Surface diffusion in C(18)-silica monolithic stationary phase

A peak parking-moment analysis method was used for the measurement of surface diffusion coefficient (D(s)) in a reversed-phase liquid chromatography (RPLC) system consisting of a C(18)-silica monolithic column and a mixture of methanol and water (70/30, v/v). The D(s) values experimentally measured were analyzed by considering the correlation with corresponding values of molecular diffusivity (D(m)) and the retention equilibrium constant (K(a)). It seems that the correlation between D(s)/D(m) and K(a) is represented by a single curve irrespective of the RPLC conditions of temperature and the type of sample compounds. The increase in K(a) is accompanied with the decrease in D(s)/D(m). Oppositely, the ratio of D(s)/D(m) increases and approaches around unity when K(a) infinitely decreases. It seems that surface diffusion is originally similar to molecular diffusion and that it is restricted due to the sample retention. These characteristics of surface diffusion are the same between the C(18)-silica monolithic stationary phase and the conventional C(18)-silica gel particles. In addition, the values of K(a) and D(s) are also comparable between them. It is concluded that basic properties concerning the retention equilibrium and surface diffusion of the C(18)-silica monolithic stationary phase are almost the same as those of the conventional C(18)-silica gel particles in spite of the difference between their structural characteristics.