MEASURING REVERSIBLE ADSORPTION-KINETICS OF SMALL MOLECULES AT SOLID LIQUID INTERFACES BY TOTAL INTERNAL-REFLECTION FLUORESCENCE CORRELATION SPECTROSCOPY/

Reversible adsorption kinetics of a cationic dye molecule at a C-18 mo dified silica surface are measured in a fluorescence correlation spect roscopy experiment. An interfacial observation volume is defined by to tal internal reflection of a laser beam which excites fluorescence emi ssion from Rhodamine 6G (R6G(+)) in adsorption equilibrium with the su rface. Spontaneous fluctuations in the population of R6G(+) molecules within the interfacial volume are observed as excess low-frequency noi se superimposed on a fluorescence transient. Autocorrelation of the tr ansient allows the temporal characteristics of the fluctuations to be examined. The rate at which fluctuations occur depends on transport of molecules to the interface and their adsorption kinetics. Adsorption and desorption rates of R6G(+) in water/methanol solutions at a C-18 m odified silica/solution interface were measured over a 3 order of magn itude range of the retention equilibrium; both rates were found to dep end strongly on the composition of the overlying solution phase.