Influence of temperature and anion type on thermophysical properties of aqueous solutions of morpholine based amino acid ionic liquids

Densities and sound velocities of aqueous solutions of N-butyl-N-methylmorpholine based amino acid ionic liquids (AAILs), including N-acetyl-L-alanine, N-acetyl-L-valinate, N-acetyl-L-leucinate, and N-acetyl-L-izoleucinate anions were measured at a temperature from 293.15 to 313.15 K at 5 K intervals and atmospheric pressure. These data were used to calculate the apparent molar volumes and the apparent molar compressibilities in the concentration range of (0.02 to 0.3) mol.kg(-1). Infinite dilution values for the volumetric and acoustic properties were evaluated by an extrapolation procedure based on Redlich - Meyer type equations. The limiting apparent molar expansibility coefficients were also obtained from the slopes of the apparent molar volume at infinite dilution versus temperature. Using the Passynski method, the hydration numbers for AAILs in an aqueous medium were calculated. The results indicate that in the studied solutions, the ion-solvent interactions are determined by the hydrophobic effect, which results in the decrease of the limiting apparent molar compressibilities and an increase of limiting apparent molar expansibility coefficients in the order: [Mor(1,4)][N-Ac-L-Ala], [Mor(1,4)] [N-Ac-L-Val], [Mor(1,4)][N-Ac-L-Leu], [Mor(1,4)][N-Ac-L-Izoleu]. The empirical slopes in Redlich - Rosenfeld - Meyer equations show that the hydrophobicity of amino acid anions is also responsible for the ion-ion interactions.

Automated summary

This study measured the properties of amino acid ionic liquids in aqueous solutions using density and sound velocity measurements, and calculated related parameters. The results indicate that the interactions between ions and solvents are primarily determined by the hydrophobic effect, and the hydrophobicity of amino acid anions also has an influence on ion-ion interactions.