Textural, Structural and Electrical Characterizations of EMIMAc Silica Ionogels and Their Corresponding Aerogels

Silica ionogels were synthesized from tetramethoxysilane (T MOS ),methyltrimethoxysilane (MT MS ) and 1-ethyl2-methylimidazolium Acetate (EMIMAc: Ionic Liquid) in different proportions .The textural characterizations showed an effect of these concentrations on the corresponding aerogels: pore size distributions and effective surfaces. The structure of the aerogels was measured with a SAXS (SmallAngle XRay Scattering) apparatus and was typical of acid catalyzed aerogels. Conductivity voltage measurements, operated on the ionogels, were carried out using an electrical 4 wire-electrodes set up. The electrical voltage temporal response of the EMIMAc silica ionogel was modelled by a RLC series circuit which characteristics depended on the synthesis.


INTRODUCTION
Ionic liquids (IL) are molten salts which are liquid at a temperature lower than 100 o C.
Since the first IL synthesis, chemists and material scientists have been highly interested in this new type of conducting liquids because of their numerous properties and potential applications like green solvent, electrolytes, sensors, catalysts or batteries [Galinski et al. Asymmetric cation, N,Ndialkylimidazolium or 1-butyl-3-methylimidazolium associated with anions such as tetrafluoroborate, were the most commonly IL used for a few years [Gupta et al. (2012)] [Shi et al. (2005)]. Here we chose to work with another imidazolium cation, 1-ethyl-2-methylimidazolium bonded to the anion acetate (EM IM Ac). This IL exhibits a high electric conductivity (2500µS/cm) at ambient temperature [www.ilco- The present study is focused on the synthesis and the characterization of EM IM Ac silica ionogels and their corresponding aerogels.
Textural and structural characterizations as nitrogen adsorption/desorption and SAXS were carried out on the aerogels. Hence, porous volume and texture were obtained together with the structure of the silica solid network.
Then, by a 4-wire-electrodes set up, we measured the temporal response of our ionogels to a voltage pulse and we modeled these responses by a RLC circuit. Up to now, only three studies on ionogels conductivity were performed [Neouze et al. (2006)  The knowledge of the texture and the structure of ionogels imply their drying under conditions which allow to obtain dry materials exhibiting properties very close to these of the ionogels. For these reasons, drying must be carried out under supercritical conditions to avoid capillary stresses which would lead to dense and cracked materials. For the same reasons and to avoid the dissolution/redeposition phenomenon which occurs at high temperature and which can induce textural modifications at a very low scale [Quinson et al. (1992)], we chose to dry the gels using supercritical CO 2 so-called COLD process. The wet gels were aged during 12h. For CO 2 supercritical drying, the liquid that impregnates the wet gels must exhibit a total miscibility with CO 2 . So, the wet gels, which contained water and the IL, were first soaked into ethanol for 15 days to obtain gels with pores mainly filled by ethanol. Then the ethanol was exchanged with liquid CO 2 at 8 o C and 6 M P a during 5 hours. Finally, temperature and pressure were raised to 38 o C and 10 M P a and after 10min, the pressure was released slowly during 12h. Samples obtained were translucent (figure 1) without any cracks and exhibited a shrinkage which depended on the composition of the gel.

Textural and structural characterizations of the aerogels
The bulk densities of the samples were evaluated from their weight and linear dimensions.
For the textural characterization, nitrogen adsorption/desorption isotherms at 77K were carried out on a Micromeritic ASAP2010. Samples were out-gassed under vacuum during 16h at 50C. The specific surface area was obtained using BET theory. Its accuracy was about 4%. For rigid porous materials, when the nitrogen relative pressure P/P 0 gets close to 0.99, the volume of adsorbed nitrogen must correspond to the porous volume [Reichenauer et al. (2001)]]. The dwell time, which is the interval of time allowing a pressure change of 0.01%, was set to 10 seconds. The pore size distributions were derived from BJH method from the desorption branches of the isotherm. SAXS (small angle X ray scattering) and WAXS (wide angle X ray scattering) experiments were performed with an in-house setup of the Laboratoire Charles Coulomb, using a X-ray tube GeniX3D from Xenocs and a Schneider 2D image-plate detector prototype, in order to estimate the fractal dimension of the materials.

Electrical conductivity measurements
These measurements were performed on ionogels using a set up described on figure 2.
The cylindrical cell allowing these measurements consisted of a Teflon mold containing the ionogel and of 4 Pt-electrodes partially immersed in the ionogel and passing through the sides of the mold. The 4 electrodes were P t wires with a section of 0.5 mm. These wires were immersed in the bulk of gels along 7mm and fixed to the mold to make sure that all the measurements were done in the same conditions. The mold was cylindrical with a depth of 5 mm and a diameter of 15 mm. The 4 wires were connected to a pulse generator 4 (Aligent 33220A, 20MHz, Arbitrary waveform generator) and to an oscilloscope (TDS2022C, 200MHz, Tektronik).
A rectangular voltage pulse was applied between 2 (channel 1) of the 4 electrodes. Then the potential difference was measured between these 2 electrodes (channel 1) and between the 2 other ones (channel 2). First, we measured the electrochemical domain of stability of the EMIMAc. We found that for an applied potential equal to 4V and an offset equal to -1.375V , there was oxido-reduction of the IL at room temperature. We checked that for an applied potential equal to 2.5V and an offset equal to -1.25V , the IL was stable.
Straightforwardly, we chose to study the electrical temporal response of our ionogel samples at this voltage. The voltage pulses had an absolute value of 3.75V (applied potential 2.5V plus offset 1.25V ), a frequency of 50Hz and pulse durations of either 20ns or 100ns.
This allowed us to measure the electrical response of the solvent (IL, HCl, water) contained in the porous network of the ionogels. Indeed, as the diameter of the electrodes was smaller than the depth of the set up (i.e. smaller than the height of the ionogel cylinder), these electrodes were always in contact with the IL contained in the pores. As we applied rectangular voltage pulses to the ionogel, we were able to analyze the temporal response of the sample.
In order to analyze the role of the silica network, a blank was first done on EM IM Ac + HCl solution. Then electrical voltage measurements were performed on the ionogel samples within 1 hour after gelling occurred.

Textural and structural Characterizations
The relative shrinkage which occurred during the different steps, from the gel to the aerogel, and the characteristics of the aerogels are presented in table II. Most of the samples exhibited low densities and high specific surface areas. The specific surface area showed a tendency to increase up to a maximum value with a higher ratio of nM T M S/nT M OS but it did not change with the IL concentration. A divergence between the measured porous volume V mp and the calculated one V cp was observed. As reported in the literature [Reichenauer et al. (2001)], when the density of the aerogel is low, it is more difficult to explore the whole porous volume by adsorption/desorption. We must also note that some IL remained inside the aerogels (∝ 5% in weight) as already described in the literature ].
Nitrogen adsorption isotherms are given in Figure 3 for three compositions. The pore size distributions, reported in Figures 4, are calculated from the isotherms. Aerogel made without M T M S displayed a larger pore size distribution. This distribution is increasingly narrow and shifted to lower diameters when the M T M S ratio is increasing (Figure 4 left).
However, distributions do not vary when the IL ratio increases (Figure 4 right).
All the ionogels synthesized are fractal ones even if the fractal domain is relatively small (one decade)(figures 5).The fractal dimension is enclosed between 2.2 and 2.5, values which are characteristic of acid catalyzed aerogels. Samples C,D and E exhibit the same behavior with a larger fractal domain than sample A or B, and a fractal value around 2.5.

Conductivity: electrical voltage characterization
The maximum of the measured amplitudes on both channels 1 and 2 are presented in  • For the system EMIMAc + HCl solution, the maximum of the response on channel 1 was lower than the input pulse but with oscillations whatever the pulse duration. In the case of responses measured on channel 2 (see section Results), the damping is due to the direction of the applied potential (perpendicular to channel 2).
What is worth noting, is the difference between the responses of the ionogels samples