LATERAL DIFFUSION OF AN ADSORBATE AT CHROMATOGRAPHIC OCTADECYLSILOXANE WATER INTERFACES OF VARYING HYDROCARBON DENSITY/

The lateral diffusion of acridine orange was investigated at the inter face of water and octadecylsiloxane (C-18) monolayers of varying hydro carbon density; The surfaces examined ranged from 24% to 100% of a ste rically complete monolayer of 3.5 mu mol/m(2) in alkane coverage. Fluo rescence recovery after photobleaching was used to measure the lateral diffusion coefficient. Acridine orange is a fluorophor known to resid e at the water/hydrocarbon interface, negligibly adsorbing to bare sil ica. Its diffusion coefficient was found to decrease 500-fold as the s urface coverage was reduced 4-fold. By use of effective medium and per colation theory, the strong density dependence was attributed to decre ased contiguity of hydrocarbon at lower coverages. The temperature dep endence of the lateral diffusion coefficient showed that the decease i n diffusion rate at lower coverage was entirely due to an increase in the entropic barrier, pointing to percolation as the mechanism of diff usion.