SPECTROSCOPIC STUDY OF THE MOLECULAR-BASIS OF WETTING OF A C-18 SURFACE BY LONG-CHAIN N-ALCOHOLS

Wetting of a chromatographic surface by long-chain n-alcohols was studied on silica plates derivatized with C-18 at chromatographic densities. Contact angle and surface tension measurements were used to determine the amount of wetting, FT-IR was used to ensure that the adsorbed alcohol layer was a submonolayer, and frequency-domain fluorescence spectroscopy was used to study the reorientation behavior of a dilute fluorescent probe in the C-18 monolayer. The experimental results reveal that the long-chain alcohols adsorb in a submonolayer, interpenetrating with the chains of the C-18 monolayer. The data show that this adsorption causes the C-18 chains to tilt toward the surface normal and become more ordered. In contrast to short-chain alcohols, which wet the interface but do not greatly affect the underlying C-18 monolayer, long-chain alcohols wet the interface but significantly affect the underlying C-18 monolayer.