Spectroscopic observation of adsorption to active silanols

Fluorescence microscopy was used to investigate the origin of strong adsorp tion 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 ads orbs strongly at lines and points on the surface. Comparison of an atomic f orce micrograph and a fluorescence micrograph of the same region show the s ame features, indicating that strong adsorption occurs at nanometer topogra phical 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 adsorp tion appear when water is replaced with acetonitrile, and the patterns rema in obvious even at surface concentrations approaching those used in HPLC. T he adsorption behavior is the same for pH 7 and pH 2, showing that these to pographical indentations behave as the putative "active silanols", Since th e size distribution of the nanometer polishing marks overlap the pore size distribution of chromatographic silica gel, these results suggest that surf ace topography is a previously unrecognized factor in the tailing of organi c bases.