Selectivity in Micellar Liquid Chromatography: Surfactant Bonded Phase Interactions. II. C-8 and Cyanopropyl

Abstract

Micellar liquid chromatography and solid state ¹³C NMR spectroscopy have been used to study the interactions of three ionic surfactants with C₈ and cyanopropyl bonded phase columns. The three surfactants, sodium dodecylsulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), and dodecyltrimethylammoniurn bromide (DTAB), are commonly used in micellar RPLC. Surfactant adsorption is found to produce distinct changes in the selectivity of the stationary phase. Specifically, the differing nature of the surfactant-bonded phase association is largely responsible for the observed differences in selectivity between SDS, CTAB, and DTAB micellar RPLC. For SDS, the association leads to the formation of an anionic hydrophilic surface layer on C₈ (as well as on C₁₈) which would explain the superior resolution achieved by SDS for hydrophilic compounds. For CTAB. small surfactant aggregates form within the C₈ stationary phase, which would explain the differences in the observed selectivity of CTAB mediated separations on C,₁₈ and C₈ alkyl bonded phases. The observed differences in the selectivity of DTAB and CTAB modified C₈ alkyl bonded phase columns towards hydrophilic aromatic compounds are probably due to the differing nature of the CTAB and DTAB C₈ bonded phase association, which suggests that hydrocarbon chain length is an important factor influencing the adsorptive behavior of these amphiphiles on hydrophilic silica surfaces. The unusual behavior of cyanopropyl bonded phase columns in SDS or CTAB micellar RPLC can be attributed to strong interactions between the polar head group of the surfactant and the cyano group of the polar bonded phase. Chemical models depicting the structure of the surfactant coated C₈ and cyanopropyl stationary phase are proposed from the NMR data, and these models are in good agreement with retention data obtained for these micellar RPLC systems.