Structure of an octadecyltrimethylammonium bromide layer at the air/water interface determined by neutron reflection: systematic errors in reflectivity measurements

Abstract

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 × 10^-4 M at 33°C and the area per molecule at the cmc 44 ± 2 Ų. 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 Å and, taking the depletion layer of water to be uniform, the width of the depleted water region was 10.5 ± 1 Å. The separations of the three distributions, chain, head, and solvent, denoted by δ_cs, δ_hs, and δ_ch, were determined directly and found to be respectively 9 ± 0.75, 9 ± 1.5, and 1 ± 1 Å. 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 Å, respectively for the corresponding three separations, δ, 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.