Micellar reversed phase liquid chromatography (RPLC) and solid state NMR have been used to study the interactions of two ionic surfactants with alkyl and cyanopropyl bonded phase columns. The two surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) are commonly used in micellar RPLC. Differences in selectivity between CTAB and SDS micellar RPLC on alkyl bonded phases can be attributed to the differing nature of the SDS-and CTAB-bonded phase association. For SDS, the association leads to the formation of an anionic hydrophilic surface layer, which would explain the superior resolution achieved by SDS micellar mobile phases for hydrophilic compounds on C-18. For CTAB, the association leads to a more hydrophobic bulk stationary phase, where the nitrogen head group is partially incorporated into the C-18 phase because of strong hydrophobic interactions between the C-18 bonded chains and the methyl nitrogens. The unusual behavior exhibited by cyano bonded phase columns towards ionogenic compounds in SDS and CTAB micellar RPLC can be attributed to strong interactions between the polar head group of the ionic surfactant and the cyano functionality of the polar bonded phase. Models depicting the structure of the surfactant modified C-18 and cyanopropyl bonded stationary phase are proposed from solid state NMR data, and these models are in good agreement with the selectivity exhibited by these bonded phase columns toward polar and ionogenic compounds in micellar RPLC. 290
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