NEUTRON REFLECTION FROM HEXADECYLTRIMETHYLAMMONIUM BROMIDE ADSORBED ON SMOOTH AND ROUGH SILICON SURFACES

Neutron reflection has been used to investigate the structure of hexad ecyltrimethylammonium bromide (C(16)TAB) layers adsorbed at the silico n/silicon oxide/aqueous interface. Separate isotropic labeling of grou ps of four methylene groups at a time made it possible to determine th e structure of the layer at a higher resolution than previously possib le. The structure of the layer was investigated on smooth and rough su rfaces. The roughness of the surface has a significant effect on the p roperties of the layer. On the rough surface the bilayer is shown to b e thicker and to be unsymmetrical in the direction of the surface norm al. The surface coverage was also found to be lower on the rough surfa ce. As in previous studies the surface was found not to be completely covered, lending support to the idea that adsorption is in the form of aggregates. The division into smaller isotopically labeled fragments shows, however, that the aggregates strongly resemble bilayer fragment s, and their overall thickness, at 32 +/- 1 Angstrom, is substantially less than the diameter of a micelle.