SELECTIVITY IN MICELLAR LIQUID-CHROMATOGRAPHY - SURFACTANT BONDED PHASE INTERACTIONS .2. C-8 AND CYANOPROPYL

Micellar liquid chromatography and solid state C-13 NMR spectroscopy h ave been used to study the interactions of three ionic surfactants wit h C-8 and cyanopropyl bonded phase columns. The three surfactants, sod ium 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 t he selectivity of the stationary phase. Specifically, the differing na ture of the surfactant-bonded phase association is largely responsible for the observed differences in selectivity between SDS, CTAB, and DT AB micellar RPLC. For SDS, the association leads to the formation of a n anionic hydrophilic surface layer on C-8 (as well as on C-18) which would explain the superior resolution achieved by SDS for hydrophilic compounds. For CTAB, small surfactant aggregates form within the C-8 s tationary phase, which would explain the differences in the observed s electivity of CTAB mediated separations on C-18 and C-8 alkyl bonded p hases. The observed differences in the selectivity of DTAB and CTAB mo dified C-8 alkyl bonded phase columns towards hydrophilic aromatic com pounds are probably due to the differing nature of the CTAB and DTAB C -8 bonded phase association, which suggests that hydrocarbon chain len gth is an important factor influencing the adsorptive behavior of thes e amphiphiles on hydrophilic silica surfaces. The unusual behavior of cyanopropyl bonded phase columns in SDS or CTAB micellar RPLC can be a ttributed to strong interactions between the polar head group of the s urfactant and the cyano group of the polar bonded phase. Chemical mode ls depicting the structure of the surfactant coated C-8 and cyanopropy l stationary phase are proposed from the NMR data, and these models ar e in good agreement with retention data obtained for these micellar RP LC systems.