P Brionnet
email: pierre.brionnet@riken.jp

H Faure

O Dorvaux

B Gall

K Hauschild

C Mathieu

E Gamelin

Characterization of a New Generation of Silicon Detector: the SIRIUS Side "Strippy-Pad" Detector

Keywords: Tracking and position-sensitive detectors, Solid-state detectors, Double Sided silicon Detectors, Charged-particle spectroscopy, alpha particle detection, Electron spectroscopy

SIRIUS (Spectroscopy & Identification of Rare Isotopes Using S3) is a detection system designed for the focal plane of S3 (the Super Separator Spectrometer), which is part of the SPIRAL2 (Système de Production d'Ions RAdioactifs en Ligne de 2e génération) facility at GANIL (France). This study presents the characterization of the Side silicon detector of SIRIUS. This new "Strippy-pad" detector design benefits from an ultra-high resistivity and the windowless technique from Micron Semiconductor Ltd. These detectors were tested at the IPHC Strasbourg on a dedicated test bench with custom fast preamplifiers coupled to digital electronics cards. Combining all these elements, we obtained resolutions as good as 13.6 keV for certain pixels of this detector for 8.8 MeV alpha particles, with an average resolution measured at 14.7 keV over the whole detector.

INTRODUCTION

One of the main challenges in modern nuclear physics is the study of nuclei far from stability, which provides strong constraints for nuclear models. Closed nucleonic shells are clear fingerprints of quantal behaviour of nuclei leading to the socalled magic numbers of neutrons or protons. The region around the doubly magic 100 Sn and the region of very heavy and super heavy nuclei are excellent nuclear structure physics cases to study these extreme magic systems and for the quest of an ultimate island of stability.

The very low production cross-section of these nuclei needs to be compensated by improving the sensitivity and efficiency of the experimental setups. The present development is performed within the S 3 [START_REF] Dechery | Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S3[END_REF] (Super Separator Spectrometer) project, which is a part of the new SPIRAL2 (Système de Production d'Ions RAdioactifs en Ligne de 2e génération) facility at GANIL (Caen, France). SIRIUS [START_REF] Dechery | Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S3[END_REF] (Spectroscopy & Identification of Rare Isotopes Using S 3 ) is the detection system designed and optimized for spectroscopic studies of rare exotic isotopes and heavy/super-heavy elements at the final focal plane of S 3 . Its compact geometry combined with new advanced technologies, such as new detector designs, customized preamplifiers and adapted digital electronics, allows for a large range of energies to be covered (from low-energy electrons, ~50 keV, to heavy fission fragments, ~100-200 MeV) with resolution (from 0.2% for the typical alpha energies and up to 1% for the heavy ions [START_REF] Dechery | Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S3[END_REF]).

Ions enter the SIRIUS focal plane detector array through the large-area tracker detector for time of flight, ion tracking as well as mass measurement. These ions are then implanted into a 10 × 10 cm 2 Double-sided Silicon Strip Detector (DSSD, also called implantation detector) segmented in 128 strips on each side. Implanted nuclei, as well as alpha particles and conversion electrons emitted by the subsequent decays, are also partially detected in the DSSD. Four large silicon detectors are placed in a "tunnel" configuration, upstream from the DSSD to detect alpha particles and electrons escaping from the DSSD. An array of five Germanium clover detectors for gamma ray spectroscopy is placed as close as possible around this "silicon box" (one behind the DSSD and 4 around the Side detectors in a cross configuration, Fig. 1 [2]). All detected decay events are individually time-stamped in dedicated digital electronics modules.

© 2021 published by Elsevier. This manuscript is made available under the CC BY NC user license https://creativecommons.org/licenses/by-nc/4.0/ Version of Record: https://www.sciencedirect.com/science/article/pii/S0168900221007555 Manuscript_292e057b730edb4cb4ef4eca4cce5597

Detector Characteristics and Design

The Side detectors of SIRIUS are part of the new generation of silicon detectors produced by Micron Semiconductor Ltd. These detectors are produced using a high resistivity silicon wafers (20-50 KΩ.cm) made from N-Type silicon with the orientation <100>. With a thickness of ~700 μm and large active area (10 × 10 cm 2 ), they have a surface capacitance of 19.71 pF.cm -2 at full depletion voltage. Finally, due to their dead layer thinner than 50 nm (typically around 30 nm, as provided by Micron Semiconductor Ltd under their 9.5P denomination [START_REF]MICRON[END_REF]) are called "windowless". This new design of "Strippy-Pad" (Fig. 2) detectors is composed of 64 pixels (pads) on the ohmic side and one large single windowless pixel side with 3% grid coverage (for good charge collection) on the junction side.

Each pad has a peripheral metallization (Fig. 3) for charge collection associated with a 30 μm wide strip connected to a bonding pad. The bonding pads are gathered into four groups (two columns of 8 pixels in each group), and are located on the same edge of the detector (Fig. 2). To avoid charge trapping under the usual insulation layer between the wafer and the metallic collection strip, the signal-tracking were designed as active strips (i.e., metallic deposit on an active volume of silicon Fig. 3, rather than the metallic deposit being insulated from the wafer by an oxide layer). These metallic strips are located between every two columns of pixels. Due to this design, the surface of each pixel, as well as the tracking length, differs from one pixel to the next. Thus, the measured pixel capacitances range from 51 pF (large pixels area and long tracking: 1.4484 cm² and 8.51 cm of tracking) down to 32 pF (smaller active area: 1.356 cm² and no tracking next to the bounding pads). This "Strippy-Pad" concept [START_REF] Faure | Proc. Conf. on Advancements in Nuclear Instrumentation, Measurement Methods and Their Applications[END_REF][START_REF] Gall | [END_REF] enables windowless operating for a pad detector by avoiding the dead layer associated with the double metal technique usually used for pads, as well as the insulation oxide layer for the metallic strip. This design also allows the connectors to be located only on one side, on the "bottom"-side of the detector frame, to ensure a very compact geometry of SIRIUS.

An innovative hybrid support frame, combining an anodized aluminium frame for an efficient cooling and a PCB for the connectors and signal transmission, were developed at the IPHC Strasbourg (Fig. 2). Good cooling and grounding properties of this detector are provided by the aluminium frame. In addition to the opening dedicated to the silicon detector wafer, and to minimize the absorption of low-energy gamma-and X-rays, a shallow volume is machined at the bottom of the aluminium frames for the positioning of a double-sided PCB board. This board bridges the signals from the detector to the output cables.

Bonding pads are placed facing the detector field plate, guard rings and pixel bonding pads. Connection to the preamplifier is made via two 40-channel compact cables onto SAMTEC connectors. The PCB board is glued onto a non-anodized recess into the aluminium frame using a silver-loaded glue to ensure good grounding connection. This hybrid frame allows for a compact mounting of the five detectors (Four Side detectors and one implantation detector) into a silicon-box geometry.

SIRIUS Detectors Test Bench

The characterization of the Side detectors of SIRIUS was performed at the IPHC Strasbourg using a dedicated test bench [ [6], page 151-155]. It is composed of a chamber for operation under controlled vacuum/temperature and a digital acquisition chain based on TNT2 cards [START_REF] Arnold | TNT digital pulse processor[END_REF]. A copper cooling plate was designed to be able to either place the ohmic (or the junction) side in front of a radioactive source inside the vacuum chamber. Four radioactive sources were available ( 207 Bi, 133 Ba, 212 Pb and a mixed 239 Pu/ 241 Am/ 244 Cm) and could precisely be positioned using a SMARACT piezoelectric XY-Θ table combined with a set of collimators.

The acquisition chain is composed of custom CR110 CREMAT preamplifiers [8] combined with digital electronics composed of TNT2 cards [START_REF] Arnold | TNT digital pulse processor[END_REF]. The custom gain of these custom preamplifiers is 173 mV/MeV (rather than 62mV/MeV, both corresponding to unloaded gain) was chosen to get a 14.5 MeV full-scale energy range (i.e. 1.25 Vpp signal) while retaining their very fast response time. For pixels with capacitances in the 30-50 pF range, the response-time of the custom CR110 is 13.2-16.6 ns (0.17 x Cd (pF) + 3 ns as given by the CREMAT specification sheet [8], where Cd is accounting for detector and cables capacitance (30 pF)) which is much faster than the ~100-150 ns needed for charge collection our silicon and could enable to probe the discrimination between alpha particle and electron through their different collection time. A dedicated 26 channel PCB motherboard was developed at the IPHC to host these preamplifiers. After the pre-amplification stage, the signals are digitized at a sampling rate of 100 MHz rate using 7 TNT2 cards [START_REF] Arnold | TNT digital pulse processor[END_REF] developed at IPHC Strasbourg. These 4 channels TNT2 boards are based on 14-bit flash ADC's operating at 100 MHz. Each channel is treated independently and in parallel. Moreover, they can generate online spectra as well as to store individual oscillograms (traces) to disc for offline treatment. The energy measurement is performed using trapezoidal shaping provided by a Jordanov algorithm [START_REF] Valentin | Digital techniques for real-time pulse shaping in radiation measurements[END_REF] implemented on the FPGA of the TNT2 board. After setting the proper pole zero correction for each individual channel, this pipe-line data processing of the energy is defined by 2 parameters: integration time, and flat top duration of the trapezoid (defined for each channel individually).

VALIDATION OF THE DETECTORS

This paper is focused on the validation of the concept of the "Strippy-Pad" detector. Two detectors were characterized during this work. The results presented here report on the first one, the characteristic of the second one being very similar.

Reduced Number of Channels by Pixel Grouping

Even though the "Strippy-Pad" detector was designed and produced with 64 pixels, it was foreseen from the beginning to reduce this number by making macro-pixels. Indeed, in order to meet the specifications of the SIRIUS collaboration, the number of channels per detector needed to be reduced, with the condition that the maximum energy resolution should remain below 20 keV for 8 MeV alpha particles. A good pixel granularity near the DSSD is required to limit the dead layer effect on the escaped alpha particles [Courtesy of A. Lopez-Martens, [START_REF] Gall | [END_REF]], and lower granularity can be accepted further away from the DSSD. It is worth pointing out that these constraints lead to the grouping of lower capacitance pixels rather than higher capacitance ones.

We established that the capacitance of the macropixels should remain under 145 pF in order to minimize the influence of the capacitance noise from the CREMAT preamplifier on the measured resolution (less than 10% of the 20 keV of the specification); considering the equivalent noise charge slope (3.8 e -RMS/pF) and the average energy for a pair creation in the silicon (3.62 eV). Moreover, the intrinsic noise of the system at 0 pF was less than 5 keV (1.7 keV coming from the preamplifier

[8])
Several possible schemes of macropixels structures corresponding to 16, 20 or 24 groups were studied. Since the structure of the "Strippy-Pad" presents four identical blocks composed of 2 columns of 8 pixels with their respective strips, the reduced schemes were designed around this elementary block. Several reduced configurations were selected by the collaboration and fully characterized. However, two were selected, based on their performances and have been used for the present work. Fig. 4 illustrates both the mapping of the macropixels used during this work (Fig. 4 (a)) and the two macropixels configurations selected (Fig. 4 (b)). The final choice between these two configurations was left to the collaboration.

Practically, the number of pixels is reduced physically by using a PCB board plugged between the connector and the Kapton cable (Fig. 5) grouping the pixels into the macropixels.

Testing Protocol

A two-step protocol was established to test the detectors upon delivery and to precisely determine the optimal operating conditions for each detector. This protocol will be applied for the validation tests of every Side detector that the collaboration order.

Depletion Voltage and Operating Voltage Determination

After a visual inspection under binocular lenses, the detector is characterized in a clean room. During this step, done in a dark room, one measures pixel by pixel the current (Keithley 6487 (set V and read I) Fig. 6.b) and capacitance (Keithley 6487 (set V) and Boonton 7200 (read C) Fig. 6.a) evolution with applied voltage. This characterization enables us to determine the full depletion voltage of the detector (VD), the possible operating voltage range as well as the good behaviour of every single pixel (not macropixel).

For this particular detector, the full depletion voltage (VD) is at 50 V from the capacitance plots (Fig. 6 (a)); Above VD, there are no more changes in the measured capacitances, capacitance stabilizes around a constant value. This value is consistent with the value given by Micron Semiconductor Ltd. as well as the value obtained from the current versus voltage plot (start of the plateau observed in Fig. 6 (b)). Thus, the range of operation of the detector was chosen to be above 1.5xVD and below 150V (according to the recommendation of MICRON Semiconductor Ltd.).

Optimal Working Point

In a second step, we tune the applied voltage and the detector operating temperature by monitoring the energy resolution of detected alpha particles. A resolution versus temperature and voltage matrix is first constructed with data from the TNT2 energy spectra using the trapezoidal rising time and flat top duration known to give good resolution from previous detectors; The pole zero compensation was optimized channel by channel once and for all at the beginning of the characterization.

The Fig. 7 illustrates such evolution of the measured energy resolution (all energy resolutions in this article will be given in FWHM). For this detector, the best resolution is obtained at 80V and -20 °C. However, this graph does not show the error bars on the measured resolution: typically below 0.5 keV on the FWHM. Thus, once the applied voltage and temperature reach the plateau (above 80 V and below -5 °C), every point is expected to behave the same regarding the measured resolution within the error bars. However, the evolution of the energy resolution is quite sensitive to change of both voltage and temperature at the edges of this operating region. Therefore, a slightly higher applied voltage was preferred and enabled higher operating temperature. The working point was then set at 90 V and -15 °C for this detector.

Once this working point is determined, we accumulated a significant number of preamplifier signal traces (~10000 per alpha line) in order to get a statistical data set for the full detector. An offline analysis code processed the recorded signal traces and was used to optimize the Jordanov [START_REF] Valentin | Digital techniques for real-time pulse shaping in radiation measurements[END_REF] parameters to obtain the best energy resolution using a 100 ns increments. Fig. 8 shows the resolution for the 8.8 MeV 212 Po alpha line as a function of the trapezoidal integration time and flat top duration. The best resolution is obtained for a quasi-triangular configuration: 4.8 μs integration time and 350-400 ns of flat top duration for this particular macropixel.

For flat top durations of 0 to 200 ns (cf. Fig. 8, corresponding to quasi-triangular shape) a drastic increase in the measured FWHM was observed as expected. This width corresponds to the signal rise time of ~100-150 ns. This effect has already been observed and discussed in [START_REF] Valentin | Digital synthesis of pulse shapes in real time for high-resolution radiation spectroscopy[END_REF]. Fig. 8 corresponds to a typical macropixel within the average resolution measured. Such an optimization has been performed for each macropixel within the same run, since our data file contains traces from the full detector. This allows us to determine the optimal parameters over the whole detector. According to this measurement, the optimal working point is for a rise time of 5 µs and a flat top time of 0.4 µs for this particular detector.

Macropixel Resolution

Macropixels were characterized with a mixed 212 Po/ 212 Bi/ 212 Po alpha emitting source obtained from the deposition of 216 Po onto a gold foil during the decay of the 232 Th through the gaseous alpha decay of the 220 Rn to avoid any source thickness effect on the alpha energy. A typical 212 Pb alpha spectrum can be seen in Fig. 9 with very good resolution within the whole alpha energy range. This spectrum was obtained at (90V,-15 °C) following the optimization of the Jordanov parameters, with an energy resolution of 13.66 keV for the 8.783 MeV 212 Po line. The fitting function used during this work is the derived from the SAMPO gamma-ray program, which is a modified Gaussian distribution [START_REF] Bland | Choosing fitting function to describe peak tails in Alpha-particle Spectrometry[END_REF].

Resolutions are reported in Fig. 10 as a function of the capacitance of the macropixel for the two selected reduced pixel configurations. The average value of energy resolutions obtained is about 14.7 keV over the full detector for the 212 Po 8.783 MeV alpha line. This figure shows a slight increase in the measured average FWHM with capacitance of about 0.6-0.7 keV on average from the low capacitance macropixel to the high capacitance ones (from 14.4 keV to 15-15.1 keV). The expected contribution of the intrinsic noise of the preamplifier is 3.8 e-(RMS)/pF, thus leading to a 3.1 keV contribution to the FWHM at 95 pF and 4.7 keV at 146 pF. Moreover, the noise contributions add in quadrature. Thus, if we take the 14.4 keV FWHM at 95 pF, the "intrinsic" FWHM will be 14.1 keV leading to a 14.9 keV at 146 pF just based on the contribution from the preamplifier which is in good agreement with the measurements.

In addition, all the FWHM values measured during these characterizations are within the specifications of the project, with every single macropixel below the 20 keV upper limit.

Determination of the Diode Orientation

The tunnel detectors collect electrons from internal conversion electrons (ICE) as well as escaping alpha particles from decays occurring within DSSD detector. The energy range of these particles is in principle significantly different, enabling a selection with a single energy deposit threshold. Unfortunately, due to the energy deposition in the DSSD while escaping and the dead layer effect in the Side detector, the residual energy from alpha particles can be pushed down to the energy range of ICE. This overlapping is particularly damaging for energies below 300 keV. Fortunately, the range of both particles in silicon differs, thus inducing noticeable difference in collection timings even with similar number of charges to be collected. The main key points in the discrimination between the electron signals and escaped alpha signals when they have similar amplitude in a Side detector (100-300 keV energy range) is the optimization of the physical characteristic of the pulse shape of the signal. In that sense, a correct choice of the orientation of the silicon detector with respect to the alpha emission (decay of implanted heavy ions in DSSD) is mandatory.

In order to investigate this point, our Strippy-pad detectors were designed with zero dead layer on both junction and ohmic sides. Fig. 11 illustrates in that sense the difference of pulse shapes observed for detection of alpha particles on the ohmic side (blue) and on the junction side (red). A difference of rise-time of about 50 ns is observed, as expected. This is due to the difference in mobility of the charge carrier inside the silicon, as well as the very small range of alpha particles in silicon.

As alpha particles are being stopped in the first few micrometers of the detector, the charge collection time will mainly be driven by one charge carrier, the opposite one being created very close to its collecting electrode. Therefore, only one type of charge carrier will drive the rise-time of the signal. Indeed, in the case of the junction-side detection the signal rise-time is shorter, as the signal will be mainly driven by the electron, compared to the ohmic implantation where the signal is due to the motion of holes through the entire thickness of the detector (Fig. 11).

We can confirm the difference in rise-time for the alpha particles: as predicted, the one on the ohmic side is slower. Indeed, the electrons have a deeper implantation depth in the silicon (~50 μm to a few hundreds of micrometers depending on their energy) than the alpha particle (few micrometers only). Thus, by using the slower signal for the alpha particle, we should have a better chance to discriminate between the two signals in the low-energy range (100-300 keV). This observation in the difference in mobility for holes and electrons inside silicon firmly points out the orientation of our detector inside the silicon box. Moreover, the fastest collection time in the detector is not at the surface of the junction side where the holes are collected immediately, but slightly deeper. In our case (90 V and -10 °C), the fastest collection is about 120 μm from the Junction side, where electrons and holes have the same collection time. Thus, if the junction side was selected to face the DSSD, the difference between alpha and electron signal of low energy would have been less than 5 ns. But, in the case of the ohmic side, this difference should be around 10-20 ns due to the slow collection time of the holes.

The optimal orientation was determined by setting the inner face inside our 3D assembly to the ohmic side, which that should give the best potential discrimination properties (slower signal) between alpha particles and electrons. This decision triggered the development of the hybrid frame for the final assembly (Fig. 1.); the design of the frame and double-sided PCB depended on the choice of orientation.

CONCLUSION:

Two new generation silicon detectors for the S3-SIRIUS focal plane Side detectors were characterized during this work. These detectors have 64 pixels on the ohmic side and a wide pixel on the junction side. A macropixel grouping of the 64 individual pixels was used to determine two 24-channel configurations for the collaboration, while maintaining the possibility to operate in single pixel mode in the future. Very good alpha energy resolution was achieved with overall resolutions (FWHM) better than 18 keV for 8.8 MeV alpha line and the best macropixels as low as 14.8 keV. These innovative detectors largely meet the SIRIUS collaboration specifications. They will be used for the physics program of S3.

ACKNOWLEDGMENT

The authors warmly thank the mechanical service of the IPHC Strasbourg for their work and support on the test bench development, on the hybrid frame design as well as the 3D mounting system of SIRIUS. We would like to address special thanks to M. Richer for the support on TNT2 operation, to the IPHC microtechnics services as well as to N. Karkour for their support and last but not least the present and former members of the S3-SIRIUS collaboration. The authors would like to express their gratitude to Micron Semiconductor Ltd for their expertise that enabled the realization of this project and to F.

Olschner from CREMAT Inc. for the developments of our customs preamplifiers.


Inês S Cardoso

Emanuelle L P De Faria

Armando J D Silvestre

Mara G Freire

Aminou Mohamadou
email: aminou.mohamadou@univ-reims.fr

Synthesis and characterization of analogues of glycine-betaine surface-active ionic liquids

Keywords: surface-active ionic liquids, analogues of glycine-betaine, synthesis, thermal properties, toxicity, critical micellar concentration

Synthesis and characterization of 8 ionic liquids with surfactant properties

• Thermal properties, micelle diameter and critical micellar concentration were determined • Toxicity towards Aliivibrio fischeri was determined • Most ionic liquids display more atractive properties than the conventional surfactant

Introduction

Surfactants, which could be non-ionic, anionic, cationic and amphoteric compounds, constitute a class of chemical compounds with unique physical properties, including their ability to completely modify surface and interfacial properties and being capable to self-assemble into micelles [START_REF] Dave | A Concise Review on Surfactants and Its Significance[END_REF]. These properties allow the application of surfactants to modify wetting and detergency features, in the displacement of liquid phases through porous media and to (de)stabilize dispersions [START_REF] Kumar | Applications of Surfactants in modern Science and Technology[END_REF]. Due to these properties, this group of chemicals is present in our everyday lives in endless ways, being applied in the textile, leather, food, cosmetic, pharmaceutical, household cleaning and other detergent industries [START_REF] Schramm | Surfactants and their applications[END_REF]. However, many conventional surfactants used in those applications require a high concentration to form stable micelles in aqueous media, which may result in a high concentration of surfactants in both municipal and industrial effluents and in serious threats to the ecosystem and ultimately to human health [START_REF] Scott | The biodegradation of surfactants in the environment[END_REF]. To overcome environmental concerns [START_REF] Bhadani | Current perspective of sustainable surfactants based on renewable building blocks[END_REF], there is a growing interest to develop new surfactants with improved surface-active properties and lower environmental impact [START_REF] Hayes | Biobased Surfactants: Overview and Industrial State of the Art[END_REF].

Amongst relatively recent surfactants, ionic liquids (ILs) have emerged as an alternative class. In short, these compounds are organic salts, composed of large organic cations and organic/inorganic anions, resulting in charge delocalization and low melting temperatures. It is also possible to adjust the solvent properties to a specific application by the proper design of the cation and/or anion chemical structure [START_REF] Sintra | Synthesis and Characterization of Surface-Active Ionic Liquids Used in the Disruption of Escherichia Coli Cells[END_REF]. ILs containing long alkyl chain substituents (usually more than eight carbons) have been reported to be surface-active ionic liquids (SAILs), allowing the design of several families and types of surfactants, such as cationic, anionic, catanionic or gemini surfactants [START_REF] Sintra | Synthesis and Characterization of Surface-Active Ionic Liquids Used in the Disruption of Escherichia Coli Cells[END_REF]. As the conventional surfactants, SAILs are able to self-aggregate or create micelles in aqueous and mixed solutions above the critical micelle concentration (CMC) [START_REF] Smirnova | Ionic liquids as surfactants, Russ[END_REF], [START_REF] Vicente | Impact of Surface Active Ionic Liquids on the Cloud Points of Nonionic Surfactants and the Formation of Aqueous Micellar Two-Phase Systems[END_REF], and even in high salt concentration solutions [START_REF] Shah | Aggregation behavior and antimicrobial activity of a micellar system of binary ionic liquids[END_REF], [START_REF] Hezave | Dynamic interfacial tension behavior between heavy crude oil and ionic liquid solution (1-dodecyl-3-methylimidazolium chloride ([C12mim][Cl] + distilled or saline water/heavy crude oil)) as a new surfactant[END_REF]. Therefore, they can be applied in a wide range of applications, such as cell disruption [START_REF] Sintra | Synthesis and Characterization of Surface-Active Ionic Liquids Used in the Disruption of Escherichia Coli Cells[END_REF], proteins solubilization [START_REF] Mester | Ionic liquids designed as chaotrope and surfactant for use in protein chemistry[END_REF], biocatalysis [START_REF] Ventura | Ionic liquids microemulsions : the key to Candida antarctica lipase B superactivity[END_REF] or extraction and purification processes [START_REF] Fernández-Stefanuto | Ionic Liquids derived from proline: application as surfactants[END_REF], [START_REF] Vicente | R-phycoerythrin extraction and purification from fresh Gracilaria sp. using thermo-responsive systems[END_REF]. For instance, the high impact of SAILs was demonstrated by Vicente et al. [START_REF] Vicente | Impact of Surface Active Ionic Liquids on the Cloud Points of Nonionic Surfactants and the Formation of Aqueous Micellar Two-Phase Systems[END_REF], who investigated the effect of different SAILs as coadjuvants upon the clouding behaviour of three nonionic commercial surfactants, proving that the nature of SAILs can significantly modify the micelles size and the temperature at which the mixture becomes turbid and phase-separate.

Despite the relevance of SAILs, the majority of the published research has focused on the study of the aggregation behaviour of cationic SAILs based on imidazolium cations, which may bring some environmental and health concerns [START_REF] Ivanković | Surfactants in the environment[END_REF]. Significant efforts have been carried out in the past years towards the design of environmentally safer SAILs, while exploring different surfactant families, namely anionic surfactants. Several IL-based surfactants comprising a biobased cation, such as those derived from cholinium, combined with counterions such as carboxylates and alkylsulfates, have gathered significant attention [START_REF] Rengstl | Effect of choline carboxylate ionic liquids on biological membranes[END_REF], [START_REF] Gehlot | Surfaceactive ionic liquid cholinium dodecylbenzenesulfonate: Self-assembling behavior and interaction with cellulase[END_REF]. Among these, Gehlot et al. [START_REF] Gehlot | Surfaceactive ionic liquid cholinium dodecylbenzenesulfonate: Self-assembling behavior and interaction with cellulase[END_REF] reported a cholinium-based surfactant synthesized from a commercial surfactant (sodium dodecylbenzenesulfonate -SDBS), demonstrating the surfactant low ecotoxicity towards freshwater microalgae of Scenedesmus genus. Biodegradable amino-acidbased SAILs showing better surfactant properties than conventional surfactants were reported by Trivedi et al. [START_REF] Trivedi | Task-specific, biodegradable amino acid ionic liquid surfactants[END_REF].

More recently, other ILs classes derived from several natural materials with improved biodegradability and low toxicity have been disclosed [START_REF] Rengstl | Effect of choline carboxylate ionic liquids on biological membranes[END_REF]- [START_REF] Singh | Synthesis and complexation of a new caffeinebased surface active ionic liquid with lysozyme in aqueous medium: Physicochemical, computational and antimicrobial studies[END_REF], e.g. analogues of glycinebetaine (AGB) ILs [START_REF] Capela | Sustainable strategies based on glycine-betaine analogues ionic liquids for the recovery of monoclonal antibodies from cell culture supernatants[END_REF], [START_REF] Pereira | Glycine-betaine ionic liquid analogues as novel phase-forming components of aqueous biphasic systems[END_REF], [START_REF] Pereira | Synthesis and characterization of analogues of glycine-betaine ionic liquids and their use in the formation of aqueous biphasic systems[END_REF], [START_REF] Gaetano | Ionic liquids derived from esters of Glycine Betaine : Synthesis and characterization[END_REF]. Glycine-betaine is a relatively low-cost natural component, which can be obtained from sugar industry by-products, collected after the saccharose extraction step, accounting with 27 wt% of molasses of sugar beet [START_REF] Goursaud | Glycine betaine as a renewable raw material to ' greener ' new cationic surfactants[END_REF], [START_REF] Belgacem | Surfactants from Renewable Sources: Synthesis and Applications[END_REF]. On the other hand, the use of glycine-betaine, as well as their derivatives, is currently acceptable in food supplements and health-care formulations [START_REF] Ferreira | Enhanced Extraction and Biological Activity of 7-hydroxymatairesinol obtained from Norway Spruce knots using Aqueous Solutions of Ionic Liquids[END_REF]. The future application of these solvents requires adequate understanding of their physical properties and due to the ''designer solvent'' character of the ILs, there is a high number of unexplored cation/anion combinations able to boost the applicability of these solvents by covering a much wider polarity and affinity range. Given their high potential and lower toxicity [START_REF] Pereira | Glycine-betaine ionic liquid analogues as novel phase-forming components of aqueous biphasic systems[END_REF], [START_REF] Pereira | Synthesis and characterization of analogues of glycine-betaine ionic liquids and their use in the formation of aqueous biphasic systems[END_REF], [START_REF] Gaetano | Ionic liquids derived from esters of Glycine Betaine : Synthesis and characterization[END_REF], AGB-ILs with surface-active properties have been raising attention recently [START_REF] Kaczmarek | Glycine betaine-based ionic liquids and their influence on bacteria, fungi, insects and plants[END_REF], [START_REF] Janek | Investigating the biomolecular interactions between model proteins and glycine betaine surfactant with reference to the stabilization of emulsions and antimicrobial properties[END_REF]. Yet, to the best of our knowledge, these have not been investigated with the dodecylbenzenesulfonate anion ([DBS] -).

Due to the potential interest of analogues of glycine-betaine ILs with surfactant properties, herein we report the synthesis and characterization of 8 anionic AGB-SAILs comprising the [DBS] -. The novel AGB-SAILs were synthesised and characterized regarding their thermal properties (melting point, glass transition temperature and decomposition temperature), critical micellar concentration (CMC), size distribution by dynamic light scattering (DLS) and ecotoxicity against A. fischeri.

Experimental Section

Materials

Sodium dodecylbenzenesulfonate (SDBS) (purity > 99%), ethyl 2-bromoacetate (purity 98%), ethyl 4-bromobutyrate (purity 97%) were acquired from Sigma Aldrich, and triethylamine (purity 99%), tri(n-propyl)amine (purity 98%), tri(n-butyl)amine (purity 99), and tri(nbutyl)phosphine (purity 95%) were acquired from Acros Organics.

Tri(n-alkyl)[2(or 4)-ethoxy-2-oxoethyl(or 4-oxobutyl)ammonium/phosphonium bromide ([R3NC2]Br or [R3NC4]Br, and [Bu3PC2]Br or [Bu3PC4]Br), were obtained by the methods earlier described [START_REF] Pereira | Glycine-betaine ionic liquid analogues as novel phase-forming components of aqueous biphasic systems[END_REF], [START_REF] Pereira | Synthesis and characterization of analogues of glycine-betaine ionic liquids and their use in the formation of aqueous biphasic systems[END_REF], [START_REF] Gaetano | Ionic liquids derived from esters of Glycine Betaine : Synthesis and characterization[END_REF]. These bromide salts were then applied to an anionic metathesis reaction protocol to prepare the dodecylbenzenesulfonate based ILs (AGB-SAILs).

Phosphate Buffer tablets (purity > 99%) was acquired from Sigma Aldrich.

Synthesis of AGB-SAILs

A hot aqueous solution of sodium dodecylbenzenesulfonate (17.42 g, 0.05 mol) in 150 mL of water was added to a solution of tri(n-alkyl)[2(or 4)-ethoxy-2-oxoethyl(or 4oxobutyl)ammonium/phosphonium bromide (0.05 mol) in water (100 mL) and heated (70 °C) for 1 h whilst stirring. The mixture was then stirred at room temperature for 12h, and then allowed to settle into two phases. The bottom IL phase was extracted by ethyl acetate (3 × 50 mL) and the organic solvent was removed by under vacuum using a rotary evaporator. The resultant sample was dissolved in dichloromethane and washed with pure water several times, until no residual bromide was detected with the use of AgNO3 test of the aqueous phase. Then, the organic phase was dried over anhydrous MgSO4, filtered, and dichloromethane was evaporated under reduced pressure at 50 °C. The oily product obtained were re-dissolved in a minimum of dichloromethane (30 mL) and 3g of active charcoal was added and the mixture was stirred at room temperature.

After 24 h, the mixture was filtrated, and the solvent was removed under reduced pressure. The obtained product was thereafter dried for 48 hours under vacuum to give the AGB-SAILs products. 1 H spectra are given in Figs. S1 to S8 in the Supporting Information, showing the presence of both the organic cations and the corresponding organic anion. Their yields and synthesis characterization are given below.

N,N,N-tri(ethyl)(2-ethoxy-2-oxoethtyl)-1-ammonium dodecylbenzenesulfonate ([EtNC2][DBS]·0.5 H2O)

Yield (20.28 g, 75%

N,N,N-tri(ethyl)(4-ethoxy-4-oxobutyl)-1-ammonium dodecylbenzenesulfonate ([EtNC4][DBS]·0.5 H2O)

Yield (23.04 g, 81%

N,N,N-tri(n-propyl)(2-ethoxy-2-oxoethyl)-1-ammonium dodecylbenzenesulfonate ([PrNC2][DBS]·0.4 H2O)

Yield (25.20 g, 80%

N,N,N-tri(n-propyl)(4-ethoxy-4-oxobutyl)-1-ammonium

N,N,N-tri(n-butyl)(2-ethoxy-2-oxoethyl)-1-ammonium dodecylbenzenesulfonate ([BuNC2][DBS]·0.3 H2O)

Yield (24.81 g, 83%

Characterization of AGB-SAILs

All AGB-SAILs samples were dried under vacuum at room temperature for at least 48h before their characterization. The water content of the dried AGB-SAILs was determined by Karl Fischer coulometry using a Metrohm 787 KF Titrino coulometer, with Hydranal 34805 and Hydranal 37817 (from Fluka) as titrant. All samples have a water content lower than or equal to 3,5 × 10 -3 in mass fraction. The water content of each AGB-SAIL was considered in the preparation of the respective aqueous solutions.

The elemental analysis (C, H, N and S contents) of all synthesized AGB-SAILs was carried out on elementary CHNS thermo-electron FLASHEA 1112 series, whereas the 1 H Nuclear Magnetic Resonance (NMR) spectra were recorded at room temperature with a Bruker AC 30 spectrometer (250 MHz for 1 H) using DMSO as solvent.

The differential scanning calorimetry (DSC) assays were carried out using a TA Instruments Q100 (under controlled nitrogen atmosphere), at a cooling and heating rate of 10 °C•min -1 . The decomposition temperature of all AGB-SAILs was established by thermogravimetric analysis (TGA) using a Netzsch TG 209 F3 Tarsus thermogravimetric analyser (under controlled nitrogen atmosphere) with samples of 10 to 20 mg. The samples were heated from 30 to 600 ºC, at a heating rate of 10 °C•min -1 .

The CMC of all SAILs was determined by electrical conductivity measurements. The electrical conductivity was determined using a SevenMulti™ conductimeter (Mettler Toledo Instruments) at 25°C, within an uncertainty of ± 0.01 mS.cm -1 . The CMC of each compound is given by the breaking point in the linear dependency of the specific conductivity as function of the IL concentration [START_REF] Ali | Synthesis and characterization of choline -fatty-acid-based ionic liquids : A new biocompatible surfactant[END_REF]. Since esters are normally stable in contact with water for long periods, unless acidic or alkaline conditions conjugated with high temperature are used, it is expected that their application in neutral media at moderate temperatures does not raise stability problems, namely hydrolysis.

Dynamic light scattering (DLS) measurements, using a Malvern Zetasizer Nano-ZS from Malvern Instruments, were carried out to evaluate the micelle size of the novel AGB-SAILs.

Aqueous solutions of AGB-SAILs were prepared above their CMC in Phosphate-Buffered Saline (PBS), at pH 7.2 and 25 ºC. Samples were irradiated with red light (HeNe laser, wavelength of 565 nm) and the intensity fluctuations of the scattering light were detected at a backscattering angle of 173° to generate an autocorrelation function. For each sample at least 4 measurements were performed, and the average size and standard deviation determined.

The ecotoxicity of the novel synthetized AGB-SAILs was determined by the standard Microtox ® liquid-phase assay [START_REF] Steinberg | A review of environmental applications of bioluminescence measurements[END_REF]. In this technique it is assessed the inhibition of the luminescence of a specific bacteria (Aliivibrio fischeri -strain NRRL B-11177) [START_REF] Silva | Ecotoxicology and Environmental Safety Sustainable design for environment-friendly mono and dicationic cholinium-based ionic liquids[END_REF]. These assays were carried out following the standard 81.9% test protocol [START_REF] Silva | Ecotoxicology and Environmental Safety Sustainable design for environment-friendly mono and dicationic cholinium-based ionic liquids[END_REF], [START_REF] Volpi | Microtox s solid phase test : Effect of diluent used in toxicity test[END_REF]. Firstly, the bacteria was exposed to a series of diluted aqueous solutions of each IL (from 0 to 81.9 wt%) [START_REF] Silva | Ecotoxicology and Environmental Safety Sustainable design for environment-friendly mono and dicationic cholinium-based ionic liquids[END_REF], [START_REF] Volpi | Microtox s solid phase test : Effect of diluent used in toxicity test[END_REF].

After 5, 15 and 30 min of exposure to each AGB-SAILs aqueous solutions, it was determined the light output of the microorganisms and compared with the light output obtained from the control.

The EC50 values, which represent a standard statistic parameter to evaluate dose-responsive relationships in which there is a 50% growth inhibition [START_REF] Noel | Significant influence of EC50 estimation by model choice and EC50 type[END_REF]. This acute test is assessed at 5, 15 and 30 min of exposure were determined through the available Microtox ® Omni™ Software [START_REF] Silva | Ecotoxicology and Environmental Safety Sustainable design for environment-friendly mono and dicationic cholinium-based ionic liquids[END_REF], [START_REF] Volpi | Microtox s solid phase test : Effect of diluent used in toxicity test[END_REF].

Results and discussion

AGB-SAILs synthesis and characterization

The several AGB-SAILs listed in Fig. 1 were sucessfully synthesized by reaction between ethyl 2-bromoacetate and triethylamine, tri(n-propyl)amine, tri(n-butyl)amine, or tri(nbutyl)phosphine, followed by anion exchange (Fig. 2), obtaining yields ranging between 75%

and 90% for [EtNC2][DBS], [EtNC4][DBS], [BuNC2][DBS], [BuNC4][DBS], [BuPC2][DBS], [BuPC4][DBS], [PrNC2][DBS] and [PrNC4][DBS]. Elemental analysis results and 1 H NMR

spectra of all AGB-ILs, summarized above, reveal the ILs successful synthesis. 1 H NMR resonance assignments and reaction yields are given above, being the respective spectra provided in the Supporting Information. Remarkably, and contrarily to the commercial SDBS which shows a melting temperature of 300 ºC [START_REF] Tyowua | Manipulation of Concentration-Conductivity Data of Sodium Dodecyl Sulphate and Sodium Dodecylbenzene Sulphonate in KCl Solution in Relation to Micellisation Parameters Dodecyl Sulphate and Sodium Dodecylbenzene Sulphonate in KCl Solution in Relation to M[END_REF], all the synthesized AGB-SAILs are liquid at room temperature, which is due to the bulky and asymmetric AGB cations with high charge dispersion, resulting in poor cationanion interactions. This a significant advantage comparing with the SDBS, allowing overcoming the energy associated to the melting enthalpy, thus improving the surfactant water solubility.

Table 1 summarizes the obtained AGB-SAILs glass transition (Tg) and thermal degradation or decomposition (Tdec) temperatures, the last one corresponding to a weight loss of 10%. The Tdec values of all prepared AGB-SAILs are within the range of 140 and 216 °C (TGA profiles given in Fig. S9 in the Supporting Information). This range is lower than the thermal stability of SDBS (380 ºC) [START_REF] Mukwada | Effect of sodium dodecylbenzene sulphonate modifier and PP-g-MA on the morphology and thermal conductivity of PP/EG composites[END_REF], being a consequence of the organic cation present. Although the Tdec values of the prepared AGB-SAILs (above 140 ºC) are lower than of the SDBS, it is still high enough to enable their use in most applications requiring the use of bio-based surfactants.

The most stable AGB-SAIL is [BuPC4][DBS], whereas [PrNC2]

[DBS] is the less thermally stable. Since all the AGB-SAILs share the same anion -[DBS] --the thermal degradation of the investigated AGB-SAILs follows the cation order:

[PrNC2] + < [PrNC4] + < [EtNC2] + < [EtNC4] + << [BuNC2] + < [BuNC4] + < [BuPC2] + < [BuPC4] + .
In general, there is a slight increase in the thermal stability of all SAILs by increasing the length of the alkyl chain lenghts of the cation.

Regarding the SAILs containing ammonium and phosphonium cations with equivalent alkyl chains, namely [BuNC2][DBS] and [BuNC4][DBS] versus [BuPC2][DBS] and [BuPC4][DBS],

the most stable SAIL is the one that holds shorter spacers. These results are in agreement with those from Gaetano et al. [START_REF] Steinberg | A review of environmental applications of bioluminescence measurements[END_REF], who verified that the degradation temperature increases with the increasing number of carbon atoms in alkyl groups grafted on glycine-betaine ILs. A more significant increase in the thermal stability is observed for the phosphonium-based SAILs when compared with the ammonium-based analogues. These results are congruent with those of other phosphonium/ammonium-based ILs, where phosphonium-based ILs display higher Tdec values [START_REF] Pereira | Synthesis and characterization of analogues of glycine-betaine ionic liquids and their use in the formation of aqueous biphasic systems[END_REF], [START_REF] Maton | Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools[END_REF], [START_REF] Kagimoto | Effect of tetrabutylphosphonium cation on the physico-chemical properties of amino-acid ionic liquids[END_REF]. Tsunashima et al. [START_REF] Tsunashima | Physical and electrochemical properties of room-temperature dicyanamide ionic liquids based on quaternary phosphonium cations[END_REF] ascribed this trend to the existence of the empty d-orbitals on the phosphorus atom and Carvalho et al. [START_REF] Carvalho | Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations[END_REF] suggested that it is a result from weaker interionic interactions due to the ionic volume increase, and that this substitution impacts bulk properties. The studied AGB-SAILs also allow to conclude that the longer the spacer, the more thermally stable is the respective AGB-SAIL. Parajó et al. [START_REF] Parajóa | Glycine-betaine-derived ionic liquids: Synthesis, characterization and ecotoxicological evaluation[END_REF]

[EtNC2] + < [EtNC4] + < [PrNC2] + < [PrNC4] + < [BuNC2] + < [BuNC4] + < [BuPC4] + < [BuPC2] + .
In general, an increase in both the aliphatic alkyl side and alkyl chains of the AGB-SAIL in the ammonium cation leads to an increase in the CMC, whereas the opposite is observed with the phosphonium-based SAILs. With the exception of [BuPC2][DBS], which presents a higher CMC (4.35 mM), the CMC values of the remaining AGB-SAILs in aqueous solutions are lower than SDBS (2.9 mM [START_REF] Ali | Synthesis and characterization of choline -fatty-acid-based ionic liquids : A new biocompatible surfactant[END_REF]), meaning that SAILs are in general highly effective surfactants. This observation is in agreement with the literature [START_REF] Vicente | Impact of Surface Active Ionic Liquids on the Cloud Points of Nonionic Surfactants and the Formation of Aqueous Micellar Two-Phase Systems[END_REF],

[10], [START_REF] Gehlot | Surfaceactive ionic liquid cholinium dodecylbenzenesulfonate: Self-assembling behavior and interaction with cellulase[END_REF], [START_REF] Ali | Synthesis and characterization of choline -fatty-acid-based ionic liquids : A new biocompatible surfactant[END_REF], [START_REF] Rao | Sodium bromide induced micelle to vesicle transitions of newly synthesized anionic surface active ionic liquids based on dodecylbenzenesulfonate[END_REF]. The decrease in the CMC with SAILs is mostly due to the large size cations, and the replacement of a more hydrated ion (Na + ) with a less hydrated and hydrophobic large organic cation, thus reducing electrostatic repulsion between the charged head groups in the surface layer, further decreasing the free energy required in the micellization phenomena. As expected, the weaker hydration of the bulky counterions allows the reduction of the electrostatic repulsion of the electrostatic repulsion between headgroups more effective, promoting micelle formation [START_REF] Jiao | Aggregation behaviors of dodecyl sulfate-based anionic surface active ionic liquids in water[END_REF].

With the exception of [BuPC2][DBS], all the novel AGB-SAILs present higher micellar stability in aqueous solution comparing to the commercial surfactant. Given that most of the novel SAILs present a lower CMC than the commercial surfactant (SDBS) it can be emphasized the higher stability of the SAILs' micelles, since the lower the CMC of a given surfactant the more stable the resulting micelles [START_REF] Kapse | Polymeric micelles: A ray of hope among new drug delivery systems[END_REF]. This is particularly important from an industrial point of view.

Table 2. CMC values for the synthesized AGB-SAILs at 25 ºC (in distilled water) compared to SDBS.

AGB-SAILs CMC (mM)

[EtNC2][DBS] 0.76 [EtNC4][DBS] 0.84 [PrNC2][DBS] 0.91 [PrNC4][DBS] 0.90 [BuPC2][DBS] 4.35 [BuPC4][DBS] 2.78 [BuNC2][DBS] 2.44 [BuNC4][DBS]
2.66 SDBS 2.90 [START_REF] Ali | Synthesis and characterization of choline -fatty-acid-based ionic liquids : A new biocompatible surfactant[END_REF] We also studied the micelle sizes of the AGB-SAILs investigated in this work, assuming the formation of spherical micelles, whose diameters, in nm, are presented in Fig. 3. When all the AGB-SAILs are compared, it can be observed that [EtNC4][DBS] forms the smallest micelles, being also the only IL that presents smaller micelles comparing to the commercial surfactant (SDBS). By opposition, the [BuNC2][DBS] forms the micelles with the largest diameter. This trend is mainly due to the head group electrostatic repulsions brought by the ionic nature of the AGB-SAIL and the higher steric effects arising from the larger bulk organic counterions [START_REF] Jiao | Aggregation behaviors of dodecyl sulfate-based anionic surface active ionic liquids in water[END_REF].

For the aqueous solutions of AGB-SAILs, two distinct behaviours were observed: a slight decrease in micelle size induced by the larger spacers in the cations or micellar growth promoted by the increase in the alkyl chain length.

It is also possible to verify that phosphonium-based AGB-SAILs promotes the formation of smaller micelles than ammonium-based counterparts. This phenomena related to the effect of the AGB-SAIL cation central atom (N vs. P) results from the charge distribution at the AGB-SAIL central heteroatom, which commands the SAIL water affinity and hydration spheres. As such, a greater charge delocalization closer to the central atom of the cation generates a stronger cation/anion interaction for the ammonium-based SAILs [START_REF] Carvalho | Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations[END_REF]. Taking into account the need of developing greener surfactants, it is mandatory to address their toxicity to foresee their potential use in the textile, leather, food, cosmetic, pharmaceutical and several detergent-related applications [START_REF] Scott | The biodegradation of surfactants in the environment[END_REF], [START_REF] Rebello | Surfactants: Toxicity, remediation and green surfactants[END_REF]. In this sense, the impact of all AGB-SAILs against the marine luminescent bacteria A. fischeri for 30 min of exposure time was evaluated, being the correspondent EC50 values with the respective 95% confidence limits summarized and compared with SDBS in Fig. 4. The EC50 values at 30 min of exposure were specifically analysed to ensure enough exposition of the bacteria and assess the full impact in the luminescence inhibition [START_REF] Silva | Ecotoxicology and Environmental Safety Sustainable design for environment-friendly mono and dicationic cholinium-based ionic liquids[END_REF], [START_REF] Volpi | Microtox s solid phase test : Effect of diluent used in toxicity test[END_REF]. According to previous studies [START_REF] Parajóa | Glycine-betaine-derived ionic liquids: Synthesis, characterization and ecotoxicological evaluation[END_REF], [START_REF] Couling | Assessing the factors responsible for ionic liquid toxicity to aquatic organisms via quantitative structureproperty relationship modeling[END_REF], it would be expected to have an increased ecotoxicity with increasing cation alkyl chain lengths, since it increases the IL hydrophobicity leading to a higher capacity to interact with phospholipid bilayers of cell membranes. This phenomenon is reported as ''side-chain effect'' [START_REF] Stolte | Effects of different head groups and functionalised side chains on the aquatic toxicity of ionic liquids[END_REF]. In addition, Parajó et al. [START_REF] Parajóa | Glycine-betaine-derived ionic liquids: Synthesis, characterization and ecotoxicological evaluation[END_REF] verified that the spacer in the cation of the AGB-SAIL plays a significant role in the toxicity of the SAIL, i. counterparts. However, in the present work, opposite trends have been observed, demonstrating that the SAIL anion is playing the major role in defining these trends. In fact, in this work and contrarily to the previously cited works, we are dealing with an anionic surfactant where the toxicity is mainly defined by the long alkyl side chain at the anion. Therefore, the opposite trends observed in what concerns the IL cation is a consequence of the cation-anion interactions and synergestic effects.

It is known that the toxicity is related with the CMC; in general, the lower the CMC, which reflects a higher surface activity, the higher the toxicity of the respective compound [START_REF] Ríos | Ecotoxicological characterization of surfactants and mixtures of them[END_REF].

However, in this work it is not possible to verify any correlation between the AGB-SAILs

Conclusions

This Although surfactants derived from natural sources are a hot topic of research, their CMC is usually lower than that displayed by conventional surfactants, a trend that is not verified in the current work. Additionally, the low CMC values of AGB-SAILs, combined with their high thermal stability and low toxicity, allows to foreseen the future research of AGB-SAILs on the development of sustainable processes.

This work opens the door to the future introduction of AGB-SAILs in industrial processes, by enlarging the available data concerning novel and more benign SAILs, reinforcing the idea that the interfacial region can then be fine-tuned according to a specific application through the combination of the cation, including the nature of the substituents introduced and other structural modifications, and the counter-ion. Another advantage is that AGB-SAILs the favourable properties of most aprotic ILs, such as high polarity, high thermal and chemical stability, low melting point and negligible vapour pressure.

Table 1. Glass transition (Tg) and decomposition (Tdec) temperatures of the synthesized AGB-SAILs and SDBS (for comparison).

Table 2. CMC values for the synthesized AGB-SAILs at 25 ºC (in distilled water).

Figure captions

Fig. 1

 1 comprises the acronym and chemical structure of these AGB-SAILs. The resonances assignment following the structures numbering. A commercially available surfactant (sodium dodecylbenzenesulfonate, SDBS) was used for comparison purposes.

Fig. 1 .

 1 Fig. 1. Chemical structure of the synthesized AGB-SAILs and of the commercial surfactant SDBS.

Fig. 2 .

 2 Fig. 2. Synthesis route for AGB-SAILs.

Fig. 3 .

 3 Fig. 3. Micelle size of AGB-SAILs in PBS (pH 7.2) at 25 ºC compared to SDBS.

Fig. 4 .

 4 Fig. 4. Microtox ® EC50 values (mg.L -1 ) for A. fischeri after 30 min of exposure time to aqueous solutions of AGB-SAILs and to a commercial surfactant (SDBS).

  e. comparing the cations [Bu3PC2] + and [Bu3PC4] + (both sharing the saccharinate anion), with the latter one being less toxic. Parajó et al.[START_REF] Parajóa | Glycine-betaine-derived ionic liquids: Synthesis, characterization and ecotoxicological evaluation[END_REF] and Pereira et al.[START_REF] Pereira | Glycine-betaine ionic liquid analogues as novel phase-forming components of aqueous biphasic systems[END_REF] also showed that the ammonium-based AGB-SAILs present a lower toxicity comparing to their phosphonium-based

  ecotoxicity and the CMC, as shown in Figure S11 in the Supporting Information, further reinforcing the role of the IL anion on determining the IL ecotoxicity towards Aliivibrio fischeri and the secondary effects manifested by the IL cation. Overall, according to the Passino and Smith classification [50], all the AGB-SAILs investigated in this work are classified as slightly toxic (at 30 min of exposure: 10 mg.L -1 ≤ EC50 ≤ 100 mg.L -1 ). Despite the fact that [BuNC4][DBS], [EtNC2][DBS] and [EtNC4][DBS] display a higher toxicity to A. fischeri than the commercially available surfactant (SDBS, EC50 = 38.72 mg.L -1 ), it should be remarked that the majority of the novel AGB-SAILs are good alternatives to the commercial SDBS surfactant in terms of toxicity and environmental risk. This fact coupled to the lower CMC values obtained with AGB-SAILs open the door to their use in a more ecofriendly and effective approach in a wide diversity of industries, such as food, cosmetic, pharmaceutical, household cleaning and other detergent industries.

  work reports the synthesis and characterization of eight new surface active analogues of glycine-betaine-based ionic liquids (AGB-SAILs) obtained by combination of analogues of glycine-betaine cations and the dodecylbenzenesulfonate anion. The respective thermal properties, namely glass transition and decomposition temperatures, were determined and discussed according to the IL chemical structure. All synthesized AGB-SAILs are liquid at room temperature, present degration temperatures ranging between 140 ºC and 216 ºC, and glass transition temperatures ranging between -53 ºC and -51 ºC. The investigated AGB-SAILs have CMC values ranging between 0.76 and 4.35 mM, where the majority of these present lowerCMC values than the respective commercial analogue (SDBS), and thus comparable or with higher micellar stability. Their toxicity against the marine luminescent bacteria A. fischeri showed that the studied AGB-SAILs are considered slightly toxic, but display a comparable or lower toxicity than the commercial surfactant. In this work, it is reported a singular synergistic effect between the IL cations and the [DBS] -anion since most hydrophobic AGB-SAILs are the less toxic.

Fig. 2 .

 2 Fig. 2. Chemical structure of the synthesized AGB-SAILs and the commercial surfactant SDBS.

Fig. 2 .

 2 Fig. 2. Synthesis route for AGB-SAILs.

Fig. 3 .

 3 Fig. 3. Micelle size of AGB-SAILs in PBS (pH 7.2) at 25 ºC compared to SDBS.

Fig. 4 .

 4 Fig. 4. Microtox® EC50 values (mg.L -1 ) for A. fischeri after 30 min of exposure time to aqueous solutions of AGB-SAILs and to a commercial surfactant (SDBS).

ammonium dodecylbenzenesulfonate ([BuNC4][DBS]·0.3 H2O) Yield

	[14 H, m, CH2(m+β+γ)]; 2.27 [6 H, m, CH2(a)]; 3.80 [2 H, d, CH2(1)]; 4.27 [2 H, q, CH2(5)];
	7.12 [2 H, d, CH(n+r)]; 7.53 [2 H, dd, CH(o+q)].
	2.2.8.	Tri(n-butyl)[4-ethoxy-4-oxobutyl]phosphonium	dodecylbenzenesulfonate
	([BuPC4][DBS])
	Yield (27.93 g, 85%). Elemental analysis: Found: C, 67.52; H, 10.84; S, 4.98%.
	Calculated for C36H67O5PS (MW = 642.95 g•mol -1 ): C, 67.25; H, 10.50; S, 4.99%. 1 H NMR,
	δ/ppm (300 MHz, DMSO-d6): 0.82-0.92 [12 H, m, CH3(δ+a)]; 1.20 [24 H, m, CH2(6+b-j)]; 1.37
	[14 H, m, CH2(β+γ+2)]; 1.75 [2 H, m, CH2(k)]; 2.21 [8 H, m, CH2(α+1)];2,45 [2 H, m, CH2(3)];
	4.11 [2 H, q, CH2(5)]; 7.11 [2 H, d, CH(n+r)]; 7.50 [2 H, dd, CH(o+q).
	Calculated for C36H67,6O5,3NS (MW = 631.39 g•mol -1 ): C, 68.48; 10.79; N, 2.22; S, 5.08%. 1 H
	NMR, δ/ppm (300 MHz, DMSO-d6): 0.97 [12 H, m, CH3(d+a)]; 1.21 [21 H, m, CH2(6+b-j)];
	1.33 [6 H, m, CH2(g)]; 1.57 [10 H, m, CH2(b+k+2)]; 1.84 [2 H, m, CH2(3)]; 2.40 [2 H, t,
	CH2(m)]; 3.20 [8 H, m, CH2(a+1)]; 4.17 [2 H, q, CH2(5)]; 7.11 [2 H, d, CH(n+r)]; 7.50 [2 H, d,
	CH(o+q)].
	2.2.7.	Tri(n-butyl)[2-ethoxy-2-oxoethyl]phosphonium	dodecylbenzenesulfonate
	([BuPC2][DBS]·)

). Elemental analysis: Found: C, 67.50; H, 10.91; N, 2.12; S, 5.60%. Calculated for C34H63,6O5,3NS (MW = 603.31 g•mol -1 ): C, 67.69; H, 10.62; N,2.32; S, 5.31%. 1 H NMR, δ/ppm (300 MHz, DMSO-d6): 0.80-0.82 [12 H, m, CH3(δ+a)]; 1.25 [27 H, m, CH2(6+γ+b-k)]; 1.60 [8 H, m, CH2(β+m); 3.45 [6 H, m, CH2(α)]; 4.35 [2 H, q, CH2(5)]; 4.50 [2 H, s, CH2(1)]; 7.11 [2 H, d, CH(n+r)]; 7.50 [2 H, d, CH(o+q)].

2.2.6. N,N,N-tri(n-butyl)(4-ethoxy-4-oxobutyl)-1-

(28.99

g, 90%). Elemental analysis: Found: C, 68.85; H, 10.92; N, 1.92; S, 5,45%. Yield (25.03 g, 80%). Elemental analysis: Found: C, 66,13; H, 10.52; S, 5.65%. Calculated for C34H63O5PS (MW = 614.90 g•mol -1 ): C, 66.41; H, 10.33; S, 5.91%. 1 H NMR, δ/ppm (300 MHz, DMSO-d6): 0.80-0.92 [12 H, m, CH3(δ+a)]; 1.16 [20 H, m, CH2(6+b-j)]; 1.42

Table 1 .

 1 reported the Tdec values of the AGB-ILs comprising the [BuNC2] + , [BuNC4] + , [BuPC2] + and [BuPC4] + combined with the saccharinate anion. Comparing these AGB-ILs with the novel AGB-SAILs here reported, it is possible to verify that the combination of those cations with the [DBS] -anion leads to a decrease in the Tdec values. Glass transition (Tg) and decomposition (Tdec) temperatures of the synthesized AGB-SAILs and SDBS (for comparison).

	AGB-SAILs	Tg (°C)	Tdec (°C)
	[EtNC2][DBS]	-53	149
	[EtNC4][DBS]	-51	153
	[PrNC2][DBS]	-52	140
	[PrNC4][DBS]	-53	143
	[BuNC2][DBS]	-53	187
	[BuNC4][DBS]	-53	193
	[BuPC2][DBS]	-51	209
	[BuPC4][DBS]	-53	216
	SDBS	n.a.	380 [37]

n.a. -not available

The AGB-SAILs CMC data were further assessed since it is a key parameter for surfactants, being given in Table

2

(and Fig. S10 in Supporting Information). CMC values of AGB-SAILs range between 0.76 and 4.35 mM, with the highest CMC corresponding to [BuPC2][DBS] and the lowest associated to [EtNC2][DBS]. The results further show that the CMC increases in the following cation order:

Acknowledgments

This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. Inês S. Cardoso acknowledges the PhD grant SFRH/BD/139801/2018.

Author Contributors

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at