Mixtures of the [TMA][EPPS] ionic liquid with methanol, ethanol, or water: thermophysical properties and molecular interactions

Self-buffering and biocompatible ionic liquids (ILs) derived from biological buffers, such as Good's buffers, have been recognized as potential innovative green agents for many practical applications. In order to develop their processing technologies, the thermophysical properties and molecular interactions of the binary systems containing tetramethylammonium 4-(2-hydroxyethyl)-1-piperazinepropanesulfonate ([TMA][EPPS]) with water, methanol or ethanol were investigated in this work. The densities and viscosities of the binary systems were measured at temperatures from 283.15 K to 333.15 K and concentrations from 0.342 mol kg(-1) to 2.048 mol kg(-1) under atmospheric pressure. The thermal expansion coefficients, the apparent molar volumes, and the solubilities of [TMA][EPPS] in water, methanol or ethanol at 298.15 K were also estimated from the experimental results of density measurements. The viscosity data of the ionic liquid solutions were correlated accurately with the Vogel-Fulcher-Tammann (VFT) equation. Moreover, the dissociation process and the solvation behavior of [TMA][EPPS] and its constituent ions in water, methanol, and ethanol were studied by using the COSMO-RS-based quantum calculations. The molecular interactions of the TMA cation, EPPS anion, or [TMA][EPPS] with solvent molecules (water, methanol, and ethanol) were also studied using DFT calculations. As a result, this IL derived from biological buffer has the ability to dissolve in water and organic solvents such as methanol and ethanol. This implies that these kinds of ILs are expected to have wide applications, not only in aqueous solution but also in organic solvent mixtures.