Single Molecule Observations of Fatty Acid Adsorption at the Silica/Water Interface: Activation Energy of Attachment

Abstract

Total internal reflection fluorescence microscopy was used to obtain real-time images of individual fluorescently labeled hexadecanoic acid molecules as they absorbed at the aqueous−fused silica interface. In contrast with laterally averaged ensemble measurements of surface coverage versus time, single-molecule observations provide direct, model-independent measurements of adsorption kinetics. Additionally, since the observations were made under steady-state conditions at extremely low coverage, effects due to transport kinetics did not influence the results, permitting accurate measurement of the fundamental attachment rate as a function of temperature, that is, the rate at which molecules are transferred to the surface from the subsurface layer. We found that the attachment rate systematically increased with increasing temperature; an Arrhenius analysis gave an activation energy of E a = 19 ± 2 kJ/mol. We hypothesize that this energy, which is similar in magnitude to the strength of an OH···O hydrogen bond, is associated with the displacement of a water molecule from the silica surface.