FOURIER-TRANSFORM INFRARED ATTENUATED TOTAL REFLECTION SPECTROSCOPY LINEAR DICHROISM STUDY OF SODIUM DODECYL-SULFATE ADSORPTION AT THE AL2O3/WATER INTERFACE USING AL2O3-COATED OPTICS

Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy was used to characterize the surface excess and molecular orientation of sodium dodecyl sulfate (SDS) adsorbed at the Al2O3/water interface. A method is described for the quantitative IR-ATR determination of Gibbs' surface excess on hydrophilic solids, in the presence of aqueous solutions ("in situ"). The method was used in an in situ linear dichroism (LD) study of the orientation of both the alkyl chains and OSO3- head groups of adsorbed SDS on Al2O3. Al2O3 Was present as a thin film (150-400 nm) on the surface of ZnSe IR internal reflection elements. This approach makes full range IR-ATR adsorption measurements feasible for solids which absorb strongly in the IR region. Adsorption isotherms were determined for SDS (2 X 10(-5) to 4 X 10(-3) M, 0.15 M NaCl) adsorption onto Al2O3 films. The adsorption densities found, 0.1-1.5 monolayer at pH 3.8, and 0.2-1.5 monolayer at pH 6.6, conform closely to literature values. Under conditions giving an average of 1.5 monolayers coverage, LD studies of the IR absorption bands of adsorbed OSO3- groups revealed an average angle between the group pseudo-C3upsilon axis and the surface normal of ca. 43-degrees. Under the same conditions, alkyl chains in adsorbed SDS exhibited an ordered packing with an angle of 45-degrees between the extended chain axis and the surface normal, with the appearance of free rotation of the chain about the extended chain axis.