SELECTIVITY IN MICELLAR REVERSED-PHASE LIQUID-CHROMATOGRAPHY - C-18 AND C-8 ALKYL BONDED PHASES

Employing a set of six vanillin compounds as retention probes, the role of the surfactant-modified stationary phase in the micellar liquid chromatographic (MLC) separation process was investigated. Specifically, the effect of surfactant adsorption on retention and selectivity in MLC was evaluated by comparing the retention behavior of the vanillin compounds in different micellar media. Surfactant adsorption was found to produce distinct changes in the selectivity of the stationary phase. In fact, differences in selectivity between cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) micellar reversed-phase liquid chromatography were found to be due to the differing nature of the SDS and CTAB bonded phase association. For SDS, the hydrophobic tail is associated with the bonded alkyl phase, with the sulfate head group oriented away from the alkyl bonded phase surface. Adsorption of SDS in the manner described would lead to the formation of a hydrophilic layer on C-18 and C-8, which would explain the superior resolution achieved with SDS for the vanillin compounds, which are also hydrophilic. For CTAB adsorbed on C-18, the nitrogen head group is probably incorporated partially or wholly into the bonded phase because of hydrophobic interactions between the methyl nitrogens and the bonded phase. Incorporation of CTAB in the manner described would ensure that much of the hydrophobic character of the original bulk phase is retained, which could also explain the longer retention time of the vanillin compounds with CTAB micellar mobile phases.