J.-P Champeaux

P Moretto-Capelle

J Renoud

L Polizzi

S Faure

D Castex

M Gianesin

E Panader

P Paquier

W Volondat

T Salbaing

J Riffaud

R Point

P Cafarelli

M Sence

Electron-Induced ionization and cationic fragmentations of the isolated molecule of 2,4-imidazolidinedione (Hydantoin) : Study of the relaxing paths thresholds

In this work, our new experimental setup has been used to study the ionization and fragmentation of the prebiotic molecule hydantoin by electron impact. Scanning of the incident electron energy allows the determination of the appearance thresholds of the cations. The vertical ionization potential was found to be in good agreement with previous data. Dissociation thresholds for the main fragmentation patterns were also measured. In parallel, thanks to quantum chemical calculations, reaction schemes compatible with the experimental results are given.

Introduction

Many complex molecules composed of carbon and hydrogen could be abundant in the interstellar medium, such as polycyclic aromatic hydrocarbons (PAHs), whose infrared signatures seem compatible with the infrared spectra emitted by some nebulae 1-4 . In the circumstellar environment, many organic molecules have been identified 5,6 . The physical chemistry of these organic molecules is not yet fully understood and is the starting point for many studies on their formation processes [START_REF] Contreras | [END_REF]8 as well as their life cycle and destruction in astrophysical media 9-11 . In the vicinity of the Sun, some molecules of biological interest have recently been detected, such as the amino acid glycine, ejected by the comet 67P/Churyumov-Gerasimenko (Chury) and detected by the Rosetta mission 12 . Such molecules, called prebiotic molecules, may have played a role in origin of life. Among them, 2,4-imidazolidinedione molecule named Hydantoin (C 3 N 2 O 2 H 4 , m=100 a.m.u) was identified with traces of glycine in samples of two meteorites, Murchinson and Yamato 13,14 , showing its presence in circumstellar environment (not detected in Chury comet). Laboratory studies by De Marcellus et al [START_REF] Marcellus | [END_REF] clearly show that hydantoin molecules can be produced by UV irradiating water ice containing methanol (CH 3 OH) and ammonia (NH 3 ). Ice grains simulate interstellar grains present in the cometary formation region. Kayanuma et al 16 also propose in their theoretical study that hydantoin can be formed from aminoacetonitrile (C 2 N 2 H 4 ) under cold astrophysical conditions, when the comets are heated by a neighboring star. Thus, if hydantoin can be assumed to be formed in space by various mechanisms, we need to consider the effect of stellar ionizing radiation and the corresponding molecular modification it can induce. The electronic structure of hydantoin (and derivatives) was investigated by Vondrack et al 17 by photoelectron spectroscopy after HeI irradiation. The ionization potential as well as the energies of the first excited states were derived. Ildiz et al. 18 isolate the molecule in Ar matrix at low temperature (10K), they realize infrared spectroscopy, which has been understood by accompanying theoretical investigation, but also by irradiation with 230 nm (5.4 eV) radiation: IR analysis after irradiation reveals three bodies fragmentation into isocyanic acid (HNCO), carbon monoxide (CO) and methylenimine (HNCH 2 ). While X-UV irradiation of prebiotic and organic molecules has been intensively studied in an astrophysical context 19 , the effect of charged particles (electrons, ions) that make up the stellar winds has received less interest. The ionization and fragmentation of the hydantoin molecule induced by interactions with minority species of the solar wind, namely the multicharged ions O 6+ and He 2+ , through a multi-ionization/excitation process has been investigated in a previous paper 20 . In this work, we study both experimentally and computationally the interaction of isolated neutral hydantoin with one of the most abundant species in the solar wind: electrons whose energies range from 8eV to 40eV. We will focus on the (e-,2e-) processes corresponding to the reaction scheme (1) above, considering only single ionization events of the molecule:

(1)
Here, e i -is the incoming electron from the stellar wind, e i -* the diffused electron after the interaction with hydantoin, C 3 N 2 O 2 H 4 +* the ionized and excited molecule, e -the emitted electron from the molecule to the continuum, A + and {B°} respectively the cationic and neutral(s) fragment(s) produced after molecular relaxation by dissociation. Single ionization as well as cationic fragmentation thresholds have been measured and will be presented in this paper. For each of the observed A + formed cations, a possible corresponding reaction scheme will be refined by theoretical investigations.

Experimental setup and data analysis

The SWEET experimental setup is a crossed-beam experiment dedicated to the study of the interactions between an isolated molecule and an energetic electron (or ion -but not used in this work -) whose energy can be adjusted to "simulate" the energy distribution of stellar winds. The entire experiment is maintained under vacuum, typically 10 -7 Torr.

The Earth's magnetic field inside the chamber is compensated using a Mu Metal shielding and external Helmholtz coils. Less than 4 mG of residual magnetic field is present in any direction inside the interaction chamber. Figure 1 shows a schematic view of the SWEET collision chamber. The electron beam is generated by a commercially available Kimball Physics electron gun. The electronic current can be measured by means of a Faraday cup which is placed in front of the electron gun, after the collision zone. The energy range of the gun is 2 eV to 2 keV, with an energy spread of 0.5 eV (given by the manufacturer). The electron beam is pulsed (KHz) for synchronization purposes; this is ensured by a constant negative potential applied to the grid electrode (Wehnelt) of the gun, on which we superimpose a positive nanosecond pulse coming from a homemade circuit detailed in Fig. 2, based on a ZTX415 avalanche transistor. The amplitude of this signal is set so that the grid potential is positive and allows the electrons to exit during the tuneable (via delay line choice) time duration of the pulse. The temporal width of the generated electron pulse was measured directly by the time-of-flight at a fixed electron energy using the SWEET high-resolution dispersive electron spectrometer (not shown in Fig. 1). An example of a 2.8 ns electron pulse at 35 kHz for 100 eV electrons obtained with this circuit is shown in Fig. 3. Commercial hydantoin powder (Sigma-Aldrich, 98% purity) is sublimated in an oven to produce the effusive jet of the neutral molecule. The geometry of the molecular jet produced by the oven was studied as a function of temperature for the same vacuum conditions as in SWEET (10 -7 Torr) and the same distance from the oven capillary to the interaction zone, using a specific setup. The optimal conditions to obtain a thin hydantoin molecular jet (less than 2 mm width in the interacting region) were found at 423 K (PT100 sensor) and used in the SWEET setup. At this temperature, assuming thermodynamic equilibrium inside the oven, the total internal thermal energy of the hydantoin molecule is 0.98 eV, evenly distributed in the different degrees of freedom of the molecule. This is not enough (statistically speaking) to change its geometry has been discussed in previous paper 20 where the activation barrier of the first conformers are calculated. It is then reasonable to assume that the hydantoin molecule will be produced in its electronic ground state (neutral) and in the absence of any conformation. The molecule in its fundamental geometry is shown in Figure 4. After each electron/molecule interaction, the cations emitted from the molecule are mass over charge analysed using a homebuilt 250mm time-of-flight mass spectrometer (TOF-MS) based on a modified Wiley MacLaren TOF with second order time focusing (see Figure 1). The extracting and spatial focusing electrodes are pulsed with an appropriate delay to the electron pulses coming from the electron gun to ensure that the electrons are out of the collision zone when the cationic fragments are extracted. This avoids perturbation of the electrons prior to interaction and ensures that all ions generated during the passage of a given electron packet are extracted simultaneously: there is no TOF difference between an ion of a given m/q formed at the beginning of the electronic pulse and the same ion formed at the end of the pulse, so that the temporal resolution is not much reduced by the temporal width of the electronic pulse.

The TOF detector consists of two 40 mm diameter microchannel plates (MCP from Hamamatsu Ltd.), resulting in a gain of ~10 7 electrons per detected ion. The MCPs are followed by two perpendicular delay lines (Roendeck Ltd.). These form a twodimensional position sensor in the two orthogonal directions (denoted X and Y) perpendicular to the TOF drift-tube axis (denoted Z). The impact position X (or Y) of an ion is directly related to its initial velocity perpendicular to the TOF axis: vx (or vy) and the vz component along z can also be derived from the applied extraction field E alond z direction:

where T 0 is the flight time of the particle of mass m without velocity.

The signals coming out of the delay line pass through an electronic pre-amplifier (home-made) and a discrimination system before being analyzed by a 120 ps resolution time-digital converter (TDC) developed by the LUMAT Federation at the Université Paris Sud, France. For each packet of electrons, an acquisition is triggered by the frequency command. The acquisition time (typically 7.6 µs) is defined by the flight time of the heaviest ion.

In order to obtain the efficiency curves of the cations presented in this work, the electron energies E are scanned from 8 eV to 20 eV with a step of 0.5 eV. A mass spectrum is recorded at each energy step and the intensity of each peak corresponding to a particular cationic molecule fragment is plotted against electron energy. To calibrate the energy scale, the presence of residual water with a known vertical ionization potential (12.62 eV 21 ) was used. Energy calibration was also performed by injecting helium, which has a known ionization potential (24.58 eV). The appearance energy thresholds (AE) of cationic species are obtained by fitting the near-threshold efficiency curves with a Wannier threshold law surimposed on a linear background 22,23 :

where E is the electron energy, k and b are the linear background coefficients, c is the slope of the threshold, and p is the Wannier exponent, which has been theoretically found to be 1.127 for the hydrogen atom 22 .

Computational details

In order to help us determine which fragmentation is the most likely to occur, quantum chemical calculations were carried out using the GAMESS code

24

. Density Functional Theory using the B3LYP functional, together with the 6-311G++(d,p) basis set, was used for all calculations and is known to be accurate for such small, organic molecules 25,26 ; in Russo et al 26 an estimate of the error can be obtained by comparing the experimental and calculated proton affinities in aniline and is about 0.15eV. In a previous paper on solar wind minority ion/hydantoin collisions, some of the calculations presented in this paper were already introduced 20 .

The relaxed geometries of each detected ion of mass m and the complementary neutral fragment(s) of mass 100-m were calculated as well as the corresponding electronic energies. In contrast to previous work, the zero-point vibrational energy (ZPVE) is not included here. This is because we focus on the electronic excitation of the molecule. This could cause small shifts in the calculated energy levels corresponding to relaxed geometries, typically a few tens of eV. We are aware that the initial physical process is ionization and molecular electronic excitation, i.e. population of electronic excited states (hole in molecular orbitals). These excited states evolve and, after complex molecular mechanism(s), fragmentation takes place. Pitzer et al.

27

, in the one-photon protonated uracil interaction in the 4.5-9.5eV range, show that the yield of a given mass shows maxima that can be correlated to excited states. Note that we only evaluate vertical excited states. The complete energy transfer mechanisms are beyond the scope of this publication. However, it is known that internal conversion mechanisms such as conical crossings are responsible for ultrafast conversion of excited states to ground state vibrational modes. Numerous studies 28 on neutral molecule shave been devoted to nucleic acid (the structure of hydantoin is very close to the pydimidic bases) due to their remarkable photostability.

For the uracil cation, Matsika 29 found a two-and three-state conical intersection in uracil + between the ground state and the D1 excited state, and between the ground D0 state and the D1 and D2 excited states. Segarra-Marty et al 30 improved on the previous results and extended the studies to the thymine cation a few years later. In their paper, the authors present the evolution of the D0, D1, D2 states potential energy surfaces and predict the evolution of the initial D2 population to the ground D0. These studies show that, through the conical intersections, the electronic excitation energy is found in the vibrational states of the molecule at the ground level. Few data exist on hydantoin molecule, however Su [START_REF] Su | Photochemistry and Photophysics[END_REF] has demonstrated a conical intersection between the ground state and the first excited state of the neutral molecule, allowing to explain the fragmentation observed by Ildiz et al 18 .

In this work we propose reaction pathways from the initial ionized hydantoin electronic ground state (after Internal Conversion process from excited states) to the experimentally observed cationic fragmentations. For this, the possible transition states along each pathway were determined and Internal Reaction Coordinate (IRC) calculations were performed to ensure the connection between the intermediate minima of the potential surface (i.e., corresponding to intermediate molecular conformations) before and after each pathway transition point. If the activation barrier for a transition state from the initial or final intermediate conformation is too high, typically >1eV, a DRC calculation, taking into account the kinetic energy release during the process, is performed to ensure that the molecular geometries between the transition points can be reached without major changes in the molecular structure such as fragmentation and rearrangement.

For each proposed reaction pathway, we tried to minimize the number of intermediate transition states (and the height of their activation barriers) required to dissociate the molecule into two fragments: a cation corresponding to the measured one and its complementary neutral fragment. The final charges of the resulting fragments are deduced from the sum of Mulliken atomic charges.

The highest energy point of each calculated reaction trajectory (transition state or dissociation energy) must be less or equal to the measured appearance energy thresholds for each cation.

Experimental results and discussion

Mass Spectrum

Mass Spectrum The mass spectrum obtained after interaction of a neutral hydantoin molecule with a 100eV electron beam is shown in Fig. 5. The incident electron energy (100 eV) is much higher than the first ionization potential (IP) of the molecule (10 eV see section 4.4). The mass spectrum shown in Figure 5 is therefore an integrated mass spectrum over this range of possible initial excitation energies.

The origin of the cation is essentially derived from the process of ionization and its fragmentation. As shown by Tian

32

, who gives a , the formation of positive-negative ion pairs is completely negligible. In our study on the fragmentation of halo-uracil by proton impact at 100 keV (same velocity than 54eV electrons), we explicitly searched for the presence of anions, which turns out to be negative despite the presence of halogen in the molecules 33 .

The five major cationic fragments discussed in this paper are labelled with their atomic masses in Fig. 5: 28, 29, 44, 57 and 72 a.m.u. The non-fragmented parent cation Hydantoin + (m/z 100 amu) is also observed. For these cations, the appearance threshold has been measured and the corresponding reaction pathway leading to these ions has been calculated. The peak corresponding to residual water (H 2 O + , 18 amu) is also observed, but is not due to hydantoin dissociation. It will be used for the calibration of the energy scale for the determination of the thresholds of the hydantoin fragments.

The relative abundances of the fragments (Branching Ratios) are in agreement with the NIST data recorded at 75 eV. Except for the 44 amu fragment, which is slightly higher than the 43 amu fragment. However, resolution effects may account for this difference.

The mass spectrum in Fig. 5 is also comparable to those obtained after the interaction of hydantoin with minority ions of the solar wind He 2+ at 8 keV and O 6+ at 30 keV 20 , but in the latter case a stronger population of Hyd + is observed as well as a smaller population of small fragments such as 28 + and 29 + . These differences can be explained by the lower amount of energy deposited in the molecule in the case of multi-charged ion collisions. This is due to the larger impact parameters involved.

Comparison with pyrimidic bases

The hydantoin molecule is very similar to the uracil molecule: one atom of the hexagonal ring is removed and the hydrogen atom attached to it is transferred to the neighboring carbon. The fragmentation of cationic pyrimidine bases produced by electron impact has been intensively studied: HNCO emission is clearly observed with uracil but no CO emission

23

. The same main channel is also found in thymine 34

. Theoretically, Zhou et al.

35

investigate ground-state fragmentation pathways of uracil cation, showing initial HNCO emission from uracil cation and the possibility of further fragmentation processes. In cytosine, strong emission of neutral CO at 70eV collision energy is observed as well as HNCO 36 which requires hydrogen transfer from -NH2; their paper presents calculations of isomerization (ring opening) scheme and dissociation path (with corresponding transition state).

In collisions with 100 keV protons A Le Padellec et al 37 , HNCO° emission is observed in uracil and thymine, 28 0 emission in cytosine, and the 28 + fragment predominates in all pyrimidine bases. In multiphoton ionization of uracil, Ryszka et al.

38

investigate this fragmentation channel and, thanks to high-resolution mass spectrometry, show that the chemical nature of the cation 28 + is HNCH + /NCH 2 + rather than CO + .

Kinetic energies spectra of fragments

The kinetic energies of the fragments can be measured using the 3D Velocity Mapping Imaging technique. The event by event mode allows us to measure the position of the impact on the detector as well as the time of flight of each cation detected. The 3D velocity vector and the mass of the cation are derived from these measurements. In Fig. 6 we present the 2D (X,Y) velocity distributions (resulting from the impact of the position on the detector) for the unfragmented hydantoin cation 100 + and for the 28 + cation for 100eV impact energy. The kinetic energy distribution is obtained from the statistical analysis of the kinetic energy (issued from 3D velocity) determined for each event For the unfragmented cation, the experimental distribution is compared with the Boltzmann distribution at T=150°C (which corresponds to the oven temperature) and good agreement is found. For the 28 + , the experimental distribution D(E lab ) is broader. This indicates a higher kinetic energy for this fragment. The experimental distribution also reflects the temperature broadening of the initial unfragmented molecule. In order to avoid this, we use a deconvolution of the energy distribution. For a given energy E and a corresponding velocity V of the fragment in the original cation frame, the energy in the laboratory frame is given by:

Where e rand is a unitary vector randomly oriented in space and V ic is the thermal velocity of the initial 100 + cation. The corresponding distribution R(E lab ,E) can be calculated and follows the distribution of V ic measured. Therefore, we can write:

where D(E i ) represents the distribution without thermal broadening. The Scofield algorithm, as presented by Sampoll 39 is used to invert these relations to obtain D(Ei). The result is shown in Fig. 6. There is an important peak at 80meV and a less intense one at 0.35eV, this could be an indication of a dissociation scheme where, for example, a transition state is involved and the available kinetic energy (KER) is distributed among the fragments.

Cationic fragments appearance energies

Fig. 7 shows the mass spectra measured at different electron energies and the measured efficiency curves (black dots) for the hydantoin cation and the five major fragments (72, 57, 44, 29 and 28 amu). The fitted curve obtained by the method described in section 2 is plotted as a red solid line and the resulting E AP is indicated by arrows. The error given by the numerical fit gives the uncertainty of each E AP value.

The measured Hyd + appearance energy corresponds to the first vertical ionization energy, i.e. the energy required to remove an electron without rearranging the geometry of the nucleus. The appearance energies are summarized in Table 1. It should be noted that only low energy thresholds have been studied and that other channels of fragmentation may exist at higher energies.

Breakdown curves

Breakdown curves vs. electron energy derived from Figure 7 are shown in Figure 8 for the studied fragmentation channels. The curves associated with 72 + (emission of neutral CO), 57 + (emission of neutral HNCO) show a broad maximum, indicating that further fragmentation occurs, while the curve associated with 100 + cation decreases continuously with electron energy, as expected. The curve associated with 44 + starts to rise from its threshold and then remains almost constant above 14eV. On the other hand, the curves associated to 29 + and 28 + cations show a continuous increase. In search of possible fragmentation schemes looking for parent and daughter ions, we have summed different channels: it seems that the sum of unfragmented cation 100 + and 29 + is constant above ≈13eV, suggesting a direct formation of 29 + directly from 100 + parent. The decrease of 72 + coincides with the increase of 44 + , the same is true for 57 + and 28 + at the threshold, but the strong increase of 28 + at higher energies is essentially related to the decrease of the population of 72 + (73%) and 57 + cation (27%).

Theoretical results and discussions on the possible reaction scheme

Initial molecular excited states

The first seven excited states of the hydantoin cation molecule have been evaluated by means of TDDFT calculation. Vertical transitions from the ground state of the neutral molecule to these excited states are summarised in Table 2 with corresponding orbital bases decompositions, excited states are clearly associated with vacancy formation in the inner molecular orbitals. 1 with Table 2, it can be seen that the appearance energies (AE) of the cationic fragments with masses 72 and 57 are compatible with the third excited state of Hyd + , whereas 44, 29 require a higher initial electronic excitation: the nearly excited state D4 of Hyd + . The composition of this cationic exited state, 90% of Homo-4, shows that this state excitation is close or equivalent to a direct initial ionization of the Homo-4 orbital from the neutral hydantoin. The 28 + a.m.u. cation has the highest AE with 14.10 eV. This requires an initial excitation greater than D5, corresponding to the initial homo-5 ionization from the neutral molecule.

Cation m/z 100 : ionization of hydantoin

The first vertical ionization potential of the hydantoin was measured to be AE -100 + = 10.04 ± 0.19 eV. This experimental result is in agreement with the quantum chemical calculation of the vertical ionization energy, which was performed with the computational methods described in section 3, and which gives E AP-100 + = 10.0 eV as shown in Fig. 7. Our measurement is also in good agreement with previous studies: the experimental study by Vondrack et al 17 using photoelectron spectroscopy gives an AE -100 = 10.20 eV, and the theoretical work by Bacchus-Montabonel 40 using CASSCF/6-311G** gives 9.91 eV, as summarized in Table 3. The relaxed geometry of Hyd + in its electronic ground state was found at 9.79 eV relative to the neutral ground state (Figure 9).

The first isomers of Hyd + were calculated. Fig. 9 shows their relaxed geometry, labeled G1 to G4, and their corresponding energies. These isomers have a relatively low activation barrier (labeled TSG1 to TSG4 in Fig. 9). They are energetically accessible from Hyd + electronic excited states that are higher than D2 for the G1 and G2 geometries and higher than D3 and D4 for the G3 and G4 geometries, respectively. In Renoud et al 20 the lowest dissociation limit to produce emission of the neutral mass 28 was attributed to loss of CO rather than HCNH/NCH2. The calculations presented in Fig. 10 are consistent with the emission of the neutral C(4)O(4) fragment: The measured AE 72 + (11.24 ± 0.18) is in good agreement (0.07eV) with the reaction scheme where TS1 is located at 11.61eV and the dissociation limit is calculated at 11.31eV.

Emission of 57 +

As mentioned in Renoud et al 20 there are three simple ways to form HNCO with consecutive atoms in the hydantoin molecule.

They are shown in red, orange and green in Fig. 11.

The calculated limits of the dissociation energies corresponding to the fragments C 2 NOH 3 + + HNCO in their relaxed geometries and at infinite distances from each other are labeled 1, 2, 3. These dissociation limits are respectively 11.21, 11.95 and 12.82eV from the neutral ground state.

Only dissociations leading to 1 and 2 can energetically match the measured AE of 11.80 ± 0.33eV for the 43°/57 + fragmentations.

The dissociation limit (3) leading to the neutral emission of HN(1)C( 2)O( 2) is located about 1 eV above the observed AE. No reaction pathway has been found for the formation of this fragment, and attempts to produce it have always resulted in the formation of OCNH 2 + .

The measured AE position indicates that the lower dissociation pathway (leading to 1) is not favoured over the pathway leading to HN(3)C( 2)O( 2), contrary to what was expected in our previous article.

Emission of 29 +

Looking for the dissociation leading to HNCH 2 + , in a previous work we calculated a dissociation limit where the neutral OC-NH-CO is formed: a 'stable' structure was found 20 with an additional C-C bond formed and the corresponding dissociation limit is 13.6eV.

However, this value contradicts our new experimental threshold measured: AE -29+ =12.190.23eV.

We have therefore studied this neutral fragment more closely: a transition state is found at 0.27 eV relative to the OC-NH-CO ground state, leading to the formation of CO+HNCO, whose energy is 1.53 eV lower than OC-NH-CO one and therefore more stable. The barrier height is quite small (and in the order of the accuracy of the calculations), so we can therefore reasonably assume that the neutral fragment OC-NH-CO does not exist but rather CO and HNCO, the corresponding dissociation energy limit being 12.05eV.

It must be noted that this mode of fragmentation has been experimentally demonstrated by Ildiz et al 18 in the neutral molecule and the conical intersection between the first excited state and the fundamental responsible for this dissociation has been calculated by Su [START_REF] Su | Photochemistry and Photophysics[END_REF] .

The measured threshold for this dissociation, AE-29+ =12.19 0.23eV, is in good agreement with the 'three-body' dissociation limit and is also very close to the thresholds leading to CO (0.9eV higher) and HNCO (0.4eV higher) discussed previously in Sections 5.3 and 5.4, respectively. In a sequential view of the dissociation process, we could then consider a first initial dissociation with an emission of CO or HNCO. This would be "followed" by a second dissociation of the remaining cations, both leading to HNCH 2 + :

For the initial CO emission, only the lowest energy scheme is retained (n°1 red see Figure 10). This is because the transition state for the higher energy scheme is much higher in energy than the measured threshold.

The same energetic argument applies to the initial neutral HNCO emission, retaining only reaction schemes (1) and (2). Corresponding calculated reaction schemes for the three channels are shown in Figure 12 and are energetically compatible with the measured threshold. Nevertheless, the reaction scheme with initial loss of HNCO after the reaction (1 in Fig. 10) is the simplest one with only one transition state. However, the breakdown curve (above 13eV) suggests a direct mechanism from hyd + , shown in Figure 12.

Emission of 44 +

The measured activation energy for 44 + emission was found to be 12.39 +/-0.5 eV. In section 5.4, the investigation of the reaction scheme of process 3 leads to the formation of H 2 NCO + instead of HNCO as shown in Fig. 13. The calculated dissociation limit and activation barrier are two high to match the measured threshold. Therefore, we investigated the initial conformation of the molecule leading to -OH group as a precursor of HNCOH cation emission: Transition states as well as dissociation limit are much higher than the experimental threshold, such reaction schemes cannot energetically correspond to the emission of 44 + cation.

The breakdown curve associated with 44 + formation (see Figure 8) begins to increase as 72 + begins to decrease, suggesting possible two-step fragmentation:

Three reaction schemes were found and are shown in Fig. 14. The red and black ones lead to NCH 2 + OCNH 2 + , while the blue one leads to HNCH + OCNH 2 + .

These reaction pathways have dissociation limits at 12.08 eV (black and red) and 12.64 eV (blue), which are all energetically compatible with the measured experimental threshold at 12.39 eV. However, we can expect that the black one should be favored, considering that the activation barrier of the red one is slightly higher than the measured threshold as well as the dissociation limit of the blue one. Finally, we can reasonably assume that the emission of 44 + fragments from hydantoin at the observed threshold corresponds to the emission of the cation OCNH 2 + rather than HOCNH + , given the numerous tentative simulations:

Emission of 28 +

The dissociation limit associated with the formation of CO + in three-body fragmentation is 16.3eV. This is much higher than the measured experimental appearance threshold for this cation, 14.10eV ± 0.43. This is close to the calculated D5 and D6 excited states of the hydantoin cation. Therefore, the 28 + fragment must be assigned to the crude formula CNH 2 + , This is in agreement with the experimental result of Ryszka et al 38 on the multi-photon ionisation of uracil, who show, by means of high-resolution mass spectrometry, that the 28 + fragment is associated with the HNCH + /CNH 2 + cation and not with the CO + one, due to the small mass difference (28.019 amu and 27.995 amu, respectively). A possible scenario is to consider, as a first step, the 57 + /49° dissociation described in 5.3 (Fig. 10) leading to the C 2 NOH 3 + cation (breaking curves analysis indicate that 27% of formation of 28 + could come from this first step) :

This is followed by a hydrogen transfer within the C 2 NOH 3 + cation before (28+ / 29°) fragmentation to form the HNCH + cation: For both reaction schemes, the highest energy point (TS1 and TS2) corresponds to a hydrogen transfer at about 13.6 eV for both cases, which is in reasonable agreement with the measured experimental appearance threshold of 14.1 ± 0.45 eV for the 28 + cation.

Since the initial transition points are energetically higher than the dissociation limits, the fragments will be able to acquire kinetic energy (this in agreement with the kinetic energy analysis by VMI in paragraph 4.3), so we have performed dynamical reaction coordinate calculations starting from TS1 and TS2 as shown in Figures 15 and16. The evolution of the trajectories (Figure 17) leads to a spontaneous fast (~200fs) dissociation into HNCH + +HCO. The kinetic energy of fragment 28 + (center of mass velocity) at the end of the DRC, is 0.09 eV for the scheme shown in Fig. 15 and0.04 eV for the scheme shown in Fig. 16 due to sharing of the total kinetic energy released between the two fragments and their rotation/ vibration. The value obtained for the first scheme is consistent with the energy of the most intense peak (0.08eV) obtained in paragraph 4.3. A second scenario is to consider the emission of CO (73%) as a first step, as suggested by the breaking curve analysis. In Figure 10, the emission of CO could be due to an energetically higher transition state located at 13.67eV, which is consistent with the measured threshold. However, no further fragmentation or hydrogen transfer is observed in the DRC calculation from the TS.

Conclusions

In this work, we have studied the fragmentation of the prebiotic hydantoin molecule induced by the impact of electrons with the help of our new experimental setup SWEET. The dissociation thresholds of hydantoin have been measured for the first time and are used to discriminate between possible reaction schemes suggested by the breakdown curves or calculated for the potential energy surface of the electronic ground state (reached by electronic states after internal transformation). These new results also refine and/or discriminate fragmentation types from solar wind ion collisions where energetic information was unavailable.

Notes and references

Fig. 1 -

 1 Fig. 1 -Schematic view of the SWEET experimental part used in this study

Fig. 2 -

 2 Fig. 2 -Electronic scheme of the fast (ns) pulse generator [Zetex,application note 1996]

Fig. 3 -

 3 Fig. 3 -Example of measured electron beam pulse width for 100eV electrons

Fig. 4 -

 4 Fig. 4 -Initial geometry of neutral Hydantoin.

Fig. 5 -

 5 Fig. 5 -100eV electron impact ionisation mass spectrum obtains from neutral Hydantoin molecule. The parent ion Hyd 1+ (at 100 amu) as well as the principal mono-cationic fragments and the residual water are labelled by their mass in atomic mass unit.

Fig. 6 -

 6 Fig. 6-Velocity distribution in the X-Y plane for Hyd + (a) and 28 + (c ). Experimental kinetic energy distribution (red), Boltzmann distribution for Hyd + (b) and deconvolution for 28+

Fig. 7 -

 7 Fig. 7 -Mass spectra as a function of the incident electron energy and corresponding measurements of appearance thresholds for observed 100,72,57,44,29 and 28 a.m.u cations

Fig. 8 -

 8 Fig. 8-Breakdown curves for the dissociation of Hyd + vs incident electron energy

Fig. 9 -

 9 Fig. 9 -Measured and calculated EAP for Hyd + , transition states energies TS(G1 to G4) relative to Hyd + and corresponding relaxed geometries G1 to G4 relative to neutral ground state. The relaxed energy of Hyd + (adiabatic ionization potential 9.79eV) is given

Fig. 10 -

 10 Fig. 10 -Energy diagram with activation energies (EAP) and reaction schemes corresponding transition states (TS1 and 2 respectively) for neural C(2)O(2) and C(4)O(4) emission after Hydantoin single ionization. The excited states of Hyd + are noted (Di for i=3 to 5). The experimental Activation energy is EAE72+

Fig. 11 -

 11 Fig. 11 -Energy diagram and reaction schemes for neutral HCNO emission after Hydantoin single ionization. The excited states of Hyd + are noted (Di for i=0 to 5). The experimental activation energy is AE43+ .

Fig. 12 -

 12 Fig. 12 -Energy diagram and reaction schemes for neutral emission after Hydantoin single ionization. The experimental Activation energy is AE29+

Fig. 13 -

 13 Fig. 13 -Reaction scheme following formation of OCNH2 + instead of HNCO in pathway (3) of figure 11

Fig. 14 -

 14 Fig. 14 -Energy diagram and reaction schemes for OCNH2 + emission after Hydantoin single ionization.

Fig. 15 -

 15 Fig. 15 -hydrogen transfer following emission of neutral HNCO (1 in Fig. 11) Starting from the two cationic isomers of 57 + labelled 1 (red) and 2 (orange) in Fig. 11, the corresponding calculated reaction path is shown in Figs. 15 and 16.

Fig. 16 -

 16 Fig. 16 -hydrogen transfer following emission of neutral HNCO (2 in Fig. 11)

Fig. 17 -

 17 Fig. 17 -Dynamical calculations showing the spontaneous emission of HNCH + after emission of neutral HNCO°

Table 1 -

 1 Measured appearance energies (AE) extracted from Fig.7

	Mass (amu)	AE (eV)
	100 (Hyd + )	10.04 ± 0.19
	72	11.24 ± 0.18
	57	11.80 ± 0.33
	44	12.39 ± 0.25
	29	12.19 ± 0.23
	28	14.10 ± 0.45

Table 2 -

 2 Calculated energies of excited states with TDDFT and corresponding main orbital contribution

	States	Vertical Excitation	Composition
		energy (eV)
	D0	10.00	100% (Homo)
	D1	10.15	92% (Homo -1)
	D2	10.67	92% (Homo -3) + 7% (Homo -2)
	D3	11.06	70% (Homo -2) + 22% (Homo -3)
	D4	13.61	92% (Homo -4)
	D5	14.25	92% (Homo -5)
	D6	14.41	91% (Homo -6)
	D7	14.84	90% (Homo -7)
	With a global shift of only 0.2eV, the calculated excitation energies agree with the photoelectron spectrum of hydantoin
	provided by Vondrak et al	17 .
	Comparing Table

Table 3 -

 3 Comparison of Hydantoin + AE values obtained experimentally and theoretically in this work with values obtained in previous studies.

	Author	Type	Method	AE(eV)
	Present work	Exp.	Electron impact	10.04 +/-0.19
	Renoud et al 20 .	Th.	DFT(B3LYP)	10.00
			6-311++G(d,p)
	Bacchus-Montabonel 40	Th.	CASSCF	9.91
			6-311G**
	Vondrak et al. 17	Exp.	Photo electron	10.20+/-0.05
			spectroscopy


I Quesada

C Julcour
email: carine.julcour@ensiacet.fr

U J Jáuregui

A M Wilhelm

H Delmas
email: henri.delmas@ensiacet.fr

REMEDIATION OF PHARMACEUTICAL WASTEWATER BY SEQUENTIAL ADSORPTION -CATALYTIC WET AIR OXIDATION ON ACTIVATED CARBONS

Keywords: CWAO, adsorption, paracetamol, activated carbon, biodegradability, pharmaceutical wastewater

The catalytic wet air oxidation (CWAO) of paracetamol on activated carbon was investigated both in an autoclave reactor and in a fixed bed column as a regenerative treatment of carbon after adsorption (sequential adsorption-oxidation process ADOX). Three activated carbons (ACs) from different source materials were used as catalysts: two microporous basic ACs (S23 and C1) and a meso-and micro-porous acidic one (L27). During the first CWAO experiment the adsorption capacity and catalytic performance of fresh S23 and C1 were higher than those of fresh L27 despite its higher surface area. This situation changed after AC reuse, as finally L27 gave the best results after five CWAO cycles. L27 also showed better oxidation performances and regeneration efficiency in the ADOX process, which could be explained by its physico-chemical properties. Finally, respirometry tests with activated sludge revealed that in the studied conditions the use of CWAO enhanced the aerobic biodegradability of reaction effluent.

Under the Directive 2000/60/EC, the European Union has identified a list of priority substances setting emission limit values and water quality objectives 1 . However, some molecules are only now being considered as potentially significant, but largely unregulated, pollutants. Those include pharmaceutical substances, which can induce biological effects on aquatic environment, whereas their long-term impact on human health even at low level is still unknown. Conventional treatment processes appear to be insufficient in eliminating sewage residuals and require additional efforts to achieve non-detectable concentrations 2 . Consequently, the occurrence of these molecules is being reported in wastewater plant effluents, rivers and lakes, and more rarely in groundwater and drinking water. In particular, paracetamol has been reported as the pharmaceutical compound present in largest concentration in French rivers 3 . The aim of this work is to study the feasibility of paracetamol degradation by a sequential process consisting in continuous adsorption on activated carbon (AC) followed by batch CWAO. The stability of the activated carbon and the biodegradability of effluents are studied.

EXPERIMENTAL Batch Autoclave Reactor for CWAO preliminary experiments

The first step of the study was the selection of suitable oxidation catalysts among three ACs from different source materials (carbon L27 from wood, S23 from coconut shell and C1 from casuarina), that were previously characterized in terms of physico-chemical properties and adsorption isotherms at room temperature. In a typical experimental run, 200 mL of 1 g/L paracetamol aqueous solution and 1 g of AC were added in a stirred autoclave (Parr Instruments), operating at 150°C and 3.5 bar of oxygen partial pressure (total air pressure of 20 bar). The experiments were performed batchwise for the liquid phase and were continuous for air, circulating with a flow rate of 60 L/h to ensure constant oxygen pressure. Prior to oxidation reactions, the ACs (0.8-1.0 mm) were saturated with paracetamol at 150°C under nitrogen atmosphere. The stirrer speed was set at 800 rpm to avoid catalyst attrition and prevent external mass transfer limitations. Five oxidation cycles were completed in each case in order to evaluate the catalyst efficiency and the biodegradability of resulting effluents.

Fixed-Bed Reactor for Adsorption-Oxidation cycles

A sequential process consisting in continuous adsorption on AC followed by batch CWAO was conducted on the same fixed bed set-up, oxidation achieving AC regeneration in mild conditions. The experimental setup was previously described 4 . The reactor was successively packed with 168 g of L27 and 340 g of S23 AC particles (1.25-1.40 mm sieved fraction). The adsorption step was operated at ambient conditions (25°C, atmospheric pressure). Solutions of 1 g/L and 2 g/L of paracetamol for L27 and S23 respectively were continuously introduced into the column by a dosing pump. The same pump was used to feed the column by recycling the reaction mixture from a pressurized tank during the oxidative regeneration step. The wall temperature was here set to 150 °C, and the oxygen partial pressure (P O2 ) was adjusted to 3.2 bar. The gas flow rate (50:50 O 2 :N 2 mixture) was 100 L/h (NTP) and the liquid flow rate was 2 L/h. Gas and liquid circulated cocurrently upward through the fixed bed. Three adsorption-oxidation cycles were completed for each AC.

Liquid sample analysis

Liquid samples for both processes were characterized using a HPLC equipped with a UV/Vis detector (Varian ProStar 310). Paracetamol was separated from oxidation intermediates on a Prontosil C 18 reversed phase column using a 90:10 mixture of acidified water (pH 2.2) and acetonitrile as isocratic mobile phase (flow rate of 0.25 mL.min -1 ). The detection of the compound was achieved at a wavelength of 254 nm.

The closed reflux colorimetric method was used to determine COD values. 2 mL of sample, diluted or not, was introduced into a commercially available digestion tube and the mixture was then heated 120 min at 150°C. The amount of oxidizing agent reduced was subsequently determined colorimetrically using a UV spectrophotometer (Hach Odyssey DR/2500) operating at 620 nm and expressed as COD.

AC characterization.

The surface area and pore volume of the carbons were measured from nitrogen adsorption isotherms at 77K, using an ASAP 2010 analyzer (Micromeritics). Specific surface area was calculated from BET plot for relative pressures between 0.01 and 0.2. Thermogravimetry analysis (TGA) was performed under nitrogen flow from room temperature to 700°C at a heating rate of 10°C/min (Q600 SDT, TA Instrument) in order to compare the amount of functional groups on the three carbons. Their acid/base properties were determined using the procedure proposed by Boehm 5 . 70 mL of 0.05 N NaOH or 0.05 N HCl solutions were added to 1 g of AC in a glass bottle. The bottles were degassed under N 2 , sealed and allowed to equilibrate for 3 days in a rotatory shaker. Then, the carbon was separated from the solution and 3 mL of each filtrate were titrated (DL 50, Mettler Toledo) using HCl or NaOH (0.05 N), as required. Each experiment was triplicated under identical conditions. The pH at the point of zero charge (pH PZC ) was measured by the pH drift method. 0.1 g of carbon was added to 20 mL of 0.1 mol/L NaCl solution, whose initial pH was adjusted with NaOH or HCl. The containers were flushed with N 2 , sealed and placed in a shaker for 24 h, after which the pH was measured. The PZC occurred when there was no change in the pH after contact with the carbon. Elemental analysis of the ACs was carried out using Thermo Finnigan Flash EA111 for CHNSO and lixiviation/ICP-AES (Jobin Yvon -Ultima 2R) for metals.

Biodegradability assessment

Respirometry tests were performed with activated sludge to assess the aerobic biodegradability and the possible inhibitory effect of paracetamol and its oxidation products. The apparatus consisted in a glass respirometry cell maintained at 26°C. Dissolved oxygen concentration and temperature were monitored continuously throughout the experiments. The respirometer was inoculated with biomass from the aeration tank of the Brax wastewater treatment plant (Toulouse, France). Sodium acetate was used as a biodegradable substrate and added before and after the injection of paracetamol or final effluents from batch CWAO. The procedure was as follows: a pulse of 98 mg COD/L of sodium acetate was first added. After the control substrate was consumed, a pulse of effluent with equivalent COD was injected. The last step consisted in the addition of the same pulse of acetate as in the first run. From integration of the Oxygen Uptake Rate (OUR) profiles, oxygen consumptions (OC) obtained with sodium acetate at the first run (sdt1) and with reaction mixture were compared. Biodegradability index was subsequently calculated according to equation 1. The toxicity of the effluent (eq. 2) was determined from the maximum values of the OUR curves (OUR max ) corresponding to the two additions of sodium acetate (std1 and std2).

OC OC 1 radability deg Bio % 1 std effluent 1 std ×         - - = (1) 100 OUR OUR OUR Toxicity % 1 std max 2 std max 1 std max ×         - = - - - (2)

RESULTS AND DISCUSSION

Preliminary studies in a Batch Autoclave Textural and chemical properties of fresh ACs: Table 1 and2 show the textural and chemical properties of the carbons. Among the investigated ACs, L27 has the highest surface area. S23 and C1 have very similar surface area but S23 is essentially a microporous carbon. The quantification of the carbon surface groups by Boehm titration reveals that L27 has also the greatest content of acidic and total surface groups. On the other hand, C1 has the greatest content of basic groups. TGA spectra confirm the results of Boehm titration with an increasing weight loss from S23 to C1 and from C1 to L27. In accordance with previous results, L27 is also found to have the lowest pH PZC and the highest oxygen content. Finally, the elemental analysis shows that all ACs have low metal contents, with a larger amount of Fe for C1. Selection of the best AC: Figure 1 shows the time concentration profiles during adsorption and oxidation using fresh ACs.

The adsorption equilibrium at 150°C was approximately reached after 2 hours when 86.7, 134.5 and 126.1 mg of paracetamol per g of AC were adsorbed on L27, S23 and C1 respectively. The adsorption of paracetamol appears here to be clearly disfavoured by the presence of acidic groups as lower paracetamol uptake is found with L27 AC despite a higher surface area. This conclusion is in accordance with those reported for phenol and some aromatic molecules for which a clear decrease in adsorption capacity is found when there are more acidic groups on the carbon surface 6,7 . An accepted mechanism for this phenomenon is the formation of water clusters through H-bonding with acidic (carboxylic) surface groups, which reduces AC adsorption capacity 7 . The adsorption results at 150°C are also in agreement with the isotherms obtained at 25°C (data not shown).

Then, after AC saturation under nitrogen, the CWAO reaction was started. The highest initial oxidation rate was obtained with fresh C1 carbon (3 mg/min) and the lowest one with L27 (1.6 mg/min). Table 3 shows the comparison of paracetamol adsorption, conversion and COD removal for the three fresh ACs. It is obvious that although a high conversion was obtained for all the ACs, the presence of acidic groups on L27 surface affected both the adsorption and the COD removal.

The stability of AC catalytic activity was tested by completing a series of five consecutive experiments (Figure 2). For S23 and C1 a large decrease of paracetamol conversion was observed after two oxidations, but it was rather constant for the last 3 cycles. For L27, paracetamol conversion was quite stable and only a slightly decrease was observed in the last cycle. In order to evaluate the role of the adsorption capacity of ACs on the degradation efficiency, the adsorbed amount of paracetamol at equilibrium after adsorption step was calculated for each catalyst and cycle (Figure 3). The adsorption capacity of L27 was more stable and it was only reduced by a factor 1.5 after five cycles. On the opposite, for S23 and C1 ACs the adsorption capacity strongly diminished during four or five cycles. There is some correspondence between the catalyst activity and the adsorption capacity as proved by the similarity of Figures 2 and3. The decrease of conversion and COD removal with each consecutive experiment, indicating the loss of catalytic activity but also the loss of adsorption capacity, is due to the deterioration of physico-chemical properties of ACs, as shown later. After five cycles the COD removal was only of 20 % or less for all the three ACs. At the end of the experimental series, the analysis of aged ACs (Table 1) showed that the BET surface area decreased by a factor of 3, 26 and 14 for L27, S23 and C1 respectively. Pore volume decreased too. Such a change in AC porous structure has already been reported during the CWAO of phenol and some of its derivatives, which has been attributed to the deposition of high-molecular weight organic compounds, blocking the access to micropores 4,8,9 . Such a phenomenom seems to be hindered by the presence of acidic surface groups 10 . Thermogravimetric analysis also reveals a different behaviour of fresh and aged catalysts. For the fresh catalysts, the weight loss between 200 and 700 °C is 14, 7 and 4 % for L27, C1 and S23 respectively due to the decomposition of some surface oxygenated groups attached to the ACs. For the aged catalysts, after water vaporization and release of physisorbed compounds (reactants and intermediates) at temperatures less than 200 °C, an even more significant weight loss is observed up to 700°C (28, 16 and 12 % for L27, C1 and S23 respectively). It corresponds thus not only to the calcination of surface groups, but also to the decomposition of some chemisorbed species of low volatilities, probably polymers formed by oxidative coupling reactions 8 .

In regard to the assessment of biodegradability of CWAO effluents, only a few works have been found to report this topic 11-14 . In order to compare the biodegradability of paracetamol and that of the effluent from batch oxidation on L27, a pulse equivalent to 98 mg COD/L of each was successively introduced into the respirometer (this COD value corresponds to 50 mg/L of paracetamol). Figures 4 and5 show the OUR profiles obtained for each experiment. After paracetamol had been added to the system (Figure 4), the oxygen consumption corresponding to sodium acetate was unchanged compared to the first run (19 mg/L). Same values of OUR max were also obtained for the two additions of control substrate.

Those results indicate that up to 50 mg/L, paracetamol is not toxic for the investigated biomass. On the other hand, the OUR profile corresponding to paracetamol is very flat, leading to an OC value equal to 2 mg/L. From equation (1), short term biodegrability of paracetamol is then estimated at 10%.

Figure 5 shows that the biodegradability of the oxidation effluent is higher than that of paracetamol solution (85% against 10% for paracetamol). No sign of toxic or inhibitory effect is detected with this reaction mixture. Although a detailed study of the reaction intermediates is not presented in this paper, it is obvious that the byproducts formed during the oxidation process are readily biodegradable substances. Figure 5. OUR profiles obtained with the effluent resulting from catalytic oxidation of paracetamol on L27.

Taken into account the catalyst preservation and the biodegradability enhancement of the effluent from L27 catalytic oxidation, the ADOX process was studied with L27. The S23 AC was also used as microporous catalyst for comparison purposes.

Sequential adsorption -CWAO of paracetamol on activated carbons in a fixed bed reactor

Contrary to an efficient application of the AD-OX sequential process for water treatment, the adsorption step was here conducted up to bed saturation in order to have a better knowledge and to estimate regeneration efficiency. While bed saturation could be achieved within one day with L27 (Figure 6), only three fourth of the theoretical capacity of S23 was obtained after 3 days when outlet concentration reached 95% of feed concentration (Figure 7). Moreover, adsorption being performed with stops during nights, the breakthrough curve of S23 shows a sharp decrease when starting feed solution owing to the slow diffusion of the molecule inside the microporous structure of S23. Following AC bed saturation, batch oxidations were performed but not detailed here. If an initially faster decrease of paracetamol concentration was observed with S23, full conversion of the molecule was only achieved within 12 hours compared to 4 hours for L27. This is in accordance with the results of the previous study performed in batch autoclave, showing L27 to be less efficient during the first oxidation, but giving better results after five CWAO cycles. More surprisingly, time-evolution of liquid phase COD exhibited a minimum, COD increasing after paracetamol was depleted. It resulted from the slow desorption of oxidation intermediates, main part of the pollutant being adsorbed on AC in this high solid loading recycle reactor.

After about 15 hours of oxidation, a 43% recovery of the initial adsorption capacity could be achieved for L27 (Figure 6), while only 13% for S23 (Figure 7, second breakthrough). On the other hand, as shown in Figure 7 (third breakthrough), a longer oxidation time could significantly improve the regeneration efficiency of S23 (up to 39% after 30 h). For L27 a large loss of AC adsorption capacity occurred between 1 st and 2 nd cycle, but afterwards, the bed efficiency remained approximately stable.

CONCLUSION A sequential -Adsorption-Batch CWAO -process has been investigated for the treatment of water effluent containing paracetamol. Three activated carbons have been compared both as adsorbent and catalyst of CWAO for their own regeneration. As a preliminary work the two steps, adsorption and batch CWAO, have been investigated in autoclave reactor to give information on AC evolution during successive adsorption and CWAO cycles. The initial catalyst activity seems to be tied to the adsorption capacity of the ACs with better (and similar) performances of C1 and S23 compared to L27 during the first oxidation. The reuse of catalyst in consecutive runs has shown a rapid deterioration in catalyst performance. After 5 CWAO cycles, a large loss of BET surface area is observed for all ACs, especially the ones exhibiting the lowest mesoporous/microporous ratios and basic character (C1 and S23). This is due to the formation of heavy compounds (polyphenols) irreversibly adsorbed on AC surface and blocking the micropores, as confirmed by TGA analysis before and after oxidation. This preliminary investigation has allowed the selection of the mesoporous AC better than the microporous ones, for the sequential AD-OX process in fixed bed reactor, due to better ageing and despite lower initial performances. Three successive AD-OX cycles with L27 have been carried out showing quasi stabilisation between runs 3 and 2 after an initial strong adsorption loss at run 2 compared to run 1. Res.,44 (25), 9513-9523. 5-Boehm, H.P. (1994) Some aspects of the surface chemistry of carbon blacks and other carbons, Carbon, 32 (5), 759-769. 6-Coughlin, R.W., Ezra, F.S. (1968) Role of surface acidity in the adsorption of organic pollutants on the surface of carbon, Environ. Sci. Technol., 2 (4), 291-297. 7-Franz, M., Arafat, H.A., Pinto, N.G. (2000) Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon, Carbon, 38 (13), 1807-1819. 8-Suarez-Ojeda, M.E., Stuber, F., Fortuny, A., Fabregat, A., Carrera, J., Font, J. (2005) Catalytic wet air oxidation of substituted phenols using active carbon catalyst, Appl. Catal. B, 58, 107-116. 9-Santos, A., Yustos, P., Rodriguez, S., Garcia-Ochoa, F. (2006) Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media. Appl. Catal., B, 65, 269-281. 10-Quintanilla, A., Casa, J.A., Rodriguez, J.J. (2007) Catalytic wet air oxidation of phenol with modified activated carbons and FE/activated carbon catalysts, Appl. Catal., B, 76, 135-145. 11-Suárez-Ojeda, M.E., Fabregat, A., Stüber, F., Fortuny, A., Carrera, J., Font, J. (2007) Catalytic wet air oxidation of substituted phenols: Temperature and pressure effect on the pollutant removal, the catalyst preservation and the biodegradability enhancement, Chem. Eng. J., 132, 105-115. 12-Suárez-Ojeda, M.E., Kim, J., Carrera, J., Metcalfe, I., Font, J. ( 2007) Catalytic and non-catalytic wet air oxidation of sodium dodecylbenzene sulfonate: Kinetics and biodegradability enhancement, J. Hazard. Mater., 144, 655-662. 13-Suárez-Ojeda, M.E., Guisasola, A., Baeza, J., Fabregat, A., Stüber, F., Fortuny, A., Font, J., Carrera, J. (2007) Integrated catalytic wet air oxidation and aerobic biological treatment in a municipal WWTP of a high-strength ocresol wastewater, Chemosphere, 66, 2096-2105. 14-Suárez-Ojeda, M.E. (2006) Catalytic wet air oxidation coupled with an aerobic biological treatment to deal with industrial wastewater, Ph.D. Thesis, Universitat Rovira i Virgili, España, 164 p.

LITERATURE CITED

Figure 1 .

 1 Figure 1. Adsorption and CWAO of paracetamol on fresh ACs. Conditions: T = 150°C, P O2 = 3.2 bar, C 0 = 1000 mg/L, w cat = 1 g.

Figure 2 .

 2 Figure 2. Conversion and mineralization of paracetamol using recycled ACs. Conditions: T = 150°C, P O2 = 3.2 bar, C 0 = 1000 mg/L, w cat = 1 g, t oxidation = 2 h.

Figure 3 .

 3 Figure 3. Adsorbed amount of paracetamol on recycledACs (mg/g AC ).

Figure 4 .

 4 Figure 4. OUR profiles obtained with paracetamol solution (50 mg/L, before batch oxidation).

Figure 6 .

 6 Figure 6. Successive breakthrough curves on L27 (C in =1 g/L, Q L = 6.5 L/h, w AC =168 g, T = 25°C).

Figure 7 .

 7 Figure 7. Successive breakthrough curves on S23 (C in =2 g/L, Q L = 6.5 L/h, w AC = 340 g, T = 25°C).

1-

  Directive 2000/60/EC. http://ec.europa.eu/environment/water/water-framework/index_en.html 2-Ternes, T.A., Meisenheimer, M., McDowell, D., Sacher, F., Brauch, H.J., Haist-Gulde, B., Preuss, G., Wilme, U., Zulei-Seibert, N. (2002) Removal of pharmaceuticals during drinking water treatment, Environ. Sci. & Technol., 36 (17), 3855-3863. 3-Le Parisien, 02/07/07. http://www.leparisien.fr/societe/le-paracetamol-en-tete-02-07-2007-2008173740.php 4-Suwanprasop, S., Eftaxias, A., Stuber, F., Polaert, I., Julcour-Lebigue, C., Delmas, H. (2005) Scale up and modeling of fixed bed reactors for the catalytic phenol oxidation over adsorptive active carbon, Ind. Eng. Chem.

Table 1 .

 1 Textural properties of ACs.

	AC	BET surface area, m 2 /g	Microporous volume, cm³/g	Mesoporous volume, cm³/g	r	Average pore diameter, Å	Apparent particle density, g/L
				Fresh ACs
	L27	1860	0.77	0.48	0.62	25	552
	S23	1180	0.47	0.05	0.11	17	1024
	C1	1230	0.53	0.26	0.49	22	799
			Aged ACs after 5 oxidations in batch autoclave
	L27	560	0.23	0.04	0.17	18	n.a.
	S23	45	0.02	0.006	0.30	21	n.a.
	C1	85	0.04	0.09	2.3	46	n.a.
	r = mesoporous/microporous ratio, n.a. = not available

Table 2 .

 2 Surface groups, pH PZC and elemental analysis of fresh ACs.

		acid	basic	total surface		%weight loss	%C %H %N %S %O %Ash %Fe %Cu
	AC	groups, mmol/g	groups, mmol/g	groups, mmol/g	pH PZC	between 200 and 700ºC
	L27	2.11	0.80	2.91	6-6.2	13.8	71.1 2.5 0.3 -19.3 8.3 0.008 0.011
	S23	0.40	0.96	1.36	9-9.1	4.2	92.3 0.6 0.3 -2.3	3.2 0.012 0.007
	C1	0.12	2.13	2.25	11	6.6	86.5 0.8 0.3 -3.4	nd 0.064 0.005

Table 3

 3

		. Paracetamol adsorption, conversion and COD
	removal with the fresh ACs (T = 150 °C, P O2 = 3.2 bar, C 0 = 1000 mg/L, w cat = 1 g, t oxidation = 2 h).
	CA	q e (mg/g)	Paracetamol conversion * (%)	COD removal * (%)
	L27	87 ± 2	89 ± 1	41
	S23	135	95	60
	C1	126 ± 4	98 ± 0	66

*calculated from initial and final concentrations (respectively COD values) of oxidation step

Acknowledgements:

The authors express their gratitude to the ALPHA-Programme, the 6th FP "REMOVALS", INP-ENSIACET, and ANR (Precodd "PHARE") for financial support. They also thank PICA (Veolia group) for supplying S23 and L27 ACs, CIPIMM for supplying C1 AC, L. Le Coq, C. Gerente (GEPEA, Nantes), C. Rouch, M. Auriol and S. Mouysset (SAP, LGC, Toulouse) for AC characterization.


F Theron

N Lesauze

A Ricard

A CONTINUOUS MICROENCAPSULATION PROCESS BY INTERFACIAL POLYCONDENSATION

Keywords: Microencapsulation, interfacial polymerization, static mixer, process intensification

A process to produce continuously microcapsules made of polyurea shell and containing cyclohexane as oil phase is proposed. The polymeric microcapsules are obtained via a two-step process involving emulsification of two immiscible fluids in static mixers followed by interfacial polycondensation in an empty pipe. The use of static mixers in turbulent flow to perform emulsification enables to obtain droplet sizes ranging from 10 to 60 µm with short residence times (0.01 -0.20 s). In order to calculate the residence time needed to bring the reaction to completion in the empty pipe, the time of encapsulation must be determined. Therefore the encapsulation step is also carried out in a stirred tank where the conversion of the monomer soluble in the aqueous phase is followed by monitoring the pH. Compared to a process where emulsification is carried out in a stirred tank, static mixers used in this study require less energy and enable to perform the microencapsulation continuously.

Microencapsulation is a widely used method to obtain products made of polymeric wall, enclosing and protecting a liquid or solid core, with controlled release properties. In general, the size of microcapsules ranges from 5 to 200 µm [START_REF] Kondo | Microcapsule Processing and Technology[END_REF]. The interest for microencapsulation is driven by many diverse applications such as carbonless copying paper with capsules diameters from several micrometers to 30 µm, medicines, perfumes and fragrances like perfumed capsules for tissue paper, and cosmetics. Since several years, microencapsulation is also used by the textile industry to propose active clothes to customers like slimming tights.

The microencapsulation device used in the studied process is the interfacial polymerisation. Two reactive monomers are first solubilised in two immiscible liquids, and react one with the other when both phases are brought into contact. The polycondensation reaction occurs at the interface between the two immiscible liquids. Hydrophobic or hydrophilic solutions or dispersions may be encapsulated by interfacial polycondensation. To encapsulate a hydrophobic material, a hydrophobic monomer A is first solubilised into this phase. Then this solution is dispersed in an aqueous continuous phase. When droplets are obtained, a hydrophilic monomer B is added to the water continuous phase. As soon as the monomer B is added, the interfacial polycondensation begins. The same method is used to encapsulate a hydrophilic material, but, in this case, an indirect W/O emulsion is prepared instead of an O/W emulsion.

Polymer walls prepared by interfacial polycondensation may be for example polyester, polyamide, polyurethane, or polyurea [START_REF] Kondo | Microcapsule Processing and Technology[END_REF]. In this study, polyurea shells are obtained from the reaction of hexamethylene diisocyanate (HDI) as hydrophobic monomer, and hexamethylene diamine (HMDA) as hydrophilic monomer. A hydrophobic liquid, cyclohexane, is encapsulated in the polyurea shell. The firstly obtained O/W emulsion is stabilized using Tween 80 as surfactant.

The aim of this study is to propose a continuous intensified process to produce polyurea microcapsules by interfacial polycondensation. The first step is performed in Sulzer SMX static mixer. Process intensification is nowadays a major concern for production of chemicals and static mixers offer the advantage to fit well continuous processes. The use of static mixer in turbulent flow enables to obtain quickly an important amount of droplets, while controlling their size distribution. The SMX mixer design is usually employed for liquid liquid dispersion in laminar flow [START_REF] Legrand | Liquid-liquid dispersion in an SMX-Sulzer Static Mixer[END_REF][START_REF] Das | Drop breakage model in static mixers at low and intermediate Reynolds number[END_REF][START_REF] Liu | Drop break up in an SMX static mixer in laminar flow[END_REF]. Here its performances are tested in turbulent flow. We also report the influence of the number of mixing elements on droplet size distribution.

In the continuous encapsulation process the addition of monomer B (HMDA) dissolved in the continuous phase and reaction with monomer A (HMDI) in the dispersed phase of the emulsion are performed in an empty pipe joined to the static mixer. To determine the pipe length, it is necessary to know the residence time in the tube that is necessary to achieve the reaction. In other words the implementation of a continuous process requires the knowledge of the reaction kinetics. Therefore in a preliminary study the encapsulation step is carried out in a stirred tank to study the reaction kinetics. In order to determine the time needed to reach 95% conversion of monomer B, the pH of the continuous phase is monitored as described previously by Yadav et al. [START_REF] Yadav | Microencapsulation in Polyurea Shell by Interfacial Polycondensation[END_REF][START_REF] Yadav | Microencapsulation in Polyurea Shell: Kinetics and Film Structure[END_REF].

A similar decoupled process using SMX static mixer has already been reported [START_REF] Legrand | Liquid-liquid dispersion in an SMX-Sulzer Static Mixer[END_REF][START_REF] Das | Drop breakage model in static mixers at low and intermediate Reynolds number[END_REF][START_REF] Hirech | Microencapsulation of an insecticide by interfacial polymerisation[END_REF]. They performed emulsification in viscous and turbulent flow regime [START_REF] Legrand | Liquid-liquid dispersion in an SMX-Sulzer Static Mixer[END_REF], and at low and intermediate Reynolds numbers [START_REF] Das | Drop breakage model in static mixers at low and intermediate Reynolds number[END_REF]. These authors assumed that the microcapsules size distribution was controlled by the emulsification step, and they only measured the size distribution of the final microcapsules slurry. Here both emulsion droplets and microcapsules are analysed to confirm this assumption.

EXPERIMENTAL Materials

Hexamethylene diamine (HMDA) and Hexamethylene diisocyanate (HDI) used as monomers were purchased from Acros Organics. The surfactant and oil phase were respectively Tween 80 and cylcohexane, and were purchased from Panreac and Acros Organics. Viscosities of continuous and dispersed phases were measured using an AR2000 rheometer (AR instruments).

Preparation of microcapsules

Figure 1 shows a scheme of the experimental set up. The emulsification step was realized in a tube equipped with Sulzer SMX static mixer made of 10 elements of 10 mm diameter, 0.67 porosity and aspect ratio D/L equal to 1. The mixture HMDI and cyclohexane was previously stirred to prepare the oily solution. The same operation was realized with distilled water and Tween 80 to prepare the aqueous solution for emulsification. The total flow rate of emulsion ranged between 204 and 600 L/h. The concentration of dispersed phase Φ was fixed to 25% in volume, and was controlled through the ratio of dispersed phase flow rate to total flow rate.

The microencapsulation step was carried out in a baffled tank of 1L, stirred with a 4 pitched blades impeller of 45 mm diameter. The stirring speed was fixed to 200 rpm. The second monomer (HMDA) was previously mixed to water -Tween 80 solution. For microencapsulation, the dispersed phase concentration was fixed to 5% in volume. In order to respect this ratio, 800 mL of continuous phase solution containing HMDA was poured in the tank, and 200 mL of emulsion was added through the tank cover to start encapsulation.

To follow the encapsulation reaction, the pH of the HMDA continuous phase was monitored during the entire experiment according to the method described by Yadav et al. [START_REF] Yadav | Microencapsulation in Polyurea Shell by Interfacial Polycondensation[END_REF][START_REF] Yadav | Microencapsulation in Polyurea Shell: Kinetics and Film Structure[END_REF].

Emulsion and Microcapsules Characterization

Particle size distribution of both emulsion droplets and microcapsules were determined using a Mastersizer 2000 (Malvern Instruments). Scanning Electron Microscopy was used to observe the microcapsules with a Leo 435 VP microscope.

Thermal properties of the polymeric membrane were analysed via differential scanning calorimetry (DSC) with a Q2000 calorimeter (TA Instruments). Before analysis, microcapsules were recovered from the slurry by filtration, washed with ethanol to remove unreacted monomers from the surface, filtered again, and dried in a vacuum oven at 25°C for at least 24 hours. About 2 mg samples were sealed in DSC pans and heated from 25°C to 400°C at a heating rate of 10°C/min. 24 experiments were performed with total flow rate ranging from 204 L/h to 600 L/h and 5 different numbers of SMX elements (n e = 2, 5, 10, 15 and 20). The dispersed phase concentration was fixed to 25% in volume. Droplet size distributions obey to a log-normal function, and are characterized in terms of Sauter mean diameter D 32 . Depending on operating parameters, D 32 ranges between 20 and 60 µm as can be seen on figure 2 showing the evolution of D 32 with n e for the different flow rates tested.

The number of SMX elements needed to reach the equilibrium between break up and coalescence mechanisms both occurring in the mixer was highlighted. As can be noticed in figure 3 showing the evolution of the droplet size distribution as a function of the number of elements for a total flow rate of 485 L/h, 10 SMX elements are enough to reach a stable droplet size distribution. The same working number of elements is obtained for every other flow rates.

At a given flow velocity, the energy dissipated per mass unit ε v can be estimated as follows through pressure drop measurement:

c D L P Q v ρ ε π ε 4 2 ∆ = (1)
Where Q is the total volume flow rate of the emulsion, ∆P is the pressure drop resulting from the liquid-liquid dispersion flow in the mixer, L the mixer length, D its diameter, ε its porosity, and ρ c the continuous phase density. The energy dissipation rate per mass unit can be regarded as a measure of turbulence intensity of any point within the mixer. This assumption has been made by Al-Taweel and Walker [START_REF] Taweel | Liquid Dispersion in Static In-Line Mixers[END_REF], who worked on liquid-liquid dispersion with Lightnin mixers. They concluded that the addition of mixing elements does not alter the turbulence intensity at a given flow velocity, but simply maintains this specific turbulence level. After fast drop disintegration in the first elements, the break up phenomenon goes on as long as turbulence is maintained, and the average drop size is achieved when the dynamic equilibrium between break up and coalescence is reached. Higher fluid velocities induce larger value of energy dissipation rate what results in faster disintegration, finer dispersions at equilibrium, and shorter residence time requirements for equilibrium.

The residence time τ in the mixer is calculated as follows:

Q L A ε Q mixer V ε τ = = (2)
Where V mixer is the volume of the tube containing the mixer, and A its cross sectional area. For 10 mixing elements and the flow rate ranging from 204 to 600 L/h, the value of τ ranges from 0.11 to 0.04 seconds what is rather low. The energy consumption per mass unit E can also be estimated from pressure drop measurements according to the following equation:

v . E ε τ = (3)
and ranges from 40 to 250 J/kg for the flow rates studied. An additional experiment of emulsification with the same system in a 1L stirred tank equipped with a Rushton turbine at 1000 rpm was carried out. It took one hour to reach the break up coalescence equilibrium in the stirred tank and the droplet size distribution was similar to that obtained with 10 SMX elements at a total flow rate of 335 L/h. The energy consumed with the stirred tank was 28,650 J/kg against 102 J/kg for the static mixer. In an economic standpoint, it is obvious that the static mixer is really interesting.

Microencapsulation

Microcapsules size distributions One of the advantages of processes involving reaction inside droplets or at their interface is the control of the final product size distribution via the emulsion droplets size distribution. Some authors [START_REF] Legrand | Liquid-liquid dispersion in an SMX-Sulzer Static Mixer[END_REF][START_REF] Das | Drop breakage model in static mixers at low and intermediate Reynolds number[END_REF][START_REF] Hirech | Microencapsulation of an insecticide by interfacial polymerisation[END_REF]] also reported the use of SMX static mixer to generate droplets, then encapsulated in a batch reactor. However they simply assumed the control of microcapsules size distribution by the emulsification step. The lack of information about the emulsion size distribution does not able to make sure that no coalescence phenomenon occurs between the mixer and the stirred tank.

During each experiment of this study, both emulsion and microcapsules size distributions were analysed. Figure 4 shows results obtained from encapsulation of an emulsion generated with a total flow rate of 485 L/h. It is obvious that the microcapsules distribution is fully controlled by the emulsion granulometry. For this experiment, emulsion droplets and microcapsules are respectively characterized by Sauter mean diameters of 21.4 µm and 20.7 µm.

Encapsulation time measurement

There are only few available experimental data on the kinetic of interfacial polycondensation. In order to determine the encapsulation kinetics, Yadav et al. [START_REF] Yadav | Microencapsulation in Polyurea Shell by Interfacial Polycondensation[END_REF] proposed to monitor either the polymer shell thickness or the concentration of one of the monomers with time. It is obvious that the more convenient method is to measure the continuous phase monomer concentration. Yadav et al. [START_REF] Yadav | Microencapsulation in Polyurea Shell by Interfacial Polycondensation[END_REF][START_REF] Yadav | Microencapsulation in Polyurea Shell: Kinetics and Film Structure[END_REF] studied experimentally the kinetic of polycondensation of HMDA and HMDI to form polyurea membranes at the interface of droplets. They used the basic nature of the continuous phase monomer (HMDA) to follow its concentration via pH monitoring. This procedure has been selected in this study to acquire experimental times of encapsulation of the polycondensation reaction. Measurements of pH of different HMDA concentration solutions with the surfactant (Tween 80) and cyclohexane (without HMDI) enabled to acquire the calibration curve for HMDA concentrations ranging from 10 -2 mol/L and 2.10 -4 mol/L. Experimental pH data obtained during encapsulation experiments were then converted in HMDA concentrations C HMDA , what enabled to calculate the HMDA conversion as follows:

( ) ( )

HMDA,0 C t HMDA C HMDA,0 C t HMDA X - = (4)
The encapsulation time is chosen as the time needed to reach 95% conversion of HMDA. With the assumptions that the polymer contains equal moles of HMDA and HMDI and that all droplets have the same diameter which value is taken equal to the D 32 of the microcapsules size distribution, the dry polymer film thickness δ is calculated as follows:

( ) D V . p . 6 32 D . t HMDA, X . HMDA,0 C . C V . B M A M t ζ δ       + = (5)
where M A and M B are the respective molecular weights of both monomers and V C and V D are respectively the continuous and dispersed phase volumes in the tank. In equation ( 5), ζp is the density of polyurea taken equal to 1,000 kg.m -3 by Yadav et al. [START_REF] Yadav | Microencapsulation in Polyurea Shell by Interfacial Polycondensation[END_REF]. The polymer film thickness value should be multiplied by the swelling factor α to represent the reality more accurately. A value of α equal to 1.6 for a similar polymer is reported by Pearson and Williams [START_REF] Pearson | Interfacial Polymerization of an isocyanate and a diol[END_REF].

Table 1 sums up the experimental conditions of each experiment, with the values of dry film thickness calculated according to equation [START_REF] Hirech | Microencapsulation of an insecticide by interfacial polymerisation[END_REF], and the times of encapsulation t 95 obtained. For the initial monomers concentrations and droplet sizes ranges tested, we reported rather low t 95 values from 20 to 73 seconds.

Figure 5 compares the X HMDA profile obtained for experiments E5 and E6. For these two runs, the Sauter mean diameters of the emulsion are almost the same and the initial monomers concentrations are equal, what enables to evaluate the reproducibility of the experimental technique used. From figure 5 it is obvious that the experimental method is rather satisfactory.

On figure 5 fluctuations of X HMDA data are noticeable during the first 10 seconds, especially for experiment E6. It corresponds to the time elapsed experimentally for homogenization of the emulsion inside the continuous phase containing HMDA.

The influence of droplets size on the encapsulation time can be assessed from t 95 obtained for the couples of experiments E3 / E10 and E4 / E9. It appears that for the same initial monomer concentrations, the time needed to reach 95% conversion of HMDA increases with the droplets size. The interfacial area available for the reaction increases when the mean droplet diameter decreases, what can explain this phenomenon. With the same initial monomer concentrations but different D 32 , a higher interfacial area means a thinner polymeric membrane at final monomer conversion. Karode et al. [START_REF] Karode | New insights into kinetics and thermodynamics of interfacial polymerisation[END_REF] showed through predictions of δ versus time that the film growth rate is very high at low times and decreases as the polymer membrane increases in thickness.

The initial monomer concentrations also have an influence on the time needed to reach 95% conversion of HMDA. This phenomenon is highlighted through encapsulation times obtained for the experiment couples E3 / E4 and E9 / E10. With the same R = n HMDI / n HMDA and the same D 32 , higher initial monomer concentrations result in lower t 95 .

Characterization of microcapsules

A SEM photograph of microcapsules obtained during this set of experiments is shown in figure 6. A microcapsule of about 7 µm is well noticeable, and is rather spherical.

In order to evaluate some thermal properties of the polymeric membranes we used differential scanning calorimetry. Figure 7 shows the DSC scan obtained for one dried sample of polymeric membrane. It shows a high melting point of about 289°C. This value is really close to the melting point reported by Yadav et al. [START_REF] Yadav | Polyureas by interfacial polycondensation: preparation and properties[END_REF] who prepared the same type of polyurea membrane enclosing cyclohexane. Figure 7 also shows an endothermic peak at a temperature just below the melting point, at about 240°C which is attributed to a cold-crystallization transition during which the polymer chains arrange themselves into the maximum ordered structure.

CONCLUSION

This study enabled to design a continuous process for the production microcapsules by interfacial polycondensation. This way of encapsulation has been decoupled in two successive steps.

The ability of Sulzer SMX static mixers to carry out emulsification in turbulent flow has been demonstrated and enabled to form droplets with size ranging from 15 to 60 µm with low residence times. We also showed that in the flow rate range tested 10 mixing elements were enough to reach the break up / The final aim of our study is to design a continuous encapsulation process where emulsification will be performed in static mixers, followed by encapsulation in an empty pipe. In order to calculate the pipe length, it is necessary to know the residence time needed, what implies the knowing of the encapsulation time. To acquire times of encapsulation we used pH-monitoring in a batch reactor to follow the continuous phase monomer concentration during the reaction. For the range of initial monomer concentrations studied, we obtained times of reaction between 20 and 73 seconds.

During these encapsulation experiments in batch reactor, we also checked the control of the final microcapsules size distribution by the emulsion firstly obtained. And analyses of the products showed similar thermal properties to those reported in the literature.

Continuous encapsulation experiments are in progress with the use of a lab-scale plate-type heat exchanger/reactors studied and optimized by Anxionnaz et al. [START_REF] Anxionnaz | Hydrodynamic Study and Optimization of the Geometry of a wavy Channel in a Heat Exchanger/Reactor[END_REF]. A wavy channel of 2 mm width, 2 mm height and about 3.5 m length is etched on each plate of the reactor. The emulsion obtained in the SMX mixer flows continuously through this channel where interfacial polycondensation is carried out. Depending on the flow conditions and time of reaction, the channel length required can be predicted.

This continuous process presents two valuable advantages compared to a discontinuous process performed in a stirred tank reactor: constant product quality and lower energy dissipation, especially for the emulsification step.

Figure 1

 1 Figure 1. Experimental set up

Figure 3 :Figure 2 :

 32 Figure 3: Effect of ne on droplet size distribution for experiments at 485 L/h

Figure 4 :

 4 Figure 4: Comparison of emulsion and capsules size distributions for experiment E6

Figure 5 :Figure 7 :Figure 6 :

 576 Figure 5: XHMDA profile for experiments E5 and E6

Table 1 :

 1 Experimental parameters employed for the preparation of polyurea microcapsules and data obtained for t95

	Experiment code	Qtot (L/h)	D32 (µm)	CHMDI,0 (mol/L)	CHMDA,0 (mol/L)	R = nHMDI/nHMDA	Film thickness δ (nm)	t95 (s)
	E3	600	19	0.0888	0.0047	1	79	20
	E4	600	17	0.0560	0.0029	1	49	47
	E5	485	23	0.0760	0.0040	1	79	39
	E6	485	21	0.0760	0.0041	1	78	42
	E9	383	30	0.0530	0.0028	1	81	73
	E10	383	32	0.0890	0.0048	1	141	58