1997 Volume 30 Issue 6 Pages 1047-1053
Adsorption characteristics of organic compounds on an octadecylsilyl (ODS)-silica gel for two mobile phases, i.e, methanol/water (70/30, v/v) and acetonitrile/water (70/30, v/v) are compared. It is confirmed that the values of adsorption equilibrium constant, isosteric heat of adsorption, and activation energy of surface diffusion are smaller in 70 vol % acetonitrile than in 70 vol % methanol. On the contrary, the values of surface diffusion coefficient, Ds, are larger in 70 vol % acetonitrile than in 70 vol % methanol. These results indicated that the interaction between ODS ligands and solute molecules was weaker in 70 vol % acetonitrile than in 70 vol % methanol. The ratio of Ds to molecular diffusivity, Dm, is about 0.3 irrespective of chromatographic conditions, i.e., the type of organic modifiers and sample compounds, and temperature. It is possible to calculate Ds from Dm, which can be estimated by various correlations previously proposed in the literature. The almost constant ratio of Ds to Dm suggests the presence of a kind of correlation between surface and molecular diffusions.
