Retention Behavior Modelization of Monoprotic and Diprotic Species in a Hydroorganic Acetonitrile/Water Mixture

The retention of ionizable solutes in reversed-phase liquid chromatography (RPLC) was predicted using retention equations for the monoprotic and diprotic species. In RPLC, with acetonitrile (ACN)/water mobile phases, the influence of pH and ACN cluster fraction can be quantitatively described by two general equations. The cluster solute solvation energies in an ACN/water mixture over a 0.50−0.80 water fraction were calculated. The energetics of the ACN cluster exchange process in the mobile phase was investigated in relation to the difference in pK a (ΔpK a) between a solute used as reference and the other solutes. A linear correlation was found between the Gibbs free energy change of the solvent exchange process and ΔpK a confirming that the solute solvation by ACN clusters was enhanced for the lesser polar solutes.