Interfacial tension effects of nonionic surfactants in reversed-phase high performance liquid chromatography
A quantitative thermodynamic model is derived to describe the retention behavior of an injected solute as a function of an added amphipathic modifier in the eluent of a reversed-phase liquid chromatographic system. In this model, the addition of surfactant to the eluent can influence the retention time of the chromatographic solute in two ways: by decreasing the interfacial tension; and, for charged surfactants, through electrostatic interactions between the charged solute and the charged surfactant in the adsorbed phase. The retention phenomena of zwitterionic, negatively charged, and positively charged compounds on a C18 column was studied using aqueous solutions containing different concentrations of nonionic and partially ionized surfactants. The results of this study indicate that addition of surfactant to the eluent reduces the interfacial tension and thus decreases the retention time of the injected solutes. When the surfactant is partially charged, there are additional strong interactions between the surfactant and charged injected solutes in the adsorbed phase. When the surfactant is uncharged, there is almost no interaction between the surfactant and charged and uncharged injected solutes in the adsorbed phase.