Study of an Adsorbed Layer of Hexadecyltrimethylammonium Bromide Using the Technique of Neutron Reflection

The structure of an adsorbed layer of hexadecyltrimethylammonium bromide (C 16TAB) has been studied at the crystal quartz/aqueous solution interface by neutron reflection. Measurements of the surface reflectivity of the quartz before adsorption showed that an 85-Å-thick layer of inhomogeneous material was present, with a scattering-length density intermediate between those of crystalline quartz and amorphous silica and containing a small fraction of water. The reflectivity of the C 16TAB layer was measured at two solution concentrations. At a bulk concentration of 3 × 10 -4 M, 51% of the surface was covered with surfactant and this rose to 65% at 9 × 10 -4 M, just below the CMC. At both concentrations the overall thickness of the adsorbed layer was 34 Å, which corresponds to an adsorbed bilayer. Partial labeling of the surfactant made it possible to resolve the structure of the bilayer into head and chain regions, but only at the higher surface coverage. The loss of this distinction at the lower coverage suggests that the bilayer consists of small islands of surfactant on the surface.