Thermophysical Properties of Five Acetate-Based Ionic Liquids

Abstract

Ionic liquids (ILs) with improved hydrogen-bonding acceptor abilities, such as acetate-based compounds, have shown great potential for CO 2 capture and biomass dissolution. In this context, the knowledge of the thermophysical properties of acetate-based fluids is essential for the design and scale-up of related processes. However, at this stage, acetate-based ILs are still poorly characterized. In this work, four thermophysical properties, specifically, density, viscosity, refractive index, and surface tension, were determined for five acetate-based ILs. Both protic and aprotic ILs were investigated, namely, N,N-dimethyl-N-ethylammonium acetate, 1-ethylimidazolium acetate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, and 1-butyl-1-methylpyrrolidinium acetate. From the temperature dependence of the measured properties, additional properties, such as the isobaric thermal expansion coefficient, the surface entropy and enthalpy, and the critical temperature, were further estimated. ■ INTRODUCTIONDuring the past decade research has emerged in various areas involving ionic liquids (ILs), and their potential applications are nowadays widespread. Ionic liquids are a group of molten salts normally composed of inorganic or organic anions, and relatively large organic cations, which do not easily form an ordered crystal and, therefore, they remain liquid at or near room temperature. 1−3 Recently, certain classes of ILs with improved hydrogenbonding acceptor capability, such as acetate-based fluids, have shown to be promising solvents for CO 2 capture 4−7 and cellulose and/or biomass dissolution. 8−11 Acetate-based fluids are able to strongly coordinate with CO 2 and hydrogen bond donator groups, such as −OH groups, and favorable results have thus been published. Moreover, acetate-based ILs present low toxicity, low corrosiveness, and favorable biodegradability. 12 Despite their undeniable interest, the thermophysical properties of acetate-based fluids are still poorly characterized. In a previous work we have determined the densities and viscosities of a series of imidazolium-based ILs which have shown potential for the dissolution of biomass. 13 Fendt et al. 14 presented the viscosities of acetate-based ILs and some of their mixtures with water and organic solvents. Qian et al. 15 explored the densities and viscosities of the protic IL 1-methylimidazolium acetate and its binary mixtures with alcohols. Additional scarce reports have addressed the measurements of densities, 16−18 viscosities, 19 refractive index, 16,20 and surface tension 18 of aprotic acetate-based ILs.In this work, the thermophysical properties of acetate-based ILs, specifically density, viscosity, refractive index, and surface tension, were measured as a function of temperature. The ILs are formed by the common anion acetate, combined either with protic (N,N-dimethyl-N-ethylammonium and 1-ethylimidazolium) or aprotic cations (1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium and 1-butyl-1-methylpyrrolydinium). Additio...

Surface tensions of ionic liquids: Non-regular trend along the number of cyano groups

“…As previously observed for other ILs [6,31,[34][35][36]57], imidazolium-based fluids combined with cyanobased anions display low surface enthalpies and entropies when compared with molecular organic compounds. This low surface enthalpies and entropy reflect a high surface organization as well as a highly structured liquid phase.…”

“…As reported in previous works [6,31,[34][35][36]57], the values for critical temperatures of ILs obtained by these approximations must be used with caution. The estimation of critical temperatures using data from a narrow temperature range, and demanding a large extrapolation, are a source of uncertainty.…”

“…The density values required for the calculation of the surface tensions from the drop image data were taken from literature [29]. Further details on the equipment and its validity to measure surface tensions of ILs can be found in previous publications [34][35][36][37]. [38] is a well-known method for a priori prediction of thermophysical properties and phase behavior on the basis of unimolecular quantum chemical calculations.…”

Wettability and corrosion of [NTf2] anion-based ionic liquids on steel and PVD (TiN, CrN, ZrN) coatings

“…Coutinho et al [20,[24][25][26][27][28][29][30][31][32] found that surface tension in a couple of imidazolium-based IL series decreases with an increase in the cation side alkyl chain length up to hexyl and thus defined the CACL (critical alkyl chain length). On the other hand, surface tension remained almost constant for ILs with aliphatic moieties longer than the obtained CACL.…”

New quantum chemistry-based descriptors for better prediction of melting point and viscosity of ionic liquids

“…[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] Once more, a multivariate correlation with several quantum chemistry descriptors along with temperature as input parameters, and ln (viscosity), as the output parameter, was developed using Eureqa software.…”