STRUCTURE OF A CATIONIC SURFACTANT LAYER AT THE SILICA WATER INTERFACE

The technique of specular reflection of neutrons has been used to study the structure of a layer of hexadecyltrimethylammonium bromide (C16TAB) adsorbed at the interface between a plate of amorphous silica and solutions of C16TAB. At concentrations of 1/3 and 2/3 the cmc (9 ˟ 10-4 M), the structure which best fitted the reflectivity data can be described either as a defective bilayer or as flattened micelles. The thicknesses of the alkyl chain and head-group regions of the bilayer were found to be 28 ± 4 and 6 ± 2 A, repectively. At 6 ˟ 10-4 M, 80% of the interface was covered with bilayer, and this dropped to 35% at 3 ˟ 10-4 M. At both coverages, the head-group area was found to be 30 ± 3 A2. © 1990, American Chemical Society. All rights reserved.