Thermophysical properties of trioctylalkylammonium bis(salicylato)borate ionic liquids: Effect of alkyl chain length

Thermophysical properties of halogen-free ionic liquids are gaining significant attention for industrial applications. In this context, trioctylalkylammonium bis(salicylato)borate (N888n-BScB) ionic liquids having variable alkyl chain length (n = 4, 8, 12, 16) are synthesized to explore the effect of chain length and temperature on their physicochemical properties. The density (ρ), refractive index (nD), and speed of sound (u) are measured in the temperature range of 293.15 to 333.15 K. The tetraoctylammonium-BScB is found to be highly dense ionic liquid, and it was attributed to the closer packing of symmetric structure of tetraoctylammonium cation. Furthermore, derived thermodynamic properties such as isentropic compressibility (βs), coefficient of thermal expansion (α), standard entropy (S°), intermolecular free length (Lf) and lattice energy (UPOT) are calculated for the N888n-BScB ionic liquids using the experimental data and shown the effect of variable chain length. This study provides a comprehensive insight on the thermophysical properties of halogen-free ionic liquids. [Display omitted] •Trioctylalkylammonium bis(salicylato)borate ILs are synthesized for evaluation of their physicochemical properties•Density, speed of sound, and refractive index are studied as functions alkyl chain length in the cation and temperature•Symmetric structure of tetraoctylammonium cation provided closer packing, consequently the higher density•Molecular volume of ionic liquids and alkyl chain length in the cation exhibited a linear correlation