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The lateral diffusion coefficient of acridine orange was measured for a series of liquid alkane/water interfaces.The data show that the lateral diffusion coefficient differs among these interfaces and that lateral diffusion is slowed by interfacial roughness. Hydrocarbon viscosity had previously been suggested to play a role: analogous measurements for a Cdwater interface indicated that the reconfiguration of the hydrocarbon controls the lateral diffusion coefficient. However, the results of this work show that hydrocarbon viscosity does not significantly influence the lateral diffusion of acridine orange at liquid hydrocarbodwater interfaces. A simple mathematical model is developed to provide a plausible explanation for the observations that interfacial roughness slows lateral diffusion for IiquidAiquid interfaces and hydrocarbon viscosity has less of an effect for liquids than for viscous monolayers.

Evanescent wave excitation of fluorescence was used to study the adsorption of Ru(bpy)(3)(2+) from aqueous solution to three types of surfaces:  bare silica, a dimethylethylsiloxane (C(2)) monolayer on silica, and a dimethyloctadecylsiloxane (C(18)) monolayer on silica. The solution pH was varied to investigate the nonpolar and electrostatic contributions to the free energy of adsorption for each surface. The pH dependence of the adsorption showed that the pK(a) is the same for each of the three surfaces, consistent with earlier conclusions that the acidity of derivatized silica surfaces is due to areas of exposed silica. The free energies of adsorption for the bare silica surface, -26.2(±0.2) kJ/mol at pH 8, was attributed to electrostatic interactions alone. The free energy of adsorption for the C(2) and C(18) surfaces, -28.5(±0.1) and -31.5(±0.1) kJ/mol, respectively, were found to have both electrostatic and nonpolar contributions, with the latter being larger by 50% for the C(2) surface and 100% for the C(18) surface. Using Gouy-Chapman theory, the surface charge densities on each of the three surfaces, calculated from the electrostatic interaction energy of Ru(bpy)(3)(2+), were found to be within the range of literature values:  8.8(±0.1) × 10(-)(7) mol/m(2) for bare silica and 1.7(±0.1) × 10(-)(7) mol/m(2) for both the C(18) and C(2) surfaces. The results demonstrate that a cationic dye can be used to probe the silanol activity of chemically modified silica surfaces. The results support the picture that these chemically modified silica surfaces are acidic due to molecular-scale areas of contact between the bare silica substrate and the aqueous phase.

The lateral diffusion of acridine orange was investigated at the interface of water and octadecylsiloxane (C18) monolayers of varying hydrocarbon density. The surfaces examined ranged from 24% to 100% of a sterically complete monolayer of 3.5 μmol/m2 in alkane coverage. Fluorescence recovery after photobleaching was used to measure the lateral diffusion coefficient. Acridine orange is a fluorophor known to reside at the water/hydrocarbon interface, negligibly adsorbing to bare silica. Its diffusion coefficient was found to decrease 500-fold as the surface coverage was reduced 4-fold. By use of effective medium and percolation theory, the strong density dependence was attributed to decreased contiguity of hydrocarbon at lower coverages. The temperature dependence of the lateral diffusion coefficient showed that the decease in diffusion rate at lower coverage was entirely due to an increase in the entropic barrier, pointing to percolation as the mechanism of diffusion.

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