Influence of temperature and alkyl chain length on physicochemical properties of trihexyl- and trioctylammonium based protic ionic liquids

Abstract

A series of trialkylammonium cation and carboxylate anion based room temperature protic ionic liquids (PILs) were synthesized by varying cation alkyl chain length to hexyl, octyl and anion chain length to butanoate, hexanoate, and octanoate. The corresponding ILs are named as trihexylammonium butanoate ([THA][But]), trihexylammonium hexanoate ([THA][Hex]), trihexylammonium octonoate ([THA][Oct]), trioctylammonium butanoate ([TOA][But]), trioctylammonium hexanoate ([TOA][Hex]), and trioctylammonium octonoate ([TOA][Oct]). The thermophysical properties such as density (ρ), speed of sound (u), viscosity (η) and refractive index (nD) were measured over a wide temperature range. The calculated parameters from experimental data such as thermal expansion coefficient, isentropic compressibility decreased linearly with the number of carbon atoms in cation or anion, whereas they increased with the temperature. Thermogravimetric analysis revealed an increase in the thermal stability of the ILs with the increase in total chain length. The shear-dependent viscosity measurements suggest that all the studied PILs showed fluid nature and Newtonian behavior at 298.15 K. The effect of alkyl chain length of cation or anion and temperature on the physicochemical properties were discussed with respect to the structure of ILs and in correlation with the reported structurally similar ILs.