Doctor of Philosophy
Sarah C. Rutan
Interactions among the stationary phase, the mobile phase, and the solute in liquid chromatography' have been studied. A strong dependence of the stationary phase properties on the mobile phase composition may arise from their interactions.
The solvatochromic comparison method, which can give estimates for the dipolarity-polarizability, the hydrogen-bonding acidity, and the hydrogen-bonding basicity of a solvent, represented by the solvatochromic parameters π+, α and β, respectively, was used to study the surface properties of silica in the presence of n-hexane-chloroform or n-hexane-ethyl ether mixtures. A high dipolarity- polarizability, a high hydrogen-bonding acidity, and a low hydrogen-bonding basicity were obtained for silica in n-hexane. The π+ and α values for silica were not affected by the addition of chloroform into n-hexane. The hydrogen-bonding basicity of silica, however, decreased with increasing amounts of chloroform. The π+value for silica decreased with increasing contents of ethyl ether. The decrease in π+ and β values for silica may have resulted from the competition between the polar solvent and the solvatochromic dyes for the strong adsorption sites on silica.
Electronic absorption spectra of a solvatochromic dye, N,N-dimethyl-4-nitroaniline, in n-hexane-ethyl ether mixtures were used to elucidate solute-solvent interactions. Target factor analysis indicated that solute-solvent interactions in a binary solvent can only be reproduced using three or four significant factors, instead of a linear combination of solute-solvent interactions in the two pure components, which is probably caused by the existence of various microenvironments in mixtures of n-hexane and ethyl ether.
Optical transmittance measurements were established to study the wetting of alkyl bonded silicas in organic solvent-water mixtures. When the percent organic solvent content in the eluent, ρ, was high, alkyl bonded silica was wetted and well solvated. With decreasing ρ, alkyl bonded silica became less solvated. When m was lower than the nonwetting limit (less than 10% of organic solvent), alkyl bonded silica became nonwetted. With increasing ρ, a nonwetted alkyl bonded silica remains nonwetted until ρ reaches the rewetting limit (around 60%), which is much higher than the nonwetting limit. Wetting hysteresis can be observed clearly in optical transmittance measurements. Slow kinetics may be a prerequisite for the appearance of wetting hysteresis. Different alkyl bonded phases have different wetting behaviors. Much longer column equilibration time is required if the stationary phase is nonwetted or not well solvated. Both the surface dipolarity and solute retention are affected by' the wetting' of the stationary phase.
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