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

[Image: see text] 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 (ΔG(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.