Abstract
Imidazolium-based chloride ionic liquids (ILs) have exhibited remarkable performance in several important applications such as biomass dissolution and extraction, but their large viscosity is a non-negligible problem. Adding molecular co-solvents into chloride ILs is effective in reducing viscosity; nevertheless, understanding of the accompanied change of thermodynamic polarity is quite few. Therefore, in this work we reported three Kamlet-Taft solvatochromic parameters, including dipolarity/polarizability π*), hydrogen-bond acidity (α) and hydrogen-bond basicity (β), for the binary mixtures of several imidazolium-based chloride ILs plus either dipolar protic solvents (water and methanol) or dipolar aprotic solvents (dimethyl sulfoxide, N,N-dimethylformamide and acetonitrile). The results demonstrated that those parameters could be altered by the structure of IL and type of co-solvent owing to the solute-solvent and solvent-solvent interactions. The structure of alkyl chain of cation had considerable impact on the π* variation of IL aqueous solution against IL concentration but hardly affected other mixtures. Moreover, remarkable preferential solvation of probes was observed for β and α in the mixtures of IL and dipolar aprotic co-solvents, whereas the hydrogen-bond interactions between IL and dipolar protic co-solvent enabled the preferential solvation to be alleviated and resulted in more linear variation of β and α against the molar fraction of IL. The results not only contribute to a better understanding of the effect of co-solvent on imidazolium-based chloride ILs, but also are instructive for improving the thermodynamic performance of IL-based applications via providing IL+co-solvent mixtures with desirable physicochemical properties.
Highlights
Nowadays, ionic liquids (ILs) have drawn a great deal of prevalence in various chemical processes because of their unique properties such as insignificant vapor pressure, wide liquid range, good thermal stability and so on [1,2,3,4]
We investigated three types of imidazolium-based chloride ILs,1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1hexyl-3-methylimidazolium chloride ([Hmim]Cl) and 1allyl-3-methylimidazolium chloride ([Amim]Cl) mixed with dipolar protic solvents or dipolar aprotic solvents
The usage of dipolarity/polarizability (*) was to describe the non-specific parts of van der Waals interaction, such as dispersion, induction and electrostatic interaction [17]. * value could be obtained from the spectroscopic shift of N,N-diethyl-4-nitroaniline (DENA) by Eq (1):
Summary
Ionic liquids (ILs) have drawn a great deal of prevalence in various chemical processes because of their unique properties such as insignificant vapor pressure, wide liquid range, good thermal stability and so on [1,2,3,4]. Imidazolium-based chlorides have better dissolving capacity towards cellulose and lignin than most of other ILs, which facilitates the processing and transformation of biomass in high efficiency [5, 6]. Their high affinity for alcohols and phenolics provides opportunities for azeotrope separations and purification of bioactive substances [7, 8]. Good solubility of metal compounds in ILs enables enhanced catalytic reaction performance to be achieved [9, 10]
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