Peak Parking-Moment Analysis Method for the Measurement of Surface Diffusion Coefficients

The peak parking (PP)-moment analysis (MA) method was applied to the measurement of surface diffusion coefficients (Ds) in reversed-phase liquid chromatography using a C18-silica gel and an aqueous solution of methanol (70 vol%). The values of Ds measured by the PP-MA method were in agreement with those obtained by the pulse response (PR)-MA method, which is one of powerful strategies for the quantitative study on the mass transfer kinetics in columns and stationary phases. It was demonstrated that the PP-MA method is an alternative for the experimental measurement of Ds. The mechanism of surface diffusion was considered on the basis of molecular diffusion by introducing a restriction energy (Er) for surface diffusion. Physical meanings of Er were discussed by analyzing the correlation between the enthalpy change due to sample retention (Qst) and Ds. It seems that surface diffusion of a sample molecule is restricted by its retention strength and by the solvent structure formation due to the hydrophobic solvation of the stationary phase surface.