Adsorbed layer structure of cationic and anionic surfactants on mineral oxide surfaces

We report on the structure of adsorbed layers of sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) on titanium dioxide (rutile) and kaolinite, and of TTAB and hexadecyltrimethylammonium bromide (CTAB) on quartz imaged by atomic force microscopy above their respective critical micelle concentrations (in the plateau adsorption region). The surfactants all form globular surface micelles on these three substrates, but under very different conditions. Adsorbed micelles of SDS are found at pH's less than the point of zero charge of rutile, whereas TTAB and CTAB adsorbed micelles are found only above the points of zero charge of rutile and quartz, respectively. Adsorbed SDS micelles are observed on the terraced edges of kaolinite particles, which are positively charged under acidic conditions. In contrast, adsorbed TTAB micelles are observed only on the anionic kaolinite basal plane.