Microscopic Probing of Silica Surfaces

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In this project, Mary J. Wirth of the University of Delaware is supported by a renewal grant in the Analytical and Surface Chemistry Program. She will study the adsorption of organic cations to silica-based chromatographic surfaces in order to understand the physical phenomena responsible for zone tailing in chromatographic separations which is a long-standing problem hindering separation of a large number of important compounds such as proteins and pharmaceuticals. Atomic force microscopy will be used to measure surface charge density over a wide pH range. Single-molecule detection of an organic cation, rhodamine 123, will be used to probe lateral diffusion and desorption rates as a function of surface charge density, which is controlled by pH. Stationary phases will be synthesized with varying silanol concentration, monolayer coverage and monolayer hydrophobicity, along with horizontally polymerized monlayers to reduce surface charge density. Adsoption isotherms will be generated from data obtained by elution of rhodamine 123 on nonporous silica gel and tested by performing the single-molecule and atomic force microscopy experiments on the same material. In this work, physical and chemical factors responsible for the currently inefficient chromatography of important classes of compounds such as pharmaceuticals and proteins will be investigated. The experiments to be done are multidisciplinary in nature, bringing together ideas from optical spectroscopy, self-assembled monolayer research, scanning probe microscopy and chemical separations. The practical impact of the work will result in faster more complete separations for analysis and preparative purposes.