LATERAL DIFFUSION OF AN ADSORBATE AT A CHROMATOGRAPHIC C-18 WATER INTERFACE

The first measurement of the lateral diffusion coefficient of an adsorbate on a chromatographic surface is reported. Pattern-photobleaching/fluorescence-recovery was used to measure the lateral diffusion coefficient of acridine orange at the interface of water and a C-18 chromatographic surface. Also, the diffusion coefficient of acridine orange in built water was measured by the relaxation of a transient holographic grating. The combined results show that the lateral diffusion coefficient of acridine orange at the water/C-18 interface is 35 times smaller than its diffusion coefficient in bulk water: 1.3 (+/-0.1) x 10(-7) cm(2)/s for the chromatographic interface vs 4.2 (+/-0.5) X 10(-6) cm(2)/s for bulk water. The slow diffusion at the water/C-18 surface is interesting in light of previous measurements showing that acridine orange reorients rapidly in the plane of the interface, with an apparent microviscosity comparable to that of bulk water. The rotational and translational diffusion behaviors; suggest that the adsorbed acridine orange occupies favored sites at the water/C-18 interface which allow it to reorient freely, but these sites exchange slowly due to the slow dynamics of the C-18 chains.