Identification of the factors affecting the retention of weak acid solutes in hybrid micellar systems with cetyltrimethylammonium bromide

Abstract

Retention behavior of ionized and unionized forms of ten weak acid solutes was investigated in hybrid micellar systems with varying cetyltrimethylammonium bromide concentrations (CTAB), acetonitrile (ACN) content and mobile phase pH range from 2.5 to 7.5. It was shown that observed retention patterns are significantly affected by CTAB concentration, ACN volume fraction and analytes' ionization state. The rationale and quantitative description for observed retention behavior was obtained by fitting k values to retention model and calculation of binding constants for both ionized and unionized species. The importance of interaction between analyte anions and positively charged CTAB molecules adsorbed onto stationary phase surface was confirmed. For all solutes, except salicylic acid and furosemide, binding to stationary phase is reduced upon ACN volume fraction increase since the solvent partially desorbs the adsorbed surfactant. Also, the interactions of analytes with micelles are diminished with increase in ACN content because it promotes decrease in micelle aggregation number and its capacity for analyte binding. The additional analysis to identify and interpret molecular descriptors which can best explain the experimental evidence and findings obtained was performed. These findings could provide a new and specific understanding of the interactions between all the components of the hybrid micellar chromatographic system and a good basis for further investigations suggesting the development of generally applicable predictors in structure-retention relationship studies in related chromatographic systems.