[Competitive binding of tetrabutylammonium and cetylpyridinium cations on C18 surfaces]

The adsorption behavior of tetrabutilammonium (TBA) and cetylpyridinium (CP) cations was studied on Hypersil 5-ODS sorbent. After establishing, that CP has about 5 fold stronger affinity to the C18 surface, than the structurally different, symmetric, spheric molecule of TBA, the adsorption from their common solution was also examined. The data of "competitive binding" seem to show, that TBA should have certain selective binding sites on the C18 surface, from which, it can not be displaced by the CP even in the case the latter is present in a 4 fold excess in the mobile phase. The maximal coverage was found 52.9% (CP) and 12% (TBA) i.e. every second of C18 groups is covered by CP cations and only 1/8th of C18 groups may interact with TBA. The accessible, proton releasing silanol content, related to the total C18 content of the column, was found very low.