Spectroscopic identification and dynamics of adsorbed cetyltrimethylammonium bromide structures on TiO2 surfaces

Infrared spectroscopy is used to identify the aggregated structures of adsorbed cetyltrimethylammonium bromide (CTAB) on negatively charged TiO2 surfaces. In particular, there are abrupt changes in the intensity of the symmetric bending mode of the CTAB headgroup, and these abrupt changes have been correlated to different aggregated structures along the adsorption isotherm. Furthermore, by measuring spectra as a function of time, it is possible to obtain information on the dynamics of the formation of aggregated CTAB structures on the surface. It is shown that aggregated hemimicellar, admicellar, and micellar structures initially adsorb through intermediate structures that have a higher percentage of CTAB molecules bound directly to charged sites on the surface. Above the critical micelle concentration (cmc), the CTAB exists as micelles on the surface when exposed to a bare TiO2 surface or to a TiO2 surface already covered with hemimicellar/admicellar adsorbed structures. In the latter case, the existing hemimicellar/admicellar structures either are transformed to micelles or are displaced from the surface.