Slow organization of cationic surfactant adsorbed to silica from solutionsfar below the CMC

The aggregation of hexadecyltrimethylammonium bromide, CTAB, on silica at p H 9.6 +/- 0.4 has been shown, using optical reflectometry and AFM, to occur at a significantly lower concentration than is seen at neutral pH. This lo wer critical surface aggregation concentration, CSAC, results from the incr eased surface charge of the silica. Although this concept is not new, AFM h as provided the first direct evidence of lateral organization of adsorbed s urfactant at an order of magnitude below the cmc. This confirms that a thre shold surface concentration of adsorbed surfactant is required to initiate surface aggregation. Specifically, on silica an adsorbed density of 0.6 mg/ m(2) is sufficient for hydrophobic interactions between neighboring adsorba tes. This promotes adsorption beyond that required to balance the surface c harge, and hence the formation of surface aggregates. In addition, slow ads orption is observed in the vicinity of the CSAC as reported recently at neu tral pH. The results confirm that surface aggregation is not a simple funct ion of the bulk surfactant concentration, but rather a complex function of surface charge characteristics in the case of oppositely charged surfactant s adsorbed to silica.