Thermodynamic characteristics of solute—solvent interactions in liquid organic salt solvents, studied by gas chromatography

Abstract

A number of thermodynamic constants were measured for five McReynolds selectivity probes and a series of aromatic selectivity probes on thirteen tetra- n-butylammonium salts and eleven 4-toluenesulfonate salts to elucidate the nature of intermolecular interactions of solute—liquid organic salts by gas chromatography (GC). The molar free energy of solution for a methylene group was used to determine dispersive contributions of individual cations and anions to retention. The partial molar, mole fraction, and molal free energy of solution and partial mole fraction enthalpy and entropy of solution were determined to estimate the contribution of various intermolecular forces to retention and to compare the magnitude of these interactions to those of non-ionic, polar phases. Weak dispersive interactions increasing with the size of the cation, strong orientation interactions of a relatively constant nature, and proton acceptor interactions that varied greatly and correlated with the properties of the anion were observed for the liquid organic salts. The magnitude of the orientation and proton acceptor interactions in several cases were much larger than equivalent interactions for polar, non-ionic phases. The infinite dilution activity coefficient and partial molar excess free energy of solution were determined for the five McReynolds probes on the tetra- n-butylammonium salts. With the exception of butanol the range of values observed were similar to literature values for polar, non-ionic GC phases.