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

The first measurement of the lateral diffusion coefficient of an adsor bate on a chromatographic surface is reported. Pattern-photobleaching/ fluorescence-recovery was used to measure the lateral diffusion coeffi cient of acridine orange at the interface of water and a C-18 chromato graphic 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 coeffic ient of acridine orange at the water/C-18 interface is 35 times smalle r 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 i nteresting in light of previous measurements showing that acridine ora nge reorients rapidly in the plane of the interface, with an apparent microviscosity comparable to that of bulk water. The rotational and tr anslational diffusion behaviors; suggest that the adsorbed acridine or ange occupies favored sites at the water/C-18 interface which allow it to reorient freely, but these sites exchange slowly due to the slow d ynamics of the C-18 chains.