FTIR and rheological studies of surfactant adsorption onto silica
The adsorption of four surfactants at the silica-toluene interface have been investigated by Fourier-transform infrared - attenuated total reflection (FTIR-ATR) spectroscopy. Specifically, hexadecylamine, 1,12-diaminododecane, didecylamine and sorbitan monopalmitate were studied. The silica surface for the spectroscopic experiments was provided by the native oxide layer on a silicon ATR prism. In order to study the CH(_2) stretching vibrations, the solvent was fully deuterated. Careful calibration of the ATR experiment enabled the contribution to the ATR spectrum made by adsorbed species to be calculated. The surface excess concentration of the amines were calculated over a range of bulk solution conentrations, enabling adsorption isotherms to be constructed. The use of polarized radiation allowed the orientation of the molecules within the adsorbed layer to be studied. The adsorbed layer of hexadecylamine showed some ordering of the aliphatic chains. There is some spectral evidence that the chains in the adsorbed layer of 1,12- diaminododecane adopt a 'looped' configuration. The effect of adsorbed layers of the molecules listed above upon the rheological behaviour of Aerosil 200 dispersions in toluene was investigated. Differences in the rheological behaviour of these dispersions have been correlated to differences in adsorbed layer structure as determined by FTIR-ATR. Due to differences in the Aerosil and ATR surface these correlations must be treated cautiously.