Estimation of Surface Diffusion Coefficient in Reversed-Phase Liquid Chromatography Using Methanol/Water Mobile Phases

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An estimation procedure for the surface diffusion coefficient (Ds) is proposed for a reversed-phase liquid chromatographic system using an octadecylsilyl-silica gel and methanol/water mixtures. The procedure is based on a restricted molecular diffusion model for surface diffusion, in which Ds is correlated with molecular diffusivity by introducing restriction energy (Er) due to an adsorptive interaction between adsorbates and adsorbents. The ratio of Er to isosteric heat of adsorption (Qst) is linearly correlated with 1/(–Qst). It is indicated that the linear correlation between γ (=Er/(–Qst)) and 1/(–Qst) can be observed when a linear free-energy relation and an enthalpy-entropy compensation effect in adsorption equilibrium are established. Though both Er and γ are empirical parameters, Ds at given temperatures and mobile phase compositions can be consistently calculated with an error less than about 20% from only one datum of adsorption equilibrium constant.
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