FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY OF THE ADSORPTION OF CETYLTRIMETHYLAMMONIUM BROMIDE AND CETYLPYRIDINIUM CHLORIDE ON SILICA

A Fourier transform infrared (FTIR) spectroscopic investigation of the adsorption of two cationic surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), on silica was conducted to identify the structural changes that occur as a function of surfactant surface coverage and state of wetness. In situ transmission and cylindrical internal reflectance FTIR spectrometry studies were performed on silica self-supporting films and colloidal suspensions, respectively. Spectroscopic results show that, in the presence of water, micelle-like, surfactant-aggregate clusters form on the silica surface, even at surfactant surface coverage as low as 12 %. Increasing the surfactant sorption density results in an increase in the size and/or the number of the surfactant-aggregate clusters. In contrast, the state of wetness appears to change the aggregation structure of sorbed surfactants as a function of coverage. At low surface coverage, adsorbed surfactant molecules change from the aggregated-cluster state to a monomer state upon drying. During this drying process, the methylene chains of the surfactant molecules change from the aggregated state to a flat, parallel orientation on the silica surface, preferring an association with the silica surface instead of with other methylene groups. Upon rewetting, the surfactant molecules revert to their original aggregate-cluster form. For high surface coverage, no significant structural or orientational change is found during the drying and rewetting processes, presumably because the surfactant molecules have reached monolayer or possibly bilayer coverages, leaving insufficient room for sorbed surfactant molecules to rearrange upon drying. These results suggest that the spectra obtained from the preparation of dry surfaces are not necessarily representative of surfactant structure and orientation in situ.