronger selectivity or changing the salt composition. The latter approach is confirmed by Table 6 Desalination performance under two different currents NaCl/Acetic acid system, organic solute concentration calculated according to Eq. ( 8 Influence of the salt composition on organic solute transfer NaCl-Na 2 SO 4 /organic solute systems, I = 2A, [inorganic salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 (acetate 5900 ppm, acetic acid 6000 ppm, phenol 9400 ppm, glucose 18,016 ppm) organic solute concentration calculated according to Eq. ( 8). 7).

To sum up, attention for a better ED performance control should concern not only process parameters (like current) but also the solution compositions (both salt and organic solutes).

Conclusions

The objective of this paper was to propose a phenomenological model in order to investigate the electrodialysis performance in desalting saline solution containing organic solute. Experiments were performed using brines of varying compositions, i.e. salt mixed with organic solute of different nature of size, hydrophilicity, charge. The mass transfer investigation for solvent and solute in electrodialysis was conducted.

Firstly, inorganic salt and water transfer was studied, and corresponding characteristic parameters (α and β) were experimentally obtained. The presence of neutral solute didn't affect the transfer of salt and water. Competition of acetate with inorganic salt for current transfer was observed, where the total salt transfer with both contributions included is still constant and equal to those without solute.

Organic solute transfer comprised contribution of diffusion, characterized by (P OM ), and additional transfer, characterized by (γ) which is proportional to the current. More precisely, the additional transfer of organic compound is convection for the neutral one owing to electroosmosis, migration for charged one. Acetic acid convection is less important than acetate migration.

Finally, a phenomenological model involving 4 characteristic coefficients was proposed, on the basis of the transfer of salt (α) and water (β) as well as organic solute, including the diffusion (P OM ) without current and the additional one (γ) with current. With this model, it is possible to calculate the variation of both the solution volume and concentration for salt and organic solute respectively, in either compartment, as a function of their initial values and of the current. The formalism of model is valid for any system. However, the parameters (for transfer of salt (α), water (β) and organic solute (P OM , γ)) depend on the membrane properties and they have to be determined experimentally. Application of the model in up-scaled system is highly possible given similar operation of electrodialysis in a quasi-steady conditions, such as comparable potential drop per cell, hydrodynamics and mass transfer in the channel, etc. [START_REF] Laktionov | Method of electrodialysis stack testing with the feed solution concentration regulation[END_REF][START_REF] Nikonenko | Description of mass transfer characteristics of ED and EDI apparatuses by using the similarity theory and compartmentation method[END_REF].

ED performance was then evaluated in a case study regarding the effect of the current and solution compositions. For a fixed salt quantity removed, a lower current gives a higher organic solute transfer. This result can be mainly attributed to the contribution of the solute diffusion, which depends on time. Additional transfer, also important in ED process, is controlled by the electrical charge. Finally, the effect of the solution compositions with a regard to both the organic solute and salt nature was discussed. Sulfate resulted higher solute transfer compared to chloride for glucose and acetic acid due to solute dehydration effect. Same trend but more important influence was observed for acetate due to anion selectivity. The reversed trend was observed for phenol. This result suggests pretreatment to further improve the ED performance such as salt composition and solution pH control.

Nomenclature

Fig. 1 .

 1 Fig. 1. Schematic drawing of ED desalting saline solution containing organic solute.

Fig. 2 .

 2 Fig. 2. Flux variation of the inorganic salt and water versus current; system consisting of NaCl and acetate, [salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 .

Fig. 3 .

 3 Fig. 3. Variation of the organic solutes flux density versus current for varying NaCl/ organic solute systems; [NaCl] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 .

Fig. 4 .

 4 Fig. 4. Variation of the concentration of salt in diluate and concentration of organic solute in concentrate versus time: influence of the current NaCl/acetic acid system, I = 2A and 6A, [NaCl] = 0.8 eq•L -1 , [acetic acid] = 6000 ppm.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Variation of the concentration of organic solute in concentrate versus the concentration of salt in diluate: influence of the solute NaCl/Organic solute systems, I = 2A, [inorganic salt] = 0.8 eq•L -1 , [organic solute] = 0.1 mol•L -1 (acetate 5900 ppm, acetic acid 6000 ppm, phenol 9400 ppm, glucose 18,016 ppm)

1 )

 1 List of symbolsC OM , D 0 , C OM , C 0 intial solute concentration in diluate and concentrate(mol•m -3 ) ΔC OM solute concentration gradient (mol•m -3 ) I current intensity (A) j s salt flux (eq•m -2 •s -1 ) j w water flux (m 3 •m -2 •s -1 ) j OM organic solute flux (mol•m -2 •s -1 ) M w molecular weight (g•mol -1 ) P OM solute permeability (m•s -1 ) S mtotal membrane area of one type, AEM or CEM (m 2 )Greek symbolsα current coefficient (eq•m -2 •s -1 •A -1 ) β electroosmotic coefficient (m 3 •m -2 •s -1 •A -1 ) γ additional transfer coefficient (m•s -1 •A -

Table 1

 1 Characteristics of the four organic solutes used in this study.

	Chemical name and formula	M w (g•mol -1 )	Log P a	pKa	pH
	Glucose	C 6 H 12 O 6	180.16	-2.93	12.28	6.0 ± 0.5
	Phenol	C₆H₅OH	94.11	1.48	9.99	5.4 ± 0.3
	Acetic acid	C 2 H 4 O 2	60.05	-0.32	4.76	3.0 ± 0.2
	Acetate	C 2 H 3 O 2 -	59.04	-	4.76	6.7 ± 0.3

a Solute hydrophobicity (P, partition coefficient between solvents of octanol and water)

[START_REF] Leo | Partition coefficients and their uses[END_REF]

.

Acknowledgements

China Scholarship Council (CSC), Ministry of Education, P. R. China is acknowledged for the scholarship provided to Le Han.


Olivier Jeannin

A.M.S H.-T Huynh

M Riel

Fourmigué

Huu-Tri Huynh

Asia Marie

S Riel

Marc Fourmigué

Chalcogen bonding interactions in organic

Introduction

The rediscovery of halogen bonding (XB) interactions in the last twenty years 1 has recently evolved toward the identification of similar properties in the chalcogen, pnictogen or even tetrel series. 2,3 Recent illustrations of chalcogen bonding are found for example in the solid state structures of benzo-2,1,3-selenadiazoles, 4 benzo-1,3-tellurazoles, 5 iso-tellurazole N-oxides, 6 or selenophtalic anhydride, 7 as well as in the use of chelating bis-tellurophene 8 or bis(benzimidazolium-selenomethyl) derivatives 9 in catalytic reactions. One striking difference with the halogen bond donors is the distribution of the electrostatic surface potential. While one single charge-depleted area, the σ-hole, is identified in the prolongation of the covalent C-Hal bond, theoretical and experimental investigations of molecules with activated chalcogen atoms have unambiguously demonstrated the presence of two such charge-depleted area, each of them in the prolongation of the two carbon-chalcogen bonds. 2,3,10 The simplest and most convincing example was already identified years ago in Se(CN)2, 11 and revisited recently. 12 Its X-ray crystal structure (Fig. 1a) shows indeed two nitrogen atoms of neighboring molecules pointing toward the σ-holes of the selenium atom, with Se•••N intermolecular distances, 2.813 (9) and 2.835 (7) Å, well below the sum of the van der Waals radii (3.45 Å), and C-Se•••N angles close to linearity (166-170°). Cocrystallization of Se(CN)2 with 18-crown-6 is also reported to afford a co-crystal (Fig. 1b), 13 where again the two σ-holes of the selenium atoms are clearly interacting with two oxygen atoms of the 18-crown-6 in a 1:1 adduct. Selenophtalic anhydride provides another example where both charge-depleted area were identified from an experimental high resolution X-ray data collection. Considering that the predictability of halogen bonding in crystal engineering strategies is essentially due to the presence of one single σ-hole in the prolongation of the C-X bond, we postulated that the ability of chalcogen atoms to provide a similar predictability despite the presence of two σ-holes can be strongly enhanced if one is able to favor one σ-hole over the other. For that purpose, the use of unsymmetrically substituted chalcogen atoms, with one electron-withdrawing (EWG) group and one electron-releasing (ERG) group should favor the presence of a stronger σ-hole in the prolongation of the EWG-Se bond. 9 Based on this assumption, organic selenocyanates appear as ideal candidates, as the selenium atom is simultaneously linked to the strongly electron-withdrawing nitrile substituent and to a more electron-releasing alkyl, benzyl or aryl group. Following earlier conclusions by Bauza et al.,3a investigations of the crystal structures of organic selenocyanates reported in CSD 14 confirmed this assumption, as detailed below. We also recently showed that molecules bearing two such selenocyanate moieties, 15 as the ortho-, meta-and para-bis(selenocyanato)xylene (Chart 1) are indeed able to act as ditopic chalcogen bond donors when faced with ditopic Lewis bases such as 4,4'-bipyridine to afford one-dimensional structures. We want here to extend this pool of benzylic selenocyanates to the tris and tetrakis-substituted derivatives (Chart 1), as described in a second part below, where their preparation and solid state structures will be reported and analyzed.

Se

Se

Se

Se

Se

Results

Analysis of reported solid state structures of organic selenocyanates

Most of the organic selenocyanates found in CSD show evidence of a chalcogen bonding interaction involving the selenium atom as chalcogen bond donor. In the following, we will described successively: (i) systems with intramolecular chalcogen bond, (ii) aromatic selenocyanates, (iii) benzylic and allylic selenocyanates, (iv) aliphatic selenocyanates. In the following, the evaluation of the strength of the chalcogen bond in all these systems will be based primarily on the reduction ration of the actual Se•••Y distance, relative to the sum of the van der Waals radii of the interacting atoms, with Se: 1.90 Å, N: 1.55 Å, O: 1.52 Å, that is

dvdW(Se•••N) = 3.45 Å, dvdW(Se•••O) = 3.42 Å, dvdW(Se•••Se) = 3.80 Å.
Intramolecular chalcogen bonding is found in three examples (Fig. 2), namely methyl 2-selenocyanatobenzoate, 16 phenacyl selenocyanate 17 and 8-(dimethylamino)-1-naphthyl selenocyanate. 18 The planarity of the three systems despite the strong steric constraints demonstrates the stabilization brought by the chalcogen bond interaction. The chalcogen bond distances are accordingly very short, with reduction ratio down to 0.72. The interaction takes place in the prolongation of the Se-CN bond, demonstrating also the selective activation of one σ-hole site on the selenium atom by the opposite nitrile group. It already confirms the interest of organic selenocyanates derivatives to activate one single strong σ-hole on the selenium atom. Cholesterol derivative ZUTTAL 3.36(2) 0.974 157.9(7) (a) Two crystallographically independent chains. (b) Two independent molecules alternating in one chain.

Aromatic selenocyanates represent the largest reported group, with an interesting series provided by phenyl selenocyanate, 19 pentafluorophenyl selenocyanate, 20 1,4bis(selenocyanato)benzene 21 and 2,4,6-tris(trifluoromethyl)phenyl selenocyanate. 22 As shown in Fig. 3, a linear Se•••N interaction involving the nitrogen atom of the C≡N group takes place, leading to the recurrent formation of chain-like motifs

•••Se(R)-CN•••Se(R)-CN•••.
We also observe a strengthening of the interaction with the most-electron-withdrawing aromatic cores, most probably attributable to an enhancement of the σ-hole on the selenium atom. We believe that such one-dimensional systems are also stabilized by cooperativity, as indeed theoretically demonstrated in model systems. 23 Structural characteristics are gathered in Table 1. A similar Se•••N interaction is also observed in 1,4-bis(selenocyanato)benzene (CSD : SECNBZ ) where it develops in two dimensions. Another interesting example are provided 3-selenocyanatopyridine 24 (Fig. 4) where the pyridinyl nitrogen atom is now engaged in the chalcogen bond, with a short Se•••NPy distance (RR = 0.824), demonstrating that the nitrile N atom can be easily displaced from the chalcogen bond by stronger Lewis bases, as already illustrated above in the Introduction with the adduct of Se(CN)2 with 16-crown-6 (Fig. 1b). The robustness of these chains is however questioned when a stronger chalcogen bond acceptor is present. This is indeed the case in 4-nitrobenzyl-selenocyanate 25 and 2-(methylselanyl)benzyl selenocyanate 28 where either oxygen atoms from a -NO2 group or selenium atom of a -SeMe group act as chalcogen bond acceptors (Fig. 6). The structure of 4nitrobenzyl-selenocyanate is particularly interesting ( Two examples involving aliphatic selenocyanates involve 1,1-bis(selenocyanatoethyl) cyclohexane 29 (CSD: GIHMEW) and a cholesterol derivative (CSD: ZUTTAL). 30 They also exhibit this chain motif but with rather weaker interactions (Table 1).

Novel tris-and tetrakis organic selenocyanates: a recurrent chain motif

Following our preliminary experiments aimed at unravel the ability of bis(selenocyanates) derivatives such as the three ortho-, 26 meta-, 15 and para-bis(selenocyanatato)xylene 27 to form one-dimensional structures upon co-crystallization with neutral ditopic Lewis bases (4,4'bipyridine), we turned our attention to the corresponding tris-and tetrakis-substituted derivatives, namely 1,3,5-tris(selenocyanatomethyl)benzene (1a), 1,3,5-tris(selenocyanato methyl)-2,4,6-trimethylbenzene (1b) and 1,2,4,5-tetrakis(selenocyanatomethyl)benzene (2).

We describe in the following their synthesis and analyze in details their crystal structures, where 1a crystallizes in the monoclinic system, space group P21/n, 1b in the monoclinic system, space group C2/c, and 1b•DMF in the triclinic system, space group P-1 with in the three structures one molecule in general position. On the other hand, tetra-substituted compound 2 crystallizes on an inversion center, in the monoclinic P21/c space group. As shown in Fig. 7ac, in the tritopic derivatives 1a and 1b, we find two SeCN groups above the benzene ring with one below. A similar geometry for 1b is found in its DMF solvate (Fig 7d). Note also the positional disorder of one selenocyanate group in 1b, with a 29:71 distribution for Se31 and Se32. The solid state organization of 1a (Fig. 8) is characterized by the occurrence of two strong Se•••N chalcogen bonds (see Table 2) leading to the formation of infinite chains 170.0(2) i (1+x, y, z). ii (0.5+x, 1.5-y, 0.5+z). iii (0.5-x, 0.5-y, 1-z). iv (-x, 1-y, 1-z). v (-x, y, 1.5-z). vi (-x, -y, 1-z). vii (-1+x, y, z). viii (2-x, -y, -z). ix (x, 1.5-y, -0.5+z). x (1+x, 1.5-y, 0.5+z). xi (2-x, 2-y, 1z). The structure of 1b is more complex (Fig. 9). Two selenium atoms (Se1, Se2 The structure of the DMF solvate of 1b is shown in Fig. 10 and it strongly differs from that of the pure compound. Indeed, the three selenocyanate groups are now engaged in short and directional Se•••N interactions running parallel to each other to form a chain, while one of the selenium atoms also interacts weakly (RR = 0.96) with the carbonyl oxygen atom of DMF. The structure of 2 (Fig. 11) is characterized by a strong Se1•••N1 interaction leading to the formation of layers. These layers are interconnected along a through two weaker interactions, one involving again Se1 as chalcogen bond donor, the other involving Se2 (see Table 2). As mentioned above, both 1b and 2 were found to also co-crystallize with solvent molecules, affording two different solvates, namely 1b•AcOEt and 2•(DMF)2. Their molecular structures are detailed in Fig. 12, their structural characteristics in Table 3.

the chain-like motif •••Se(R)-CN•••Se(R)-CN•••
•••Se1- C1≡N1••• Se1-C1≡N1••• and •••Se2-C2≡N2••• Se2-C2≡N2•••
In 1b•AcOEt, the AcOEt molecule is disordered on two equivalent (50:50) positions. In the solid state (Fig. 12a), the carbonyl oxygen atom competes now with the nitrile groups to engage in a chalcogen bond with one of the three selenium atoms of the 1b molecule. As a consequence, the infinite chain motifs observed above in the structure of 1b (Fig. 9) are now cut into a defined segment of three chalcogen bonds in the order (O11, O12) 2) i (0.5-x, 0.5-y, 1-z). ii (-0.5+x, -0.5+y, z). iii (-x, -y, 1-z). iv (1-x, -y, 1-z)

•••Se2- C≡N2•••Se3-C≡N3•••Se1-C≡N1,
Compound Interaction Se•••Y (Å) RR C-Se•••Y (°) 1b•(AcOEt) Se1 N3 i Se2 O11 ii Se2 O12 ii Se3 N2 iii

Conclusions

We have demonstrated here that organic selenocyanates provide a tutorial example of chalcogen bond donors. Compared with symmetrical selenides where experimental and theoretical evidences confirm the presence of two equivalent σ-holes located in the C-Se-C plane in the prolongation of the C-Se bonds, the unsymmetrical character of R-Se-CN derivatives strongly favor the σ-hole in the prolongation of the NC-Se bond. Recurrent features are found from this extensive set of crystal structures based on organic selenocyanates. The most striking one is the formation of one-dimensional superstructures derived from the complementary nature of the R-SeCN molecules, with the lone pair of the nitrogen interacting with the selenium σ-hole through a linear C-Se•••NC interaction and a reduction ration (RR) around 0.86-0.87. This interaction is however relatively weak since it is displaced by pyridinyl nitrogen atom or with carbonyl (or nitro) oxygen atom, leading then to slightly stronger chalcogen bonds, with RR values down to 0.82. The observed chain-like structures are also a probable consequence of an extra stabilization brought by cooperativity. Also, the structure of 2•(DMF)2 with two ortho CH2SeCN groups bonding to the carbonyl oxygen atom of the DMF (Fig. 12b) demonstrates that such ortho-substituted derivatives can adapt their geometry to act as chelating systems.

Such ditopic or tritopic chelate structures have been recently considered in halogen bonded systems, 31 either for anion recognition purposes 32 or for catalytic applications. 33 In these reported examples however, complex structural motifs have to be elaborated to orient two (or three) iodine atoms in a convergent interaction. The example of this selenocyanate derivative 2 in its DMF solvate demonstrates that such a goal can most probably be reached in much simpler molecules than these complex poly-iodinated ones. Work is also underway to test these assumptions. 1,3,5-tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene, 1,2,4,5tetrakis(bromomethyl)benzene were obtained from Aldrich or Across and used as received.

Experimental Section

Synthesis

Synthesis of 1a.

A solution of potassium selenocyanate (0.32 g, 2.24 mmol, 4 equiv) in acetone (5 mL) is added dropwise over a period of 10 minutes on a solution of 1,3,5tris(bromomethyl)benzene (0.2 g, 0.56 mmol, 1 equiv) in acetone (5 mL). The solution gets cloudy and a solid appear. The reaction is monitored by TLC (eluent petroleum ether/Ether 1/2). After completion of the reaction (typically 25 minutes), the mixture was filtered. The filtrate was evaporated under reduced pressure. The white solid was washed twice with 15 mL of warm water (40°C) in ultrasonic bath during 5 minutes. The solid was filtered and dried overnight at 80°C. M.p. 168 °C, yield 45%. 1 H NMR (300 MHz, d6-acetone) δ 7.51 (s, 3H), 4.48 (s, 6H). 13

Synthesis of 2.

A solution of potassium selenocyanate (0.38 g, 2.6 mmol, 6 equiv) in DMF (5 mL) is added dropwise over a period of 10 min in a solution of 1,2,4,5tetrakis(bromomethyl)benzene (0.2 g, 0.4 mmol, 1 equiv). An orange colour appears quickly and the solution gets cloudy. The reaction is monitored by TLC (eluent petroleum ether/Ether 1/2). After completion of the reaction (typically 25 minutes), addition of 15 mL of warm water (40°C) precipitated a solid. The solid was washed twice with 15 mL of warm water (40°C) in ultrasonic bath during 5 minutes. The solid was filtered and dried overnight at 80°C. White solid, dec T>160°C (yield 94%). Recrystallization was performed from PhCN by vapor diffusion of Et2O. When DMF is used rather than PhCN, a DMF solvate is obtained instead formulated as 2•(DMF)2. 1 H NMR (300 MHz, DMSO-d6) δ 7.38 (s, 2H), 4.45 (s, 8H). 13 C NMR (300 MHz, DMSO-d6) : 136.8 (C=C), 133.8 (=CH), 104.9 (CN), 29.6 (CH2). 77 Se NMR (d6-

7

 7

Fig. 1

 1 Fig. 1 Detail of the solid state structure of (a) Se(CN)2 , and (b) Se(CN)2•(18-crown-6) adduct in Se(CN)2•(18-crown-6)1.5. Chalcogen bond interactions are indicated as orange dotted lines.

  Reported (top) and novel (1a, 1b, 2) benzylic selenocyanates

Fig. 3

 3 Fig. 3 The chain-like structures formed by chalcogen bonding in (a) phenyl selenocyanate, (b) pentafluorophenyl selenocyanate, and (c) 2,4,6-tris(trifluoromethyl)phenyl selenocyanate.

Fig. 4

 4 Fig. 4 Chalcogen bonding in 3-selenocyanatopyridine Benzylic selenocyanates are easily prepared from benzyl halide and potassium selenocyanates. Many examples were therefore reported and some of them were structurally characterized. Benzylselenocyanate 25 itself and the three ortho-, 26 meta-, 15 and parabis(selenocyanatato)xylene 27 have been reported. As shown in Fig. 5, they all exhibit the recurrent •••Se(R)-CN•••Se(R)-CN••• chain-like motif. The Se•••N intermolecular distances are comparable to those reported above in aromatic selenocyanates.

Fig. 5

 5 Fig. 5 Recurrent •••Se(R)-CN•••Se(R)-CN••• chain-like motif in benzylic selenocyanates such as (a) benzylselenocyanate (hydrogen atoms were omitted), (b) ortho-bis(selenocyanatato)xylene, (c) metabis(selenocyanatato)xylene, and (d) para-bis(selenocyanatato)xylene

Fig. 6

 6 Fig. 6 Detail of the supramolecular motifs developed by (a) 4-nitrobenzyl-selenocyanate, and (b) 2-(methylselanyl)benzyl selenocyanate (Hydrogen atoms were omitted).

  mentioned above is also acting as a powerful supramolecular motif. Both 1a, 1b and 2 were prepared from the reaction of the corresponding tris(bromomethyl) and tetrakis-(bromomethyl)benzene derivatives with KSeCN (see Exp. Section). In the presence of AcOEt or DMF, 1b and 2 also crystallize as solvates, formulated as 1b•AcOEt, 1b•DMF and 2•(DMF)2. We will first describe the crystal structures of 1a, 1b, 1b•DMF and 2 where only Se•••NC interactions are found, while the structures of 1b•AcOEt and 2•(DMF)2 will be described in a second part, as they also involve Se•••O chelating interactions with the solvent molecules (see below).

Fig. 7

 7 Fig. 7 Molecular structures of (a) 1a, (b) 2, (c) 1b and (d) 1b in 1b•(DMF).

  running along the a direction (Fig 8a). The third selenocyanate group connects those chains along b through a weaker •••Se3-C3≡N3••• Se3-C3≡N3••• chalcogen bond.

Fig. 8

 8 Fig. 8 Solid state organization of 1a showing (a) the chalcogen-bonded chains running along a, and (b) the lateral Se3•••N3 interactions.

  ) interact with nitrogen atoms to generate a chain running along a [1 -1 0] direction. A third disorder selenium atom (Se31) makes a very short Se•••Se contact with a neighboring chain running along the [1 1 0] direction. As a consequence of this interchain Se•••Se interaction, one nitrogen atom (N2)is not engaged in a short contact.

Fig. 9

 9 Fig. 9 Solid state organization of 1b showing the chalcogen-bonded chains running along a.

Fig. 10

 10 Fig. 10 Solid state organization of 1b•DMF

Fig. 11

 11 Fig. 11 Projection view along a of one layer in 2, built out the strongest Se1•••N1 chalcogen bond.

  with the N1 nitrogen atom not engaged in chalcogen bonding. The structure of the DMF solvate, 2•(DMF)2, reveals another facet of these benzylic selenocyanate derivatives. Indeed, as shown in Fig 11b, the molecule, located on inversion center, actually binds with the carbonyl oxygen atoms of the two DMF molecules, with two neighboring selenium atoms interacting with the same oxygen atom, providing a very attractive chelating system.

Fig. 12

 12 Fig. 12 Details of the supramolecular organization in (a) 1b•AcOEt, and (b) 2•(DMF)2.

  Organic selenocyanates form recurrent chain-like motifs •••Se(R)-CN•••Se(R)-CN••• through short and linear chalcogen bonding Se•••N≡C interactions. A chelating motif is also observed in a DMF solvate with two neighboring CH2-SeCN groups linked to the DMF oxygen atom.

°2.723 A °2.456 A °RR = 0.796 RR = 0.712

	MeO	O 2.561 A FAGGAV Se O CN AHEQIN	Se	CN	Me 2 N GIYJUV Se CN
		RR = 0.749
	Fig. 2 Details on intramolecular chalcogen bond interactions in three reported examples (with CCDC
	REFCODE)

Table 1

 1 Details of the structural characteristics of the chalcogen bond interactions in reported compounds. RR stands for reduction ratio and is given by the ratio of the observed interatomic distance over the sum of the van der Waals radii of interacting atoms.

	Compound	REFCODE Se•••N(O,Se)	RR	C-Se•••Y	Ref
			(Å)		(°)
	Se(CN)2	UQAXUH	2.813(9)	0.815	166.3(2)
			2.835(7)	0.822	170.4(2)
	Se(CN)2•1.5 (18-crown-6)	QUHYAV	2.873(2) (O)	0.840	167.2(1)
			2.938(10) (O)	0.856	161.2(1)
	Intramolecular:
	Methyl	AHEQUIN 2.561(1) (O)	0.749 170.34(6)
	2-selenocyanatobenzoate
	phenacyl selenocyanate	FAGGAV	2.723(6) (O)	0.796 152.0(1)
	8-(NMe2)-1-naphthyl	GIYJUV	2.456(2)	0.712 172.65(7)
	selenocyanate
	Aromatic selenocyanates:
	PhSeCN	CIBFUP	3.023(3) (a)	0.876	172.9(1)
			3.065(4)	0.888	166.1(1)
	p-(SeCN)2C6H4	SECNBZ	3.06(2)	0.887 162.32(8)
	C6F5SeCN	BATDIJ	2.958(10) (a)	0.857	175.6(8)
			2.964(9)	0.859	172.1(4)
	C6H2(CF3)3SeCN	KABTEN	2.883(6) (b)	0.836	173.1(2)
			2.968(5)	0.860	166.4(2)
	3-(SeCN)pyridine	WERYAT	2.843(10)	0.824 174.0(2)
	Benzylic selenocyanates:
	Benzylselenocyanate	CIGGOO	2.997(18)	0.869 167.1
	ortho-bis(SeCN)xylene	NARBIR	2.985(8)	0.865	172.9(3)
			2.969(9)	0.860	172.6(3)
	meta-bis(SeCN)xylene		2.965(24) (a)	0.859	175.9(7)
			3.017(24)	0.874	175.7(7)
			3.010(24)	0.872	172.9(7)
			3.015(24)	0.874	174.1(7)
	para-bis(SeCN)xylene	POXYEH	2.997(18) (a)	0.860	174.4(8)
			3.022(18)	0.876	171.8(5)
	4-nitrobenzyl-selenocyanate	CIGGEE	3.005(7) (O)	0.879	166.2
			3.174(9) (O)	0.928	163.9
	2-(MeSe)benzylselenocyanate	YUNSIK	3.467(1) (Se)	0.912 163.4(1)
	Aliphatic selenocyanates:
	1,1-bis(selenocyanatoethyl)	GOHMEW 3.199(2)	0.927 145.7(8)
	cyclohexane

Table 2

 2 Structural characteristics of chalcogen bonds in the crystal structures of 1a

, 1b, 1b•(DMF) and 2.

	Compound	Interaction	Se•••Y (Å)	RR	C-Se•••Y (°)
	1a
	intrachain	Se1	N1 i	3.023(5)	0.876	168.5(2)
		Se2	N2 i	3.011(5)	0.873	169.3(2)
	interchain	Se3	N3 ii	3.243(8)	0.941	134.8(2)
	1b
	intrachain	Se1 N31 iii	2.986(18)	0.865	171.4(4)
		Se1 N32 iii	3.292(10)	0.954	155.8(2)
		Se2 N1 iv	3.188(4)	0.924	177.4(2)
	interchain	Se31 Se31 v	3.078(5)	0.810	178.6(5)
		Se32 N1 vi	3.336(6)	0.967	155.8(3)
	1b•(DMF)
	intrachain	Se1 N1 i	2.960(4)	0.858	175.4(1)
		Se2 N2 i	2.964(2)	0.859	176.1(1)
	with DMF	Se3 N3 vii	2.972(3)	0.861	173.8(1)
		Se1 O1 viii	3.284(6)	0.960	163.8(1)
	2
	intra layer	Se1 N1 vii	3.085(5)	0.894	164.0(2)
	inter layer	Se1 N2 viii	3.202(20)	0.928	167.6(2)
		Se2 N1 ix	3.370(14)	0.977

Table 3

 3 Structural characteristics of chalcogen bonds in the crystal structures of the solvates 1a, 1b, 2

  Synthesis of 1b. Procedure adapted from Lari et al..34 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene (0.098 g, 0.25 mmol) was added to a 50 mL round bottom flask, dissolved in 20 mL of acetone, and purged with Ar for 10 minutes. Potassium selenocyanate (0.209 g, 1.503 mmol) was dissolved in acetone (7 mL). KSeCN solution was added dropwise and stirred for 1 hour under inert atmosphere. The reaction was filtered and condensed under vacuum to leave a crude off-white, beige powder. The crude powder was dissolved in the minimal amount of DMF, precipitated out with H2O and filtered to give a white powder that was recrystallized by vapor diffusion of ether into a solution of 1b in ethyl acetate, leaving

	C NMR (300 MHz, d6-acetone): 138.9 (C=C), 129.3 (=CH), 102.1 (CN), 31.6
	(CH2). 77 Se (d6-DMSO, 25˚C): 314.26 ppm (3 Se, s). Elem. Anal. Calculated for C12H9N3Se3:
	C, 33.36; H, 2.10; N, 9.72 %. Found: C, 33.97; H, 2.46; N, 9.31%.
	Synthesis of 1b. clear, colorless crystals (0.094 g, 81%). Using DMF instead of AcOEt afforded the DMF
	solvate. 1 H (300 MHz, d6-acetone 25˚C): 8.6234-8.6114 (6H, S), 7.5059-74938 (9H, s). 13 C
	(300 MHz, d6-acetone, 25˚C): 138.8764, 132.7166, 102.1961, 29.7677, 17.0772. 77 Se (d6-
	DMSO, 25˚C): 250.45 (3 Se, s). Elem. Anal. Calcd. for C15H15N3Se3: C, 37.99; H, 3.19; N, 8.86
	%. Found: C, 37.82; H, 3.34; N, 8.75%.

Table 4

 4 Crystallographic data.

		1a	1b	1b•DMF	1b•EtOAc	2	2•(DMF)2
	CCDC	1820888	1820891	1820889	1820890	1820892	1820893
	Formula	C12H9N3Se3	C15H15N3Se3	C18H22N4OSe3	C19H23N3O2Se3	C14H10N4Se4	C20H24N6O2Se4
	Formula moiety	C12H9N3Se3	C15H15N3Se3	C15H15N3Se3,	C15H15N3Se3,	C14H10N4Se4	C14H10N4Se4,
	FW (g.mol -1 )	432.10	474.18	C3H7NO 547.27	C4H8O2 562.28	550.10	2(C3H7NO) 696.29
	System	monoclinic	monoclinic	triclinic	monoclinic	monoclinic	triclinic
	Space group	P21/n	C 2/c	P-1	C 2/c	P 21/c	P -1
	a (Å)	5.9630(2)	18.8209(9)	5.9559(5)	18.5055(17)	5.4173(12)	9.224(5)
	b (Å)	23.1856(8)	10.1135(5)	10.0181(8)	10.3314(9)	13.172(3)	9.382(5)
	c (Å)	10.0548(4)	17.9834(7)	17.6631(13)	23.327(2)	11.795(2)	9.591(5)
	α (deg)	90.00	90.00	79.153(3)	90.00	90.00	108.086(15)
	β (deg)	93.046(2)	99.039(2)	82.977(3)	107.551(4)	97.451(14)	116.283(14)
	γ (deg)	90.00	90.00	84.524(3)	90.00	90.00	102.687(15)
	V (Å 3 )	1388.17(9)	3380.5(3)	1024.47(14)	4252.2(7)	834.5(3)	642.3(6)
	T (K)	296(2)	296(2)	150(2)	150(2)	296(2)	296(2)
	Z	4	8	2	8	2	1
	Dcalc (g.cm -1 )	2.068	1.863	1.774	1.757	2.189	1.800
	µ (mm -1 )	7.933	6.525	5.400	5.209	8.792	5.74
	Total refls	9778	15377	31696	48703	6327	14597
	θmax (°)	27.491	27.491	27.505	27.553	27.638	27.506
	Abs corr	multi-scan	multi-scan	multi-scan	multi-scan	multi-scan	multi-scan
	Tmin, Tmax	0.627, 0.924	0.243, 0.593	0.495, 0.994	0.575, 0.901	0.810, 0.916	0.184, 0.532
	Uniq. refls	3183	3860	4678	4906	1923	2942
	Rint	0.0346	0.0464	0.0517	0.0604	0.0655	0.0519
	Uniq. refls	2299	2312	4335	3914	1281	2248
	(I > 2σ(I))
	R1	0.0513	0.0381	0.0316	0.0342	0.0439	0.040
	wR2 (all data)	0.1066	0.0991	0.0706	0.0816	0.1185	0.0906
	GOF	1.097	0.891	1.198	1.06	0.914	1.118
	Res. dens. (e Å -3 )	0.765, -0.727 0.505, -0.636 0.865, -1.3	0.535, -1.471	0.745, -0.632	0.891, -0.941

Acknowledgements

Financial supports from (i) ANR (Paris, France) through contract ANR-17-CE07-0025-02, (ii) Rennes Métropole (Decision A17.612) and (iii) the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States are acknowledged. We also thank CDIFX (Rennes) for access to X-ray diffraction facilities and C. Orione (Scanmat Rennes) for the 77 Se NMR experiments.

Crystallography

Data were collected on an APEXII, Bruker-AXS diffractometer at room temperature for 1a, 1b, 2, 2•DMF, and on D8 VENTURE Bruker AXS diffractometer at 150 K for 1b•DMF and 1b•EtOAc. Both diffractometers operate with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods using the SIR92 program, 35 and then refined with full-matrix least-square methods based on F 2 (SHELXL-2014/7) 36 with the aid of the WINGX program. 37 All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions.

Crystallographic data on X-ray data collection and structure refinements are given in Table 4.

CCDC 1820888-1820893 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

Conflicts of interest

There are no conflicts to declare.