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Unusual Three Stage Multi-layer Formation During Adsorption Of An Anionic Fluorinated Surfactant onto Germanium

Rong Xing
Chemical and Materials Engineering Department
University of KY
Lexington, KY

Wednesday, January 24, 2006 10:00 am
Ben Bandy Conference Room
UK Center for Applied Energy Research

The adsorption of surfactants and formation of surface aggregates at solid/liquid interface play an important role in many industrial processes. This work deals with investigations into the adsorption kinetics and layers formation of fluorinated surfactant tetraethylammonium perfluorooctylsulfonate (TEA-FOS) from aqueous solution onto hydroxylated germanium using in situ polarized attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The adsorption is monitored at a series of bulk solution concentrations spanning from well below to above the critical micelle concentration (CMC; 1.0 mM). The kinetics of adsorption is followed by monitoring the intensity of the fluorocarbon bands.

The orientation of the fluorocarbon director with respect to the germanium surface is determined by circular dichroism measurements of CF2 stretching bands. At bulk concentrations ranging from 10 % of the CMC to at least 500 % of the CMC, the adsorption occurs in an unusual sequence of three stages leading to multilayer formation. AFM images of either quenched film or in situ multilayer growth on mica indicate a multilayer formation mechanism consisting of growth of submonolayer, discrete nucleation of hydrophobic patches and coalescence of adsorbed aggregates. Based on the ATR-FTIR and AFM results, the mechanism of three stage multilayer formation was proposed.