STRUCTURE OF AN OCTADECYLTRIMETHYLAMMONIUM BROMIDE LAYER AT THE AIR-WATER-INTERFACE DETERMINED BY NEUTRON REFLECTION - SYSTEMATIC-ERRORS IN REFLECTIVITY MEASUREMENTS

The adsorption of octadecyltrimethylammonium bromide (C18TAB) at the air/water interface has been studied by surface tension and neutron reflection measurements. The critical micelle concentration (cmc) was found to be 3.1 X 10(-4) M at 33-degrees-C and the area per molecule at the cmc 44 +/- 2 angstrom 2. The results for the area per molecule from the two methods agreed. The structure of the C18TAB layer was determined at the cmc by measuring the neutron reflectivity profiles of six isotopic compositions. Assuming the distributions of chain and head-group regions to be Gaussian in shape, their widths were found to be respectively 17 +/- 1 and 13 +/-3 angstrom and, taking the depletion layer of water to be uniform, the width of the depleted water region was 10.5 +/- 1 angstrom. The separations of the three distributions, chain, head, and solvent, denoted by delta(cs), delta(hs), and delta(ch), were determined directly and found to be respectively 9 +/- 0.75, 9 +/- 1.5, and 1 +/- 1 angstrom. The method of analysis of reflection data has been improved and possible sources of systematic error have been fully analyzed. Application of the improved analysis to data from C14TAB at the same surface coverage leads to values of 7 +/- 0.75, 6 +/-1.5, and 1 +/- 1 angstrom, respectively for the corresponding three separations, delta, in C14TAB. The separation of the distributions for the two surfactants and the widths of head-group and solvent distributions are found to be in good agreement with a recent computer simulation. The width of the chain distribution, on the other hand, is found to be larger than from the simulation.