Study of mixed-mode sorption of a dye on heterogeneous chromatographic interfaces

Single-molecule spectroscopy, fluorescence correlation spectroscopy, atomic force microscopy, and microfluidics were used to probe the heterogeneous adsorption and lateral diffusion of molecules at chromatographic interfaces. This research is the first to report the direct observation of adsorption of molecules to topographical sites existing on chromatographic surfaces. In addition, an analytical expression to the autocorrelation function was derived to accurately model the interaction of molecules with a heterogeneous surface, where both lateral diffusion and rare strong adsorption occur.; Strong adsorption sites having a topographical origin exist on fused silica. Since there is a considerable amount of overlap between the topographical indentations of fused silica and the pore size distribution of silica gel, strong adsorption to silica gel likely has a contribution from topography. Topography had not previously been considered as a factor in the silanol activity of silica. The topographical origin could be the explanation for why endcapped silica retains its high efficiency after a significant amount of stationary phase has been degraded: steric hindrance would be enhanced in nanometer indentations. The topographical origin is consistent with the observation that a homogeneously redeposited silica gel surface has less silanol activity. Efforts to synthesize silica gel having fewer active silanols, or a reproducible number of active silanols, might benefit by addressing the topography of the silica gel surface specifically.; Fluorescence correlation spectroscopy was used to determine that the surface of C₁₈ on silica has at least four types of adsorption sites for C₁₈ DiI in 60% acetonitrile/water. The most prevalent site involves weak adsorption that allows the dye to laterally diffuse. Three specific adsorption sites exist, with desorption times on the order of tens of milliseconds, seconds, and tens of seconds, and these sites have comparable dye concentrations of adsorbed dye. The specific adsorption sites having desorption times longer than one second are clustered at topographical pits on fused silica, where specific adsorption events are observable at a high rate locally. Microfluidics was used to probe the interface of C₈ modified chromatographic silica gel particles. This study of chromatographic silica gel particles agreed with the results obtained on fused silica coverslips.