Studies on Behaviors of Interactions between Zwitterionic Surfactants and Inorganic Ions by Using an Ion Chromatographic Technique

An ion chromatographic method was proposed for studying the interactions between zwitterionic micelles and inorganic ions. Pseudomicelles obtained by adsorption of zwitterionic surfactants on octadecylsilica (ODS) were used as the stationary phase; inorganic ions, namely, the targeted ions, were used as the sample; and pure water or the electrolyte solution was used as the mobile phase. The zwitterionic surfactants, used as the stationary phases in this study, were N -dodecyl- N, N -dimethyl-3-ammonio-1-propanesulfonate (C 17 H 37 NO 3 S) and N -dodecylphosphocholine (C 17 H 38 NO 4 P). The inorganic ions, used as the targeted ions, in this study were, SO 4 2 -, Cl -, NO 2 -, Br -, NO 3 -, ClO 3 -, I -, SCN -, Na +, K +, NH 4 +, Mg 2+, Ca 2+, and Ba 2+ . The circumstantial results of separation of these targeted ions were found to be strongly relative to the patterns of the stationary-phase/mobile-phase combinations. C 17 H 37 NO 3 S/water and C 17 H 38 NO 4 P/water each separate both cations and anions (via the separation of ion combinations). C 17 H 37 NO 3 S/electrolyte separates only the anions, while C 17 H 38 NO 4 P/electrolyte separates only the cations. These results strongly support the conclusion that the zwitterionic micelles interact with both the cations and anions and the adsorption (attraction) of the cations/anions has diminished or even reversed the polarities of the pseudomicelles. The author has proposed a binary electrical double layer model and theorizes on formation of “temporary ion pairs” to explain the possible mechanism.