Activity coefficients, interfacial tensions and retention in reversed-phase liquid chormatography on LiChrosorb RP-18 with methanol-water mixtures

Abstract

Literature data on activity coefficients of various solutes in water, of some tetraalkyl compounds in methanol-water mixture and of water in organic solvents have been correlated with the product of the molecular surface area of the solute and the solute-solvent interfacial tension at ambient temperature. The conditions for which this relationship holds are examined.

The retentions of apolar solutes have been measured on LiChrosorb RP-18 using methanol-water mixtures as eluents at 25°C. The results are discussed in terms of a monolayer adsorption model (according to Locke-Everett) and in terms of the adsorption model based on the solvophobic interaction theory. The important role of solute activity coefficients in the eluent on solute retention is shown quantitatively. The affinity of the adsorbent towards solutes is shown to be a result of apolar interactions in the RP-18 phase and of polar interactions in the interfacial layer of adsorbed methanol. The influence of both contributions is illustrated.