Spectroscopic Observation of Adsorption to Active Silanols

Fluorescence microscopy was used to investigate the origin of strong adsorption sites on silica. Fluorescence from a cationic dye, 1,1‘-dioctadecyl-3,3,3‘,3‘-tetramethylindocarbocyanine perchlorate, was imaged at the chemical interface between fused silica modified with chlorodimethyloctadecylsilane and water or acetonitrile. The fluorescence images reveal that the dye adsorbs strongly at lines and points on the surface. Comparison of an atomic force micrograph and a fluorescence micrograph of the same region show the same features, indicating that strong adsorption occurs at nanometer topographical indentations. Fluorescence recovery after photobleaching shows that recovery occurs through repopulation of the same adsorption sites. The rate of adsorption to the sites was found to be much slower for water than for acetonitrile as the mobile phase. End capping greatly reduces the number of strong adsorption sites, which indicates that exposed silica is the origin of irreversible adsorption. Similar but brighter patterns of strong adsorption appear when water is replaced with acetonitrile, and the patterns remain obvious even at surface concentrations approaching those used in HPLC. The adsorption behavior is the same for pH 7 and pH 2, showing that these topographical indentations behave as the putative “active silanols”. Since the size distribution of the nanometer polishing marks overlap the pore size distribution of chromatographic silica gel, these results suggest that surface topography is a previously unrecognized factor in the tailing of organic bases.