A kinetic parameter concerning mass transfer in silica monolithic and particulate stationary phases measured by the peak‐parking and slow‐elution methods

Mass transfer in monolithic C 18 ‐silica stationary phases and C 18 ‐silica gel particles was studied. A traditional kinetic parameter, γ s D s , which is a diffusion coefficient of solute molecules in the stationary phase, was measured by two unusual approaches, i. e. , peak‐parking and slow‐elution methods. The correlation between the ratio of γ s D s to molecular diffusivity ( D m ) and the retention factor ( k ) was represented by one common curve, irrespective of the RPLC conditions. A similar curved profile was also observed between another kinetic parameter ( D Ls ), which is related to the axial diffusive molecular migration in the stationary phase, and the retention equilibrium constant ( K a ). The values of D Ls and K a were calculated from those of γ s D s and k , respectively. The ratio of D Ls / D m increases with decreasing K a and seems to approach around unity when K a is infinitely small. The dependence of D Ls on K a was also studied from extra‐thermodynamic points of view. The linear correlation between ln D Ls and ln K a suggests the existence of a kind of linear free energy relationship between the mass transfer in the stationary phase and the retention equilibrium. Because these characteristics of D Ls are similar to those of the surface diffusion coefficient ( D sur ), D Ls seems to correspond to D sur .