Study on physicochemical properties of hydroxyl-functionalized ionic liquids

Abstract

A thorough understanding of the physicochemical properties of ionic liquids (ILs) is significant for the design and synthesis of the desired ILs. In this study, three hydroxyl-functionalized ILs were designed and synthesized, and their density, viscosity, surface tension and refractive index were measured in the temperature range of 293.15–333.15 K, and it was found that they all decreased with the increase of temperature. By using the measured physicochemical property data and the corresponding empirical equations, the molecular volumes of the three ILs were calculated and the contribution of one (CH2-) group to the molecular volume was found to be 0.0262 nm3. The viscous flow activation properties were discussed and ΔH≠ > ΔS≠T was found for the three ILs, indicating that enthalpy effect is the main source of viscous flow resistance. Surface energy and surface entropy were calculated, and it was found that surface entropy is the main factor determining surface properties. The free volume was calculated, and it was found to increase with temperature, which could provide more space for light to pass through the medium, leading to a decrease in refractive index with temperature. Simultaneously, the influence of cation and anion structures on the properties of the ILs was also discussed.