A comparative study on the interactions of [bmim][NTf₂] ionic liquid with selected four- to seven-membered-ring lactones

Highlights: • Physico-chemical properties of potential solvents for Li-ion batteries were studied. • Nature of interactions between studied [bmim][NTf₂] and lactone were discussed. • Volumetric and viscosimetric properties depend on the ring size of the lactone. • Diluted solutions of studied IL system requires less energy to begin viscous flow. - Abstract: Experimental densities and viscosities of binary mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid, [bmim][NTf₂], with selected lactones, namely: β-butyrolactone (BBL), γ-valerolactone (GVL), δ-valerolactone (DVL) and ε-caprolactone (ECL), are measured at atmospheric pressure (p = 0.1 MPa) in the temperature range from T = (293.15 to 323.15) K over the whole composition range. From the experimental density and viscosity results, we data calculated excess parameters and fitted using Redlich–Kister's polynomial equation. The thermal expansion coefficients are also derived from the acquired experimental density values. Derived density and viscosity are compared with available literature data for the corresponding {[bmim][NTf₂] + GBL (γ-butyrolactone)} binary mixtures. Volumetric and viscometric parameters indicate different interactions and geometrical fitting of the lactone molecule with ionic liquid depending on the number of carbon atoms of the lactone. Thermodynamic parameters of viscous flow are also calculated. The influence of the molecular solvent structure and effects of the lactones properties on the nature of interactions in the studied binary mixtures with [bmim][NTf₂] are discussed from the obtained volumetric and viscosity values and also density functional theory calculations provided in this work.