Selectivity in micellar liquid chromatography: Surfactant bonded phase interactions .1. C-18

Micellar liquid chromatography and solid state C-13 NMR spectroscopy have been used to study the interactions of three ionic surfactants with the C-18 alkyl bonded phase. The three surfactants, sodium dodecylsulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), and dodecyltrimethylammonium 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 monomer-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 C-18 which would explain the superior resolution achieved by SDS for hydrophilic compounds. For CTAB or DTAB adsorbed on Cls, the nitrogen head group is probably incorporated (at least partially) in the bonded phase due to hydrophobic interactions between the methyl nitrogens and the C-18 alkyl bonded phase. Chemical models depicting the structure of the surfactant coated C-18 stationary phase are proposed from the NMR data, and these models are in good agreement with retention data obtained for these micellar RPLC systems.