aprotic ionic liquids as a deep

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A reciprocal binary mixture of protic/aprotic ionic liquids as a deep eutectic solvent: physicochemical behaviour and application towards agarose processing† Pankaj Bharmoria,a Krishnaiah Damarla,a Tushar J. Trivedi,a Naved I. Malekb and Arvind Kumar*ac Apart from structural tuning, the desired properties of ionic liquids (IL) can be achieved through judicious mixing of two or more ionic liquids. Herein we have investigated the alterations in the physicochemical properties of protic/aprotic ILs, (2-hydroxyethylammonium formate/1-butyl-3-methylimidazolium chloride) upon reciprocal binary mixing. Melting point analysis of the mixtures at various mole fractions showed their deep eutectic solvent nature. The variation in physical properties like density, speed of sound, and viscosity have been measured and utilized to derive the volume of mixing, isentropic compressibility, and activation energy of the viscous flow. Unlike the binary mixtures of ILs having common cations or anions, the investigated reciprocal binary mixture showed significant non-ideality, normally desired to take advantage of improved solvent properties. The polarity and ion–ion interactions have been studied through solvatochromic parameters (normalized Reichardt's parameter, dipolarity/ polarizability, and hydrogen bond donor and acceptor coefficients) derived using solvatochromic probes.

Received 25th October 2015 Accepted 9th November 2015

The prepared binary mixtures have been utilized for the dissolution of a gelling biopolymer agarose and

DOI: 10.1039/c5ra22329f

formation of ionogels. Dissolution of agarose has been correlated with the solvatochromic parameters and the viscoelastic behaviour of ionogels is discussed in light of rheological measurements. The work

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gives fundamentally useful insights into tuning the properties of ILs for a specific application purpose.

Introduction Ionic liquids (ILs) are salts which are entirely comprised of ions and have melting points (Tm) around or below 100 C.1,2 ILs on

a

Academy of Scientic and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute, Salt and Marine Chemicals Division, G. B. Marg, Bhavnagar-364002, India. E-mail: [email protected]; [email protected]; Fax: +91-278-2567562; Tel: +91-278-2567039

b

Applied Chemistry Department, S.V. National Institute of Technology, Surat-395 007, India

c Salt and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, India

† Electronic supplementary information (ESI) available: Fig. S1 (1H-NMR spectra of [HEA][HCOO]), Fig. S2 (13C-NMR spectra of [HEA][HCOO]), Fig. S3 (LCMS spectra of [HEA][HCOO]), Fig. S4 (1H-NMR spectra of [C4mim][Cl]), Fig. S5 (13C-NMR spectra of [C4mim][Cl]), Fig. S6 (LCMS spectra of [C4mim][Cl]), Fig. S7 (images of IL mixtures), Fig. S8 (DSC thermograms), Fig. S9 (composition vs. melting point plots), Fig. S10 (density plot), Fig. S11 (speed of sound plot), Fig. S12 (viscosity plots), Fig. S13 (isentropic compressibility plots), Fig. S14 (Arrhenius plots) of [HEA][HCOO] + [C4mim][Cl] mixtures, Fig. S15 (UV-vis spectra of the dyes), Fig. S16 (ionogel images), Fig. S17 (G0 and G00 vs. temperature plots of ionogel), Fig. S18 (DSC thermograms of ionogels), Fig. S19 (comparative G0 and G00 plots as a function of u), Fig. S20 (G0 and G00 vs. g% plots of ionogel), Fig. S21 (schematic showing intermolecular hydrogen bonding), Tables S1–S3. See DOI: 10.1039/c5ra22329f

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account of their unique accessible physicochemical properties have a huge potential for an array of applications. Application specic properties of ILs can be nely tuned by; (i) choice of desired cations or anions in an IL or (ii) by altering the structure of the cation or anion. Further improvements in the desired properties can be achieved through the synergism between various ions either by direct synthesis of so called “Double Salt Ionic Liquids (DSILs)” through direct mixing of multiple ions3 or by making mixtures of two or more ILs at different mole fractions. While the DSILs have been dened as “salts composed of more than two types of ions, liquid at low temperature (99%) were procured from Sigma-Aldrich, and N,N-diethyl-4-nitroaniline (97%) was purchased from Oakwood Products Inc. Agarose (Type II-A) was purchased from Sigma Aldrich. Solubilization and ionogel formation of agarose Dissolution of agarose in neat ILs and ILs mixtures was carried out in 7 ml glass vial (equipped with Teon coated cap) with continuous stirring in the glove box under an inert atmosphere of N2. The temperature of the dissolving process was controlled to 1 C. Finely powdered agarose was added to either neat ILs or mixed ILs in small lots till the resulting IL solutions become clear. Dissolution experiments were conducted at 70 C and were monitored visually. For ionogel preparation agarose (5%) was dissolved in preheated native or mixed ILs which formed a viscous solution at 70 C and gelled upon cooling to room temperature. Measurements The melting point of mixtures and native [HEA][HCOO] was determined from differential scanning calorimetry measurements using a NETZSCH DSC204F1 (Germany) thermal analyser in nitrogen atmosphere. Measurements were performed between 150 and 28 C at a heating rate of 3 C min1. In a typical experiment 2.5 mg of sample taken in alumina crucible was cooled to 150 C using liquid nitrogen, maintained at 150 C for 2 min and then heated again from 150 to 28 C. The melting point of native [C4mim][Cl] (0 to 150 C at 10 C min1) and ionogels (0 to 120 C at 5 C min1) were measured using METTLER TOLEDO DSC 822e (Japan) in nitrogen atmosphere. The 13C-NMR and 1H-NMR spectra of ILs were measured from Bruker 500 MHz spectrometer. For [HEA] [HCOO], C6D6 was used as an external solvent and for [C4mim] [Cl], the DMSO was used as solvent. Electrospray ionization mass spectrometry (ESI-MS) was done with Q-Tof micro™ Micromass, U.K. Elemental analyses were performed on PerkinElmer series II-2400-CHNS. The Karl-Fischer analysis was done in Mettler Toledo DV705, T50 Autotitrator using dry methanol as solvent and Karl-Fischer reagent. Density (r) and speed of sound (u) of various IL mixtures were measured using an Anton Paar (model DSA5000) vibrating tube density meter with a resolution of 5 106 g cm3 and 0.01 m s1. The temperature of the apparatus was controlled to within 0.01 K by a built-in Peltier device that corresponds to an uncertainty in density of 0.0002%. The reproducibility of the results was conrmed by performing the measurements in triplicate.


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Viscosity (h) measurements were carried out on Anton Paar Automated Microviscometer (AVMn) which is based on falling ball principle. Temperature of the apparatus was controlled to within 0.1 K by a built-in Peltier device. Solvatochromic parameters were determined by measuring the wavelength corresponding to absorption maximum (lmax) of solvatochromic probes (Reichardt's dye, 4-nitroaniline and N-Ndiethyl-4-nitroaniline) at 298.15 K, with a concentration of about 10 mM, in IL mixtures, using a UV 3600 Shimadzu UV-visNIR spectrophotometer with a thermocell attached to it. The stock solutions (1 mM) of the probes were prepared in absolute methanol. For each measurement 0.016 g of the probe solution was taken in a quartz cuvette (path length 1 cm), evaporated under vacuum and diluted to 1.6 g with various IL mixtures to make the nal concentration 10 mM. For complete mixing the solutions were stirred for 12 hours on magnetic stirrer. To avoid the impurity of water to maximum level during mixing the mixtures were heated at 80 C until the weight became constant and then stored in vacuum desiccator containing P2O5 as desiccant for 24 hours before measurement. Viscoelastic measurements of gels were performed on an Anton Paar Physica MCR 301 rheometer, USA, using the parallel plate PP50/PPTD200 geometry (50 mm diameter; 0.1 mm gap). The frequency dependences of dynamic storage (G0 ) and loss (G00 ) moduli were examined in the linear viscoelastic regime (predetermined at each temperature). The temperature dependences of G0 and G00 were measured with strain amplitude of 5% and a frequency of 0.1 rad s1, with the heating rate of 0.5 C min1 from 25 C to 90 C. The temperature was controlled by a Viscotherm VT2 circulating water bath.

Results and discussion Deep eutectic solvent behaviour DSC thermograms showing melting points of [C4mim][Cl], [HEA][HCOO] and [HEA][HCOO] + [C4mim][Cl] mixtures at different mole fractions are shown Fig. S8A–E (ESI†). The plot of melting temperature vs. composition of eutectic mixture and native ILs is shown in Fig. S9 (ESI†). The melting points of all the mixtures are in the range of 90 C to 101 C, which are lesser than that of native [C4mim][Cl] (65 C)26 and [HEA] [HCOO] and (88.5 C). This observation indicated the deep eutectic solvent like behaviour of mixtures generally observed for 2 : 1 choline chloride and ethylene glycol mixture.27 Physical properties Experimental density (r), speed of sound (u) and viscosity (h) of [HEA][HCOO](x1) + [C4mim][Cl](x2) mixtures at 298.15 K, 308.15 K and 318.15 K are plotted as a function of x1 in Fig. S10–S12 (ESI†). Density (r) of a material is dened as the molar mass of material per unit volume (r ¼ M/V). In the present study, the r of mixtures increased non-linearly as function of x1 and decreased with an increase in temperature (Fig. S10, ESI†). The density of a material depends upon the packing of ions in a unit volume which is further controlled by size and shape of the ions along with ion–ion interactions. The non-ideality in a mixture arises

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due to the spacing among the ions caused by improper packing. The extent of non-ideality in a mixture can be estimated from the excess molar volume, calculated from the experimental r using eqn (1). ðx1 M1 þ x2 M2 Þ x1 M1 x2 M2 VmE ¼ Vm Vmid ¼ (1) þ r r*1 r*2 where Vm and Vid m are the real and ideal molar volumes of the solutions; r, r*1 and r*2 are the density of component 1 and component 2 of the mixture; M1, M2 and x1, x2 are the molar masses and mole fractions of component 1 and component 2 respectively. VEm, calculated at 298.15 K for the binary mixture is plotted in Fig. 1. Unlike the reported binary mixtures of imidazolium or pyridinium ILs having either cation or anion in common,8–11 large positive deviation in VEm with a maximum at x1 ¼ 0.37 is observed (Fig. 1). The large positive VEm has indicated the inefficient packing of ions per unit volume in the mixtures, which can be accounted to the large difference in size and dominance of physical interactions over the chemical/specic interactions between the dissimilar ions. It is to be noted that the long-range Coulomb forces between the charges may lead to highly ordered ionic lattices and may be responsible for the observed deviations. Therefore, we monitored the rigidity of mixture solutions, maintained by different bonding interactions from isentropic compressibility (Fig. S13, ESI†) calculated from experimental r and u using Newton–Laplace equation (ks ¼ 1/u2r). The ks of liquids is dened as the propensity of liquid to sustain external pressure. The ks decreased as function of x1 at all the studied temperatures whereas at a particular composition it increased with the rise in temperature for all the mixtures. This behaviour indicates an increase in solution rigidity with an increase in concentration of [HEA][HCOO] which can be accounted to better packing of ions as a consequence of lesser size differences between ions of two ILs. The increase in ks as a function of temperature is due to the weakening of bonding interactions caused by an increase in thermal motions.

Fig. 1 Excess molar volume (VEm) of [HEA][HCOO](x1) + [C4mim][Cl](x2) mixtures at 298.15 K.

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Flow properties of the solution mixtures are a good indicator of the prevailing interactions in the systems and have a direct relation with their applicability, specically in case of diffusion dependent processes such as electrochemistry28 and in gas absorption.29 Since the cations of both the ILs in a mixture are unsymmetrical, the activation energy of viscous ow (Ea) could be calculated from the ts of temperature dependence of h at different mole fractions (Fig. S14, ESI†) using Arrhenius like law.30,31 ln h ¼ ln hN Ea/RT

(2)

where h is viscosity, Ea is activation energy of viscous ow, R is universal gas constant and T is working temperature of the system in Kelvin. The calculated Ea as a function of x1 at 298.15 K is plotted in Fig. 2. A signicant decrease in Ea with the addition of [HEA] [HCOO] to [C4mim][Cl] has indicated the reduction of electrostatic interactions between the ions of the ILs. The electrostatic interactions are least around the equimolar composition, which has been indicated by a minimum in Ea at x1 ¼ 0.494. The eutectic mixture also showed lowest melting temperature at this composition. Such a low viscous conducting mixture can be useful as solvent in electrochemical processes.28 Polarity behaviour Polarity of solvents is direct indicator of their specic and nonspecic solvation capabilities. The solvatochromic parameters, ET(30), EN T , p*, a and b gives an ideal opportunity to map the polarity of ILs. The symbol, EN T is normalized Reichardt's parameter which indicates the solute–solvent interactions and polarity of the solution. The EN T has been calculated from the wavelength corresponding to maximum absorption of Reichardt's dye using the eqn (3).32 1 EN T (kcal mol ) ¼ (ET(30) 30.7)/32.4

(3)

where the parameter ET(30) ¼ 28 591/lmax(RD) in kcal mol1 is the Dimroth–Reichardt parameter which indicates both dipolarity/polarizability and acidity of the solvent.32

Fig. 2 Activation energy of viscous flow (Ea) of [HEA][HCOO](x1) + [C4mim][Cl](x2) mixtures.


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The dipolarity/polarizability (p*) parameter is derived from the variation in maximum absorption energy of the dye which induced the local electric eld generated by the solvent. The parameter is affected by the dye–solvent interaction with the increase in mole fraction of one of the component.33 p* was calculated by measuring the lmax of dye, N-N-diethyl-4nitroaniline (DENA) in different IL mixtures using the eqn (4).34–36 p* ¼ (27.52 nDENA)/3.183

(4)

The parameters, a and b indicates the hydrogen bond donor capacity (HBD) and hydrogen bond acceptor capacity (HBA) of solvents and were calculated by measuring the variation in lmax of Reichardt's dye (RD), DENA and nitroaniline (4NA) in ILs mixture using eqn (5)37,38 and (6).36,39 a ¼ ET(30) 14.6(p* 0.23) 30.31/16.5

(5)

b ¼ 1.035nDENA n4NA + 2.64/2.8

(6)

In eqn (5) and (6) nDENA (kilokeyser, 103 cm1) ¼ 10 000/lmax and nNA (kilokeyser, 103 cm1) ¼ 10 000/lmax are the wave numbers corresponding to maximum absorption wavelengths of the dyes, DENA and 4NA in the IL mixtures. The experimental solvatochromic parameters plotted as a function of [HEA][HCOO] mole fraction (x1) are shown in Fig. 3 and corresponding values are provided in Table S1 (ESI†). The wavelength corresponding to absorption maximum (lmax) of different dyes in the IL mixtures at various mole fractions are given in Table S2 (ESI†) and corresponding UV-vis spectra are shown as Fig. S15 (ESI†). The calculated values of p*, a and b of the pure ILs are within the experimental error to the literature values.32 The EN T parameter increased with an increase in x1 which indicates the rise in polarity of the mixtures in going from [C4mim][Cl] to [HEA][HCOO]. The higher interactions between phenoxide group of RD and hydrogen atoms of the ammonium head group of [HEA][HCOO] compared to that with of C(2) or C(3) and C(4) of imidazolium head group is accountable for this

behaviour.40 The calculated dipolarity/polarizability parameter (p*) is >1 thus suggested the fact that both protic and aprotic ILs are highly polarizable. The presence of molecular ions having delocalized electrons demonstrates high polarizability of the mixture.40 A small decrease in p* till x1 ¼ 0.30 has indicated the decrease in coulombic interactions between [C4mim][Cl] and dye due to more favourable [C4mim][Cl]–[HEA][HCOO] interactions compared to [C4mim][Cl]–dye interactions. Beyond x1 ¼ 0.30 the increase in p* is driven by more favourable dye–[HEA] [HCOO] coulombic interaction.40 The observed decrease in the value of b as a function of x1 showed the reduction in hydrogen bond accepter capacity of the mixtures which can affect their ability to dissolve biopolymers.23 Since the b depends upon the anion of ILs, the decrease b can be accounted to the lowering in concentration of Cl anion which is a better proton acceptor compared to HCOO anion. The parameter a increased as a function of x1 which indicated the rise in hydrogen bond donor capacity of the mixtures. The maximum deviation from linearity in a is seen around x1 0.40, where the activation energy of viscous ow is also quite low and electrostatic interactions among ions are suggested to be signicantly reduced in the mixture. At higher mole fractions (x1 $ 0.40) the values of a reached saturation and was closer to that of pure [HEA][HCOO]. The constancy in a showed that hydrogen bonding donor capacity hardly changes by the imidazolium cation in [HEA][HCOO] rich region of the mixture.

Agarose solubility and ionogels Agarose along with other biopolymers has been investigated for dissolution in ILs for functionalization, blending, and ionogel formation for various electronic applications.35,41–44 We have investigated the solubility and gelling behaviour of agarose in IL mixtures at various mole fractions and compared the solubility with b parameter (Fig. 4).

Fig. 4 Plot showing variation in solubility of agarose at different

Plot showing variation of solvatochromic parameters (EN T , a, b, p*) of [HEA][HCOO](x1) + [C4mim][Cl](x2) mixtures. Fig. 3


compositions of [C4mim][Cl] + [HEA][HCOO] (inset shows the correlation of solubility and b parameter).

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It has been observed that the solubility of agarose decreased with an increase in mole fraction of [HEA][HCOO] in general. However, a small increase has been observed in the mixture composition range of 9 : 1 to 3 : 7 and then from 1 : 9 to pure [HEA][HCOO]. One of the most basic questions of biopolymer dissolution in ILs is whether dissolution has a linear correlation with the hydrogen bond accepter capacity (b)? Recent studies of cellulose dissolution in ILs have shown that ILs with high b value possesses higher dissolution capability for cellulose.23 The correlation was also abided by various 1 : 1 mixtures of ILs for agarose dissolution.24 In the present system, though solubility of agarose has been found to be higher at higher b value but did not follow the exact linear correlation. This observation indicated that b parameter may not be the sole criterion to explain solubility behaviour of biopolymers in ILs. Therefore, other factors such as solution polarity, hydrogen bond acidity and secondary effects such as anion size and geometry may also be playing a role in solubility as the mixed ILs at different ratios exhibits different a and p* values and signicantly different geometries. It has been reported in different studies,45–47 that hydroxyl group on alkyl functionality of ionic liquid reduces the solubility of cellulose and the protic cation prevents cellulose solubilisation entirely in many cases. The protic cation prevents cellulose dissolution entirely in many ILs due to the stronger interactions between cations and anions, thus reducing the propensity of ionic liquids to dissolve cellulose. Similar behaviour is observed in the present case wherein the solubility of agarose decreased with an increase in protic part of the mixture ([HEA][HCOO]). The increase in solubility of agarose between 9 : 1 to 3 : 7 can be accounted to the role played by polarity, hydrogen bond acidity, secondary effects such as anion size and geometry other than b value.48 Since biopolymer based ionogels have potential applications in so electronic devices, we prepared agarose ionogels by dissolving 5% of agarose in neat and mixed ILs at different compositions and by cooling the IL–agarose-sols to room temperature (Fig. 5 and S16 ESI†). The melting temperature (Tm) of the ionogels was calculated from the crossing point of dynamic storage (G0 ) and loss moduli (G00 ) vs. temperature plot (Fig. S17 ESI†) and mid-point of DSC thermograms (Fig. S18 ESI†). In the rheology measurements at a composition of 3 : 7 ([C4mim][Cl] : [HEA][HCOO]), the system exhibited very low melting point. Around this concentration

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a signicant departure from ideality in volume of mixing and reduced ion–ion interactions have also been observed from physical properties. The comparative Tm of ionogels observed from DSC are higher than that observed from rheology measurements (Table S3 ESI†). This behaviour can be accounted to the fact that in DSC the gels were initially cooled to 0 C, before heating to 120 C thus attaining more ordered arrangements of molecules leading to higher melting temperatures.49 Upon comparing the trends in Tm of eutectic mixtures (Fig. S9 ESI†) with that of gels from DSC (Fig. S18 ESI†) measurements it has been found that trends is similar up to half way mark, beyond which the Tm decreased further in gels whereas increased slightly in eutectic mixtures. On the other hand trends are similar when compared with the Tm obtained from rheology measurements (Table S3 ESI†). It is to be noted that rheology is more authentic technique than the DSC to measure the Tm of ionogels. Therefore, the ions of ILs in eutectic mixture must have interacted with agarose in a structure they attained in the mixture for gel formation. The viscoelastic behaviour of the gels has been studied from the dynamic shear measurements by calculating the storage (G0 ) and loss moduli (G00 ). The G0 and G00 in the viscoelastic materials is the measure of stored energy of the elastic portion and dissipated energy of the viscous portion. The frequency dependence of dynamic storage (G0 ) and loss moduli (G00 ) of ionogels is shown in Fig. 6A and B. The higher difference in magnitude of G0 and G00 indicates solid like behaviour of the prepared ionogels.24 A comparative analysis of the G0 and G00 at u ¼ 98 rad s1 (Fig. S19 ESI†) have shown that the prepared ionogels exhibits solid like behaviour which keep on decreasing as we move towards ionogels prepared in ILs with low b value. The strength of the gel microstructure was investigated from strain dependence of dynamic storage (G0 ) and loss moduli of (G00 ) of ionogels in the range 0.1 to 100% at a frequency of 100 rad s1 (Fig. S20, ESI†). All the prepared ionogels showed linear viscoelastic behaviour. These results show the resistance of gel microstructure to any deformation even at high % strain. Like melting temperature observed from rheology the G0 and G00 of ionogels in the strain range of 0.1 to 100% were lowest for ionogel prepared in 3 : 7 ([C4mim][Cl] : [HEA][HCOO]) mixture wherein a high non-ideality was observed in physical properties. The gel strength then increases with an increase in [HEA] [HCOO] concentration and can be attributed to the

Fig. 5 Images of agarose ionogels prepared by dissolving 5% agarose in native and mixtures of [C4mim][Cl] + [HEA][HCOO] at different compositions.

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Fig. 6 Frequency dependence of (A) bulk modulus (G0 ) and (B) loss moduli (G00 ) of agarose ionogels prepared at different compositions of [C4mim][Cl] + [HEA][HCOO] mixtures.

enhancement of inter-molecular hydrogen bonding between agarose hydroxyl groups via ions of [HEA][HCOO]. The [HEA] [HCOO] has a hydrogen bond donor site in cation ([HEA]+) and hydrogen bond acceptor sites in the anion, ([HCOO]). Therefore, the [HEA]+ involves in inter-molecular hydrogen bonding with the agarose oxygen whereas, the [HCOO] involves in hydrogen bonding with hydrogen's of agarose hydroxyl group. The hydrogen bonding between ionic liquid and biopolymers has earlier been reported from NMR50 and simulation studies.51,52 The schematic of agarose–ionic liquid hydrogen bonding interactions is provided in Scheme S1.†

Conclusion We conclude that reciprocal binary mixing of protic/aprotic ILs ([HEA][HCOO]/[C4mim][Cl]), resulted in deep eutectic mixtures over the whole composition range. The mixtures showed a signicant departure from ideality in the volume of mixing due to the predominance of dispersion forces. The activation energy of viscous ow indicated reduced levels of electrostatic interactions among the ions, which were lowest at the equimolar composition. Such a low viscous IL mixture can nd potential applications in electrochemistry28 and gas absorption.29 The solvatochromic behaviour of the mixtures showed that the mixtures are highly polar throughout the composition range. The hydrogen bond donor capacity (a) increases whereas the hydrogen bond acceptor capacity (b) decreases with the increase of [HEA][HCOO] concentration in the mixture. The solubility of agarose in the eutectic mixtures was checked and found to increase with an increase in value of b but did not follow the linear correlation generally observed for biopolymers dissolution in different ILs. The prepared ionogels of agarose showed a solid like behaviour. The gels strength was found to be lowest for ionogel prepared in 3 : 7 ([C4mim][Cl] : [HEA] [HCOO]) wherein high non-ideality was observed in physical properties and increased aer that as a function of [HEA] [HCOO]. The present results re-establishes the fact that physical properties and solvating ability of ILs towards biopolymers can be improved by judicious mixing of ILs with ions of diverse nature.1,3


Acknowledgements PB and NIM acknowledges DST, Government of India for nancial support from the projects (SB/S1/PC-104/2012) and (SR/FT/CS-014/2010) respectively. TJ is thankful for nancial assistance by CSIR, Government of India (EMR-II/2545/2011). The analytical division of CSIR-CSMCRI is acknowledged for sample characterization.

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99252 | RSC Adv., 2015, 5, 99245–99252

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