The silica-gel surface and its interactions with solvent and solute in liquid chromatography

Abstract

The multilayer formation of water on silica gel is discussed, and experiments indicating the existence of three layers of water are described. The interaction of active silica gel with solvent is also considered. Activated silica gel as used in chromatography appears to contain one strongly hydrogen-bonded water molecule per silanol group which can dispersively interact with non-polar solvents forming a monolayer. In contact with a polar solvent, however, the hydrated silanol group can hydrogen-bond to the polar solvent and form a strongly held solvent layer on top of which a bilayer of polar solvent can form by polar interactions with the first layer. The mechanism of solute interaction with activated silica can be described as follows. Solutes interact with the multilayer surface in a chromatographic column in two ways. If the solvent layer is weakly held by dispersive forces as in the case of a non-polar solvent, the solute can displace the solvent layer and interact directly with the hydrated hydroxyl groups. If, on the other hand, the solvent is polar and is strongly held by hydrogen-bonding forces to the hydrated silanol group, solutes may associate directly with the polar solvent layer but not displace it unless the solute has a polarity similar to the solvent in which case it is consequently eluted at a high k′ value.

At low concentrations of polar solvent only a small amount of the second layer of weakly held solvent is formed and thus the interaction of a solute with the surface will be with the primary layer of polar solvent. Under such circumstances, changes in retention resulting from changes in solvent composition will reflect changes in solute interactions with the mobile phase and not with the surface of the stationary phase. Such a system has been examined; it was shown that the probability of polar interactions in the mobile phase were directly related to the concentration of polar solvent, and this was substantiated by results obtained from the examination of liquid/liquid distribution systems. Evidence was also provided that indicated that the magnitude of polar interactions was related to the polarizability per cm³ of the interacting substances.