Understanding ion–ion and ion–solvent interactions in aqueous solutions of morpholinium ionic liquids with N-acetyl-L-alaninate anion through partial molar properties and molecular dynamics simulations

Abstract

Amino acid ionic liquids (AAILs) provide a low toxicity, biodegradable alternative to conventional ionic liquids, while also maintaining solubility in water. Densities and sound velocities of aqueous solutions of four amino acid ionic liquids (AAILs), based on the N-alkyl-N-methylmorpholinium ([Mor1,R], R = 2, 3, 6, 8) cation and N-acetyl-L-alaninate ([N-Ac-L-Ala]) anion were measured at T = (293.15–313.15) K and at atmospheric pressure. These data were used to derive the apparent molar volumes and the apparent molar compressibilities in the concentration range of (0.02–0.3) mol kg−1. Established extrapolation procedures were applied to estimate the values of the limiting apparent molar volumes and the limiting apparent molar compressibilities. The limiting apparent molar expansibility coefficients were obtained and the hydration numbers for AAILs in aqueous solution were calculated using Passynski’s method. The results indicate that ion–solvent interactions in the studied solutions are determined by the hydrophobic effect and the charge effect, which are both dependent on temperature. At low temperatures the ion–solvent interactions are determined by the hydrophobic effect which results in the decrease of the limiting apparent molar compressibilities with an elongation of the alkyl chain of the cation of AAIL. At high temperatures the charge effect is dominant due to the decreasing limiting apparent molar compressibilities. On the other hand, ion–ion interactions are dominated by the hydrophobic effect regardless of the temperature. Molecular dynamics simulations were also performed and applied in interpretation of experimental data at atomistic detail. Computed limiting apparent molar volumes correlate well with the values derived from experiment. The apparent hydration volume, obtained by subtracting the intrinsic volume contribution, is large and positive, indicating hydrophobic hydration and not electrostriction is the principal mode of ions’ influence on the solvent. Analysis of radial distribution functions confirms insignificant share of ion–ion interactions via contact ion pairing. The solvation shells signify predominantly hydrophobic hydration, with locally strongly hydrated functional groups.