Molecular-Level Interactions in Binary Mixtures of 1-Ethyl-3-methylimidazolium Ethylsulfate and Propane-1,2-diol: The Interplay between Intermolecular and Intramolecular Hydrogen Bonding.

In this study, volumetric properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate ([C 2 mim]C 2 H 5 SO 4 ), propane-1,2-diol, and their binary mixtures were studied by measurements of density and viscosity. The excess molar volume ( V m E ), dynamic viscosity deviation (Δη), and excess molar Gibbs free energy of activation for viscous flow (Δ G m * ) were calculated and fitted with the Redlich-Kister (RK) type polynomial equation. The results suggested that intermolecular interactions are weaker in the mixture compared to the pure components and the interactions decrease with increasing mole fraction of [C 2 mim]C 2 H 5 SO 4 . The thermodynamic activation parameters were also calculated from the Eyring equation, and their variations with mole fraction of [C 2 mim]C 2 H 5 SO 4 were correlated to the molecular-level interactions. The near-infrared (NIR) spectroscopic measurements were carried out in the temperature range from 293.15 to 333.15 K. The raw NIR data were analyzed further by two-dimensional correlation spectroscopy and principal component analysis. When [C 2 mim]C 2 H 5 SO 4 was introduced to the propane-1,2-diol system, the stronger intermolecular hydrogen bonds were destroyed. Propane-1,2-diol and [C 2 mim]C 2 H 5 SO 4 produce some weaker hydrogen bonds, but the effect of breaking hydrogen bonds predominates. On the basis of volumetric and NIR spectroscopic investigations, molecular-level interactions are predicted. The interplay between intermolecular and intramolecular hydrogen bonding decides unique molecular-level interactions and dictates enhanced thermodynamic properties of the binary mixtures to make them tunable for a multitude of applications.