Thermodynamic studies of a zwitterionic stationary phase in hydrophilic interaction liquid chromatography

A zwitterionic 3-P,P-diphenylphosphonium-propylsulfate stationary phase called HZI was synthesized for hydrophilic interaction liquid chromatography (HILIC). A set of 15 solutes including apolar aromatic compounds, positively charged β-blockers, polar nucleic acid bases and negatively charged compounds was used to prepare van't Hoff plots between 10 and 70°C with HILIC mobile phases containing between 10 and 20% v/v 20mM or 50mM ammonium acetate buffer (pH 4.1) in acetonitrile. The unique Π-Π interaction capability of the HZI phase is evidenced by the low but significant retention of the apolar aromatic compounds in the HILIC mode as well as by the strong retention of the aromatic containing β-blockers. The linear van't Hoff plots gave the ΔH° and ΔS° thermodynamic changes of the solute transfer from the mobile to the stationary phase. Plotting the ΔH° values versus the corresponding ΔS° ones produced a line excluding the three apolar aromatic solutes and a negatively charged compound. The excluded compounds corresponded to curved van't Hoff plots. If the non-linearity of van't Hoff plots is classically explained by changes in solute transfer interactions, we showed that it could also be explained by a temperature induced change in the hydrated stationary phase volume seen by the solutes.