| Abstract
| - The adsorption of tetraethylammonium perfluorooctylsulfonate (TEA-FOS) from aqueous solution ontohydroxylated germanium is studied using in situ polarized attenuated total reflection Fourier transform infraredspectroscopy (ATR-FTIR). The adsorption is monitored at a series of bulk solution concentrations spanningfrom well below to above the critical micelle concentration (CMC; 1.0 mM). The kinetics of adsorption isfollowed by monitoring the intensity of the fluorocarbon bands. The orientation of the fluorocarbon directorwith respect to the germanium surface is determined by circular dichroism measurements of CF2 stretchingbands. At bulk concentrations ranging from 10% of the CMC to at least 500% of the CMC, the adsorptionoccurs in an unusual sequence of three stages. Initially, rapid adsorption occurs within 200 min, leading tocoverage of a monolayer or less. A long period of slow adsorption follows, during which we hypothesize thatsurfactant molecules form clusters, some of which serve as nuclei for multilayer growth. This stage concludessuddenly with an acceleration in the rate of adsorption, which eventually leads to multilayer formation. Becausethis is an anionic surfactant adsorbing onto a negatively charged surface at pH 6, the tetraethylammoniumions must mediate the interactions between the surfactant headgroups and the surface. The dichroismmeasurements show that TEA-FOS is initially oriented randomly or somewhat parallel to the surface, butover time adopts an orientation somewhat normal to the surface. This behavior is consistent with initialadsorption at isolated sites, followed by aggregation into isotropic admicelles, and finally growth into flattenedadmicelles. The sudden onset of accelerated adsorption can be explained either by autoaccelerating adsorptionor nucleation and growth of a hydrophobic multilayer structure.
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