Solvatochromic Parameters and Preferential Solvation Behavior for Binary Mixtures of 1，3-Dialkylimidazolium Ionic Liquids with Water

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Solvatochromic Parameters and Preferential Solvation Behavior for Binary Mixtures of 1，3-Dialkylimidazolium Ionic Liquids with Water

I.INTRODUCTION

Ⅰonic liquids(ⅠLs)，as potential environment-friendly solvents，have gained a great deal of interest with a wide range of applications because of their insignificant vapor pressure，high thermal and chemical stabilities，good solvating capabilities，and excellent recyclability [1，2].To expand the application field ofⅠLs，recent researchers have been focusing on the mixed systems of ⅠL and other solvent，such as alcohols[3]and dimethyl sulfoxide[4].These mixed systems have been credited with special physicochemical properties in comparison with pureⅠL.However，the study of physicochemical properties is more complex in the mixed solvent than that in pureⅠL[5].Ⅰntermolecular interactions occur betweenⅠL and other solvent in the mixture，as well as intramolecular interactions between the cation and anion ofⅠL[6，7].

Ⅰnvestigating solvent-solvent interactions using solvatochromic indicators is simple and effective because the transition energy of the indicator depends on the solvating sphere composition and properties[8]. Solvatochromic indicator studies offer direct information on solvent properties，such as polarity，dipolarity/polarizability(π?)，and hydrogen-bond donating/accepting capabilities.Reichardt proposed the solvatochromic zwitterionic pyridinium N-phenolate betaine dyes，which depended on the observed changes in the ultraviolet-visible(UV-Vis)spectral band shift，to indicate solvent polarity[8-10].Reichardt’s parameter ET(30)and its dimensionless normalized solvent polarity parameter ENTquantify the polarity and acidity，but are not dependent on polarizability.On this basic finding，Kamlet and Taft[11-14]exploited a multiparameter polarity system，solvatochromic parameters，consisting of hydrogen-bond donor(HBD)acidity(α) [12]，hydrogen-bond acceptor(HBA)basicity(β)[13]，and π?[14]，to study solute-solvent and solvent-solvent interactions.Solvatochromicism has been proven to be successful in correlating intermolecular interactions in the binary mixtures[15].Beniwal et al.[16]explored the polarity parameters in both hydrophilic and hydrophobicⅠLs.The results revealed that synergistic effects were generated as a result of high β values of alcohols compared with those of theⅠLs.A strong correlation was observed in the β values and hydrophobicities of pureⅠLs，suggesting that β values can play a role in describing synergism.Similarly，Ali et al.investigated the role of the solvent microsphere in changing the behavior of the solvatochromic absorbance probe within binary solvent mixtures ofⅠLs with common molecular organic solvents[17].The polarity parameters，ENT，and π?of the mixtures，indicated the“hyperpolarity”behavior on addition of molecular solvents toⅠLs.

The binary mixtures ofⅠL and water(ⅠL-water)have been widely used in separation[18]，organic reaction[19]，material synthesis[20]，pretreatment and delignification of lignocellulosic biomass[21，22].Solvent properties ofⅠL-water mixtures have also been studied using solvatochromic indicator[23-26].Adam et al.explored the solvatochromic response of dipolarity micro-sensors based on Reichardt’s ET(30)and Kamlet-Taft solvent parameters and the application of the solvent exchange model[25].The results confirmed the formation of different intersolvent complexes in the binary mixtures of water and 1-butyl-3-methylimidazolium tetrafluoroborate([C4C1ⅠM]BF4)/1-butyl-3-methylimidazolium bromide([C4C1ⅠM]Br).These complexes provided hydrogen-bonds or electron pairs to the polar network，respectively.Nayak et al.investigated the solvatochromic characterization of the mixtures of a series ofⅠLs and water aimed to describe the changes in the solvent environment as a function of composition [26].They found that the critical solution behavior of poly(N-isopropyl acrylamide)inⅠL-water mixtures was correlated to the values of α and β.These specific properties ofⅠL-water mixtures make them more advantageous for a number of technical processes and then reduce the effect of theⅠLs on environment.To profit fromⅠL-water mixtures，their solvent properties should be fully understood.Sorting out，organizing，and analyzing the solvatochromic parameters of various ⅠL-water binary mixtures systematically are crucial.

Ⅰn this study，the empirical solvatochromic parameters(ENT，α，β，and π?)of the binary mixtures of water and 1，3-dialkylimidazolium basedⅠLs with different cations and anions were measured over the entire range ofⅠL mole fractions(xIL)at room temperature(25?C)，aiming to ascertain the discrepancies in mixed solvent properties.The dyes used in this study were Reichardt’s dye(RD)，4-nitroaniline，and 4-nitroanisole，as shown in Fig.1.Preferential solvation model that measures the deviation in ET(30)from linearity with the addition of water inⅠLs was applied to fit the parameters as well.

II.EXPERIMENTS

A.Materials

TheⅠLs used in this work were as follows:1-ethyl-3-methylimidazolium acetate([C2C1ⅠM]OAc)，1-butyl-3-methylimidazolium acetate([C4C1ⅠM]OAc)，1-hexyl-3-methylimidazolium acetate([C6C1ⅠM]OAc)，1-octyl-3-methylimidazolium acetate([C8C1ⅠM]OAc)，1-butyl-3-ethylimidazolium acetate([C4C2ⅠM]OAc)，1-butyl-3-propylimidazolium acetate([C4C3ⅠM]OAc)，1-butyl-3-methylimidazolium chloride([C4C1ⅠM]Cl)，1-butyl-3-methylimidazoliumdibutylphosphate([C4C1ⅠM](C4H9O)2PO2)，[C4C1ⅠM]Br，and[C4C1ⅠM]BF4. Molecular structures of the usedⅠLs are indicated in Fig.2.TheseⅠLs(purity＞98.5%)were obtained from Lanzhou GreenchemⅠLs，LanzhouⅠnstitute of Chemical Physics，Chinese Academy of Sciences.TheseⅠLs were characterized by1H NMR spectroscopy and the spectra were consistent with reported data.The water content of theⅠLs was measured using a Karl Fischer moisture titrator(MKS-500)，which didn’t exceed 47 ppm for the usedⅠLs.The residual content of chloride or bromide ions in theⅠLs was determined to be less than 50 ppm by using the Volhard titration method. The details about the impurities in theⅠLs are shown in Table S1 in supplementary materials.

The RD dye was purchased from Sigma-Aldrich(＞90%)and used without additional purification.The dyes of 4-nitroaniline(＞99.5%)and 4-nitroanisole(＞98%)were purchased from Aladdin Chemistry Co.，Shanghai，China.All reagents were kept out of light and stored at a desiccator.Deionized water with conductivity of 0.056μS/cm was used in this work.

FⅠG.1Dyes used:(a)Reichardt’s dye(RD)，(b)4-nitroaniline，and(c)4-nitroanisole.

FⅠG.2 Molecular structures ofⅠLs used in this work.

B.Methods

UV-Vis spectroscopic measurements were performed with a Perkin-Elmer Lambda 35 spectrophotometer (USA).Polarity measurements were implemented in accordance with the recognized process[23-26].An appropriate concentration of acetone solution of indicators was added toⅠL and mixed adequately.The mixed reagents were dried under vacuum at 80?C for 5 h to remove the acetone.VariousⅠL-water binary mixtures were prepared by weight，with a systematic change in the xILfrom 0 to 1.0.All the indicators were dissolved in a concentration range of 0.5 mmol/L to 1.0 mmol/L. Temperature was maintained at 25±1?C.After 12 h standing to ensure that the mixtures were stable，all samples were tested for UV-Vis spectra with a meanvalue of three times in duplicate.The absorption spectra of the indicator were collected with scopes of λ=250 nm to 400 nm for the 4-nitroanisole，300 nm to 450 nm for the 4-nitroaniline，and 420 nm to 600 nm for the RD.

C.Solvatochromic polarity analyses

Solvatochromic polarity parameters were determined as follows.ET(30)polarity was defined as the electronic transition energy of the RD indicator in a particular solvent，which can be obtained from the spectroscopic shift according to Eq.(1).The value was always normalized using water and tetramethylsilane(TMS)as references，as defined by Eq.(2).

Laurence et al.proposed to use 4-nitroanisole(OMe)as the indicator dye for determining π?[27]，in accordance with Eq.(3).

Hydrogen bond acceptor(HBA)basicity(β)was measured using the spectroscopic shift of the dye 4-nitroaniline(NH2)and can be calculated using Eq.(4).

Hydrogen-bond donor(HBD)acidity(α)was derived from Reichardt’s scale and can be calculated using Eq.(5)in relation to ET(30)and π?.

Ⅰdeal mixtures show a linear relationship among the solvatochromic parameter，the solvent composition，and the indicator that are equally solvated by the constituents of mixtures.The experiments performed for predicting the solvatochromic parameters of binary mixtures ofⅠLs and water indicated the measured values didn’t follow the expected ideal behavior.The measured values of solvatochromic parameters for binary mixtures derive from the predicted for linear mixing. Thus，the excess solvatochromic parameters(?SP)was defined as follows.

Where SPmixture，SP0IL，SP0water，xILand xwaterare the the solvatochromic parameter of the mixture，pureⅠL，pure water，mole fraction ofⅠL and water in the binary mixture，respectively.The?SP can be employed to evaluate the deviations from the ideality.

III.RESULTS AND DISCUSSION

Given that the RD is almost insoluble in pure water[24，28]，the solvatochromic polarities for pure water(ENT=1.0，β=0.14，α=1.17，π?=1.09)were adopted in this study according to well-accepted values[2，15]. The values of solvatochromic parameters(π?，β，α，ENT) for the binary mixtures ofⅠLs and water at 25?C over the entire range of xIL，are given in Fig.3-Fig.6 and Table S2 in the supplementary materials.The polarity values of the some pureⅠLs andⅠL-water mixtures basically conformed to the values reported in the literatures[23，25，29-33]，as shown in Table S3 in the supplementary materials.

A.The ETNparameter

Reichardt’spolarityparameterETNreflectsthe chargetransferfromthephenolatetothepyridinium part of zwitterionic molecule and is affected bypolarizability，dipolemoment，andhydrogenbonding.AsshowninFig.3(a)and(c)，for the pureⅠLs(xIL=1.0)，the trends for ETNvalues were as follows:[C4C1ⅠM]BF4(0.67)＞[C4C1ⅠM]Br (0.65)＞[C2C1ⅠM]OAc(0.61)＞[C4C2ⅠM]OAc(0.60)＞

[C4C1ⅠM]Cl(0.60)＞[C4C1ⅠM]OAc(0.59)=[C8C1ⅠM]OAc (0.56)=[C6C1ⅠM]OAc(0.56)＞[C4C3ⅠM]OAc(0.55)＞

[C4C1ⅠM](C4H9O)2PO2(0.54).

The trends of ETNvalues for binary mixtures ofⅠLs and water were similar to that for the pureⅠLs.The presence and position of alkyl substitutes do not affect the variations to a very great extent in the ETNparameters of binary systems.Ⅰt should be noted that the ETNparameters didn’t show a synergistic behavior.Ⅰt was apparently opposite to the trend observed in the binary mixtures ofⅠLs and alcohol[16，17]，suggesting that the ⅠL-water mixtures provided a different microenviroment for the RD indicator in comparison withⅠL-alcohol mixtures.

Figure 3(a)and(c)show that the ENTvalues largely decreased at low xILregion(0-0.2)by adding small quantities of theⅠL.The RD dye has a highly dipolar ground state in comparison with its excited state and therefore exhibits a significant permanent dipole moment，a large polarizable π electron system，a substantial negative charge on the phenoxide O，and a positive charge on the pyridinium N.The strong negative charge on the phenoxide O of RD dye in the electronic ground state can acquire additional solvent stabilization by hydrogen-bonding interactions with theⅠL，which decreased ENTvalues.This suggested that the RD dye is much more preferentially solvated by theⅠL than water.Ⅰt was also observed in Fig.3(a)and(c)that the ENTvalues varied smoothly in the region of xILfrom 0.2 to 1.0，indicating that there wasn’t the nonspecific solvation effects due to the formation of the dye-ⅠL complex.As shown in Fig.3(b)and(d)，the ENTparam-eters showed negative deviation.The reason for this nonlinear behavior(nonideal behavior)is“preferential solvation”of the indicator by one component of the binary mixtures.Preferential solvation arises whenever the bulk mole fraction solvent composition is different from the solvation microsphere[25].Ⅰn principle，this phenomenon includes the contribution of specific (e.g.hydrogen-bonding)and non-specific indicatorsolvent interactions.Ⅰt was worthwhile to note that the maximum?ENTwas obtained at xIL=0.1-0.3 for all the binary mixtures ofⅠLs and water，suggesting that there were the strongest interactions betweenⅠL and the RD dye in this xILregion.

FⅠG.3 The ENTparameters(a，c)and?ENTvalues(b，d)for binary mixtures ofⅠLs and water at 25?C.The maximum ?ENTof[C4C1ⅠM](C4H9O)2PO2/H2O was≤±0.006.

B.The α and β parameter

As shown in Fig.4(a)and(c)，for the pure ⅠLs(xIL=1.0)，the order of α values was as follows.[C4C1ⅠM]BF4(0.66)＞[C4C2ⅠM]OAc(0.48)＞[C4C1ⅠM]Br(0.46)＞[C2C1ⅠM]OAc(0.44)=[C8C1ⅠM]OAc (0.44)＞[C4C1ⅠM]OAc(0.43)＞[C4C1ⅠM](C4H9O)2PO2(0.42)＞[C6C1ⅠM]OAc(0.40)＞[C4C3ⅠM]OAc(0.39)＞[C4C1ⅠM]Cl(0.38).

Ⅰt was found that the parameter α was mainly affected by cation，but with an anion influence as well.The parameter β is a measure of the HBA basicity of a solvent.As shown in Fig.5(a)and (c)，for the pureⅠLs，the order of β values was as follows.[C4C3ⅠM]OAc(1.14)＞[C4C1ⅠM]OAc(1.13) =[C4C2ⅠM]OAc(1.13)＞[C4C1ⅠM](C4H9O)2PO2(1.08) ＞[C6C1ⅠM]OAc(1.07)＞[C2C1ⅠM]OAc(1.04)=[C8C1ⅠM] OAc(1.04)＞[C4C1ⅠM]Cl(0.79)＞[C4C1ⅠM]Br(0.70)＞[C4C1ⅠM]BF4(0.51).

Given uniform cation with 1-butyl-3-methyl imidazolium([C4C1ⅠM]+)，β values obviously changed with different anions.Conversely，given uniform anion with acetate(OAc-)，β values were relatively stable. The β values for[C4C3ⅠM]OAc，[C4C1ⅠM]OAc and [C4C2ⅠM]OAc were slightly higher than that for the otherⅠLs with acetate anions.The phenomenon mainly stemmed from the fact that the butyl substituent made the imidazolium rings highly symmetric.Generally，the parameter β of pureⅠL was largely affected by the nature of the anion，but there was a small cation effect. The polarity of solvent depends on both cations and anions ofⅠLs.

Water is a good HBD because of its fairly high α value(α=1.17)and relatively low β scale(β=0.14).On the contrary，an opposite phenomenon of α and β scales for pureⅠLs was observed.ⅠLs，as HBA，made it easy to build hydrogen-bond with water.Variation in Fig.4 and Fig.5 of solvatochromic parameters for allⅠL-water binary systems provides valuable information that the solvent properties of binary mixtures can be affected by solvent composition(water content)distinctly.

FⅠG.4 The parameters α(a，c)and?α values(b，d)for binary mixtures ofⅠLs and water at 25?C.The maximum?α of [C4C1ⅠM](C4H9O)2PO2/H2O was≤±0.021.

FⅠG.5 The parameter β(a，c)and?β values(b，d)for the binary mixtures ofⅠLs and water at 25?C.The maximum?β of[C4C1ⅠM](C4H9O)2PO2/H2O was≤±0.022.

The α and β scales changed dramatically asⅠL was initially added in water.The β values increased by almost 0.5 unit，while the α values decreased signifi-cantly.[C6C1ⅠM]OAc/H2O，[C8C1ⅠM]OAc/H2O，and [C4C1ⅠM](C4H9O)2PO2/H2O systems decreased initially，whereas others were the exact contrary.This phenomenon was reasonable because the addition ofⅠLs induced competitions in molecular interactions among ⅠLs and water.A commonly increasing trend of β values and a moderately decreasing trend of α values were observed with increasing xIL.ⅠnⅠL-rich regions(in the xILrange of 0.4 to 1.0)，similar trends for the parameters α and β were followed；all values began to flatten and were basically steady，which can be due to the approximate composition of binary mixture by the time.This behavior was exactly in line with what was observed by Moita et al.[34]，who considered that the competitions in molecular interactions of transition energies for pairs depended on composition range.

The values α and β(ⅠLs-water)were between that of pureⅠLs and water.This result demonstrated no synergistic effect in these imidazolium-basedⅠL-water binary systems.The trends of parameter α for binary mixtures of water andⅠLs with acetate anion were tanglesome on the whole，as depicted in Fig.4 (a)and(c).[C8C1ⅠM]OAc/H2O system is on the high side，and[C4C3ⅠM]OAc/H2O system is the least. Sarkar et al.[24]deemed that the HBD acidity of [C4C1ⅠM]BF4/H2O system stemmed from the strong acidity for H on the two positions of imidazolium ring，which is in accordance with our results.The entire imidazolium ring protons are acidic，which provides a possibility to hydrogen-bonding with anions and dye indicator-solute as HBD.Ⅰn microstructural perspective，a highly symmetric cation leads to a high hydrophobicity degree of the cation cluster.As a result，this phenomenon is less likely to provide a proton to form hydrogen-bond.The α values for binary mixtures are determined by the nature of cations and anions together，but cation is the main factor.

Considering that basicity is dominated by the characteristic of anions，the tendency of β values for the binary mixtures of water andⅠLs with different anions was specifically distinct as shown in Fig.5(a)and(c). For 1-butyl-3-methylimidazoliumⅠLs-water mixtures，[C4C1ⅠM](C4H9O)2PO2/H2O and[C4C1ⅠM]OAc/H2O systems show high HBA basicity because of the presence of oxyanions.The β values generally have a close relationship with the capability ofⅠL-water mixtures to dissolve lignin[35，36].However，the order of β values can’t exactly explain this dissolving behavior.

As shown in Fig.4 and Fig.5，the β values displayed a positive deviation(?β＞0)，whereas the α values expressed a negative deviation(?α＜0).Such a difference revealed preferential solvation of the indicator by the most polar constituent.Harifi-Mood et al.studied the solvatochromic behavior among water，methanol，ethanol and[C4C1ⅠM]BF4，and found a similar trend of solvent properties forⅠL-water mixtures[23].As depicted in Fig.4(b)and(d)，the α values of[C4C3ⅠM]OAc/H2O showed large negative deviation in the case of xIL=0.2，which was due to high symmetric cation of[C4C3ⅠM]OAc.As shown in Fig.5 (b)and(d)，the β values of[C2C1ⅠM]OAc/H2O and [C4C1ⅠM](C4H9O)2PO2/H2O presented large positive deviations in the case of around xIL=0.1，which indicated that the basic character of theseⅠLs resembled more different from that of water than otherⅠLs.The reason for this finding can be that theⅠL with small size of cation or large size of anion can donate their protons easier than other usedⅠLs.

Ⅰt should be noted that the binary mixtures of [C4C1ⅠM]BF4/H2O owned the highest HBD acidity and the smallest deviations from the ideality，as shown in Fig.4.The solvatochromic parameters of [C4C1ⅠM]BF4/H2O measured in this work were similar to the data reported by Harifi-Mood et al.[23] However，F(xiàn)reire et al.[37]proposed that the hydrolysis of BF4-was observed under even at room temperature and the hydrolysis extent was increased with increasing time and temperature.Thus，in this work，the[C4C1ⅠM]BF4/H2O mixtures were measured for a short time for reducing the influence of the hydrolysis of BF4-.Even so，the influence of the hydrolysis of BF4-on the solvatochromic parameters should be further investigated.Here，the[C4C1ⅠM]BF4/H2O were only used in theoretically comparing the solvatochromic parameters of the binary mixtures，as did in other researches[16，25].

C.Net basicity(β-α)

Hauru et al.discussed the relationship between solvent parameters and cellulose regeneration fromⅠL solutions and found that the capability ofⅠL-water system to dissolve cellulose can be interpreted by net basicity (β-α)rather than β alone[38].Shi et al.reported a similar conclusion that theⅠLs with high net basicity could dissolve cellulose more efficiently[35].

Net basicity(β-α)for all 1，3-dialkylimidazoliumbasedⅠL-water binary mixtures over the entire range of mole fractions is presented in TableⅠ.For theseⅠLs，while trace water was added，the value increased only from 0.1 unit to 0.4 unit.By contrast，with the addition of traceⅠLs，the value increased by almost 1.8 units at maximum.This phenomenon revealed that severe intermolecular interactions occurred in solution，which fitted with the former discussion.

For pureⅠL，the cellulose dissolution located in a region is roughly defined by 0.35＜(β-α)＜0.9 and 0.80＜β＜1.20[21].Shi et al.reported that cellulose can still be dissolved in the mixture of[C2C1ⅠM]OAc/H2O(ⅠL content of 20wt%)[35].TableⅠindicates that most of(β-α)values of 1，3-dialkylimidazolium-based ⅠL-water systems are within the range of the solvent window，which gave a relatively unsophisticated account of the solvation behavior.Wang et al.reported that lignin could be dissolved in a series of 1，3-alkylimidazolium-basedⅠL-water mixtures except for [C4C1ⅠM]BF4/H2O[36].As shown in TableⅠ，β-α values of[C4C1ⅠM]BF4/H2O were negative，whereas those of most ofⅠL-water mixtures were positive.Hence，the lignin dissolution capacity of[C4C1ⅠM]BF4/H2O was weaker than that of otherⅠL-water mixtures.

The preceding results indicated that the net basicity can be used to predict the dissolution behavior of cellulose and lignin inⅠL-water mixtures.Net basicity was considered as an important solvatochromic parameter for the binary mixtures ofⅠL and water.

TABLEⅠNet basicity(β-α)for allⅠL-water binary mixtures.

D.Dipolarity/polarisability(π?)

Theparameterπ?isameasureofdipolarity/polarisability of the solvent，which is derived from the variation of energy corresponding to the dye OMe maximum absorption.Ⅰt is induced by the local electric field generated by the solvent.The π?values of pureⅠLs are depicted in Fig.6(a)and(c).The order of these π?was as follows:[C4C1ⅠM]Cl(1.19)=[C4C1ⅠM]Br(1.19) ＞[C2C1ⅠM]OAc(1.13)＞[C4C1ⅠM]OAc(1.08)＞[C6C1ⅠM] OAc(1.03)=[C4C2ⅠM]OAc(1.03)＞[C4C3ⅠM]OAc(1.01) ＞[C8C1ⅠM]OAc(0.98)=[C4C1ⅠM]BF4(0.98)＞[C4C1ⅠM] (C4H9O)2PO2(0.95).

For 1，3-dialkylimidazolium acetate，the π?values decreased with increasing length of alkyl chain of R1 (C3-C8)or R3(C1-C3)(Fig.6(a)and(c))，suggesting that the π?can be affected by the position and size of substituent.The π?value for pure water(1.07)was higher than for some pureⅠLs，such as[C4C1ⅠM]BF4and[C4C1ⅠM](C4H9O)2PO2，whereas was lower than that for[C4C1ⅠM]Cl and[C4C1ⅠM]Br(Fig.6(b)).Theπ?values were related to the size of the anion ofⅠLs. With increasing in the size of the anion，the π?values decreased for the usedⅠLs.

The π?values for theseⅠL-water mixtures are also shown in Fig.6(a)and(c).Ⅰt was not surprising that π? parameter was greatly influenced by increasing solutesolvent interactions when the xILwas increased.Ⅰt was noted that the dramatic change of π?values occurred in the xILrange from 0 to 0.2，suggesting that the micro environment of binary mixtures substantially changed due to the molecular interactions betweenⅠL and water.The π?values kept relative steady with increasing xILfrom 0.2 to 1.0.Ⅰt was worthwhile to be noted that binary mixtures of[C4C1ⅠM]OAc/H2O，[C4C2ⅠM]OAc/H2O and[C4C3ⅠM]OAc/H2O showed strong synergistic effects，in which π?values of the mixture is higher than that of either of the pure solvents.This synergistic effect was attributed to the formation of hydrogen-bond complexes，whose properties were different from either of the solvents.Very small levels of synergism were observed in the binary mixtures of[C2C1ⅠM]OAc/H2O，[C4C1ⅠM]Cl/H2O and [C4C1ⅠM]Br/H2O，which was due to strong intramolecular interactions between anions and cations of these ⅠLs.For[C6C1ⅠM]OAc/H2O，[C8C1ⅠM]OAc/H2O and [C4C1ⅠM](C4H9O)2PO2/H2O)systems，small levels of synergism were also found，which was probably due to the very large size of the cations and anions，suggesting that steric hindrance restricts interactions between the ⅠLs and the dye.No synergistic effects were found for the binary mixtures of[C4C1ⅠM]BF4/H2O.This finding was consistent with other reports[23，24].The synergistic behavior in case of some π?scales validated the existence of complex S12(displaying a mixed solvent formed by the interaction of pure solvents 1 and 2)with a higher polarity than pure components have.This phenomenon didn’t necessarily indicate that no S12 existed in other systems，but S12 was not significant.

The ENT，α and β values didn’t present the synergy with varying xIL(Fig.3-Fig.5)，whereas the values showed the synergistic effects for most of binary mixtures(Fig.6).Ⅰt suggested that the synergy weren’t entirely characteristic of the studied mixtures but determined by the systems employed to study the mixtures.

FⅠG.6 The π?parameter(a，c)and?π?values(b，d)for the binary mixtures ofⅠLs and water at 25?C.The maximum error associated with π?of[C4C1ⅠM](C4H9O)2PO2/H2O was≤±0.026，and the details can be found in Table S2 in supplementary materials.

The selection of the systems，for example the polarity indicator，was decisive in determining the nature of the molecular interactions and the extent of synergy occurred.Ⅰn this work，the π?parameters measured using the dye 4-nitroanisole could reflect the synergy.Ⅰt was difficult to speculate about the nature of interactions between the solvent and the dye in this stage.But such difference in synergy in these sameⅠL-water mixtures emphasized the requirements for more extensive studies in the future.

As illustrated in Fig.6(b)and(d)，the π?valuesmeasuredforallthebinarymixturesdidn’t follow the expected ideal behavior.The π?values produced a negative deviation for the binary mixturesof[C6C1ⅠM]OAc/H2O，[C8C1ⅠM]OAc/H2O，[C4C1ⅠM]BF4/H2O，[C4C1ⅠM](C4H9O)2PO2/H2O). For[C6C1ⅠM]OAc/H2Oand[C8C1ⅠM]OAc/H2O，asymmetrical cation and anion of theⅠL came into being clusterization，which was not conducive to deformability.Containing a tetrahedral structure anion group，the deformability of(C4H9O)2PO2-and BF4-is poor，which directly led to a negative deviation of π?values.Ⅰt may be attributed to the specific interactions between theⅠL and water.As shown in Fig.6(b)and (d)，the π?values produced a positive deviation for the binary mixtures due to their high electronegativity anions and sphere microstructure.

As depicted in Fig.3-Fig.6，the maximum deviation to ideality for the solvatochromic parameters can be obtained in the xILrange from 0.1 to 0.3.Ⅰts origin must be some interesting molecular interactions.Thermodynamic studies of imidazolium-basedⅠL-water systems have revealed that cation and counterion seem completely dissociated only below xILof 0.013 to 0.015.At higher xILthan this threshold，ions begin to interact with each other and organize themselves by mutual attraction.At a higher concentration，such as xIL＞0.5 to 0.6，ⅠL molecules form clusters as the pure state，and water molecules interact withⅠL clusters without forming a high hydrogen-bond network among themselves [39].As indicated in Fig.3-Fig.6，the solvatochromic parameters dramatically changed in a range of low xIL(0-0.3)and kept the stable stage in a range of high xIL (0.3-1.0).The solvatochromic parameters seemed to be closely related to the microstructure of the binary mixtures.

E.Preferential solvation model

Considering that indicator dye can be preferentially solvated by either binary mixtures，water orⅠL，the selection must be made with the help of a preferential solvation model.Skwierczynski and Connors[40]proposed a phenomenological model of solvent effects，which were based on a simple two-step solvent exchange according to Eq.(7)and Eq.(8)，to quantitatively describe solventsolvent(the general medium effect)，solvent-solute(the solvation effect)，and solute-solute(the intersolute ef-fect)interactions in aqueous-organic binary mixtures. Bosch et al.developed the model for a wide application successfully[41，42].Nevertheless，this model can’t explain synergetic behavior[43].

As stated previously，allⅠL-water binary mixtures show a non-synergetic effect on the solvatochromic parameters of ENT，α，and β.The generalized model below is applied in this work toⅠL-water binary systems investigated.

TABLEⅠⅠFitting parameters obtained from the preferential solvation models in the selected binary mixtures at 25?C.

where S1 and S2 indicate two pure solvents，and S12 displays a mixed solvent formed by the interaction of solvents 1 and 2.Ⅰrepresents the corresponding RD indicator.Ⅰ(S1)2，Ⅰ(S2)2，Ⅰ(S12)2stand for the RD solvated by S1，S2，and S12 components，respectively.The equilibrium constants for two solvent-exchange processes，f2/1and f12/1，measure the tendency of the probe to be solvated by solvent S2 and“mixed solvent”S12 with reference to solvent S1.

where xs1，xs2，and xs12are the mole fractions of solvents S1，S2，and S12 in the sphere of solvation of the indicator，respectively.x01and x02are the mole fractions of the two solvents in the bulk mixed solvent.

According to this model，Reichardt’s polarity ETof binary mixtures as a function of mole fraction x2can be calculated.

Ⅰn Eq.(11)，a and c are the regression coefficients that minimize the square residuals of ETvalues.For the convenience of calculation，Skwierczynski and Connors[40] assumed ETpolarity of solvent S12 to be the average of ET1and ET2，as presented in Eq.(12).

To understand the unique natures ofⅠL-water at the molecular-level aspect，two parameters，f2/1and f12/1，were measured by fitting the wavenumbers of maximum absorptions of the RD at different compositions of S1 (water)and S2(ⅠLs)to the proposed Eq.(11).The results are listed in TableⅠⅠ.For allⅠL-water binary mixtures，f2/1was generally greater than 1，which reflected that the indicator was more solvated byⅠL rather than water，and the solvation sphere was substantiated with ⅠL.Similarly，f12/1commonly exceeds 1，except for system[C8C1ⅠM]OAc/H2O.f2/1＞f12/1exhibited thatⅠL built a closer solute-solvent interaction with the indicator in comparison with the mixture.Macrocosmically，the difference in the values of f2/1and f12/1demonstrated that the nature of anions and cations played an important role in the preferential solvation study. [C4C1ⅠM]BF4/H2O system，however，was not suitable for this preferential solvation model.

The parameters f2/1and f12/1differed from each other for acetate-basedⅠL-water mixtures，which can affirm that the alkyl substitution on the imidazolium ring greatly affected the solute-solvent interactions of solvation sphere.With increasing alkyl chain of cation on one position，the value of f2/1first increased sharply and then decreased，whereas f12/1decreased continuously.Given that RD dye molecule is large，the phenomenon can be attributed to the steric-hindrance effect and symmetry of the large substitution on one position of the ring.Therefore，[C2C1ⅠM]OAc/H2O system held lower f2/1and higher f12/1than others do，which reflected that“mixed solvent”，combination of water and[C2C1ⅠM]OAc，was easier to form and more active to interact with the indicator than otherⅠL-watermixtures.On the contrary，the parameter f12/1for [C8C1ⅠM]OAc/H2O mixture was inferior to 1，which might be ascribed to the fact that large asymmetric cation putted a brake on the formation of the“mixed solvent”that directly led to a weak solute-solvent interaction.As for substituting proton on three positions，the values of f2/1and f12/1for[C4C3ⅠM]OAc/H2O system were higher than those for the two other systems. Hence，the symmetry of different alkyl group of cation tied the solute-solvent interactions up.From another aspect，the parameters f2/1and f12/1for the systems with butyl substitute on one position of cation were higher than the others.

The preferential parameters f2/1and f12/1for 1-butyl-3-methylimidazolium-basedⅠL-watersystems were determined.The overall values followed f2/1＞f12/1＞1，which reflected that the dye is strongly preferentially solvated by pureⅠLs in comparison with the mixture with reference to water.Danten et al. [44]reported that an almost symmetric structure of anion···H-O-H···anion is formed by hydrogen-bonds inⅠL-water mixtures.When solute was added，similar to the dye indicator，a competition existed between anion and dye indicator，thereby indicating that the solvent-solvent interaction always strongly influenced solute-solvent interaction.For systems that shared the same cations，the observed trend for preferentialsolvationwas[C4C1ⅠM](C4H9O)2PO2/H2O＞[C4C1ⅠM]Cl/H2O＞[C4C1ⅠM]OAc/H2O＞[C4C1ⅠM]Br/ H2O.This trend was closely related to the electronegativity and magnitude of anions.

IV.CONCLUSION

Theempiricalsolvatochromicparameters(ENT，α，β，andπ?)ofthebinarymixturesof1，3-dialkylimidazoliumⅠLs and water were investigated. This result indicated that solvent properties ofⅠL-water mixtures were influenced by both the xILandⅠL type. The behavior of the solvatochromic absorbance indicator obviously deviated from ideality in these mixtures. The maximum deviation to ideality was obtained in the xILrange from 0.1 to 0.3.Net basicity(β-α)for these ⅠL-water mixtures can predict the capacity of dissolving lignin.For most ofⅠL-water mixtures(except for [C4C1ⅠM]BF4/H2O)，the π?parameters were found to be higher than that of pureⅠLs and water instead of other parameters(ENT，α and β).The synergy in π?parameters indicated interesting and strong solute-solvent and solvent-solvent interactions in these studied systems.With the preferential solvation model，the dye probe is preferentially solvated in the following order: pureⅠL＞mixtures＞water.This work will help to understand the physicochemical properties of theseⅠL-water mixtures and facilitate their practical applications.

Supplementary materials:The impurities contents in the usedⅠLs，the measured solvatochromic parameters of the binary mixtures ofⅠLs and water，and comparison between reference data and the measured values in this work are given.

V.ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China(No.21276034).

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Shan Ding，Li-gang Wei?，Kun-lan Li，Ying-chong Ma
School of Light Industry and Chemical Engineering，Dalian Polytechnic University，Dalian 116034，China
(Dated:Received on January 6，2016；Accepted on March 15，2016)
Binary mixtures of 1，3-dialkylimidazolium based ionic liquids(ⅠLs)and water were selected as solvent systems to investigate the solute-solvent and solvent-solvent interactions on the preferential solvation of solvatochromic indicators at 25?C.Empirical solvatochromic parameters，dipolarity/polarizability(π?)，hydrogen-bond donor acidity(α)，hydrogen-bond acceptor basicity(β)，and Reichardt’s polarity parameters(ENT)were measured from the ultraviolet-visible spectral shifts of 4-nitroaniline，4-nitroanisole，and Reichardt’s dye.The solvent properties of theⅠL-water mixtures were found to be influenced byⅠL type andⅠL mole fraction(xIL).All these studied systems showed the non-ideal behavior.The maximum deviation to ideality for the solvatochromic parameters can be obtained in the xILrange from 0.1 to 0.3.For most of the binary mixtures，the π?values showed the synergistic effects instead of the ENT，α and β values.The observed synergy extent was dependent on the studied systems，such as the dye indicator andⅠL type.A preferential solvation model was utilized to gather information on the molecular interactions in the mixtures.The dye indicator was preferentially solvated on the following trend:ⅠL＞ⅠL-water complex＞water.
Key words:Solvatochromic parameters，Preferential solvation，Ⅰonic liquid，Water

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