Structure of the cetyltrimethylammonium surfactant at fused silica/aqueous interfaces studied by vibrational sum frequency generation.

Utilizing vibrational sum frequency generation (SFG), we characterized the structure of adsorbed cetyltrimethylammonium chloride (CTAC) at the silica/aqueous interface in the presence of 10 to 500 mM NaCl and as a function of surfactant surface coverage. For low ionic strengths (10 mM NaCl), results indicate that adsorbed aggregates do not change conformation with increasing surface coverage. Instead, the surfactant adsorbs as micelle-like structures at concentrations considerably lower than surface saturation and the CMC. At high ionic strengths (300-500 mM NaCl), the structure of the adlayer is considerably different: The SFG results indicate that for 30 microM bulk CTAC the surfactant packs with fewer gauche defects in the hydrocarbon backbone, which is attributed to reduced Coulomb repulsion between the positively charged surfactant headgroups, and the results also indicate that CTAC adsorbs as monomers at low surface coverage but then rearranges into aggregates at higher surface coverage.