XPS film thickness and adsorption studies of alkyltrimethylammonium bromides and organosilanes on silica surfaces

Abstract

The film thickness of organic molecules adsorbed onto a substrate may be predicted by a simple comparison of relative atomic concentrations if a well characterised film is used to form a calibration curve. Values of the attenuation length determined by X-ray photoelectron spectroscopy (XPS) are therefore not required. Cetyltrimethylammonium bromide (CTAB) is ideal as a calibration standard, because the formation of a surface layer below the critical micelle concentration (CMC) is found to be readily monitored by a comparison of the relative atomic concentrations of nitrogen and bromide. Ex-situ XPS results show direct evidence of the presence of surface nitrogen and absence of surface bromide when silica plates are removed from CTAB solutions at concentrations below the CMC. Results suggest that the structure of surface layers of CTAB determined ex-situ are not representative of the reported structure and orientation in-situ. At concentrations above the CMC, both nitrogen and bromide are found to be present on the surface. Angle resolved XPS of CTAB films formed above the CMC show that bromide ions do not simply occupy the outermost surface layer, which would be expected for a bilayer, rather a film of unknown structure is formed which is different from the surface of pure solid CTAB. For octyltrimethylammonium bromide and dodecyltrimethylammonium bromide, adsorption is limited to a narrow range of concentrations below their CMC values. The film thicknesses of the CTAB layers are compared with those previously reported for films of long chain alcohols adsorbed onto silica.