DETAILED STRUCTURE OF THE HYDROCARBON CHAIN IN A SURFACTANT MONOLAYER AT THE AIR-WATER-INTERFACE - NEUTRON REFLECTION FROM HEXADECYLTRIMETHYLAMMONIUM BROMIDE

We have determined the structure of a monolayer of hexadecyltrimethylammonium bromide (C16TAB) adsorbed at the air/water interface at its critical micelle concentration (0.9 mM) at two levels of resolution achieved by isotopic labeling. We have labeled the surfactant molecule in blocks of four atoms (C4D8 or C4D9) and in blocks of two atoms (C2D4 or C2D5). We have measured the roughness of the layer and the set of separations between fragments labeled in blocks of two or four carbon atoms. The roughness of the layer was determined by measuring the width of the labeled fragment of the layer from reflection profiles of the series of compounds "0" C2mdC16-2mdTAB in null reflecting water (nrw) where m has all values from 0 to 8. The width tends to a limiting value of 14 Å as m decreases to zero, and this corresponds to the roughness of the layer (defined as the width of a Gaussian at 1/e of its height). The separations between the different labeled fragments of the chain and between the fragments and the solvent were determined using a previously developed model independent method. When C16TAB is divided into eight C2 fragments, the layer is characterized fully by eight internal separations in the alkyl chain and any one between a part of the chain and the water. We have made 39 determinations of fragment separations, from which a least squares fit gave the 9 independent structural parameters. The values of these show that the first half of the carbon chain, (nearest the head group) is oriented closer to the surface normal than the outer parts of the chain, which are progressively oriented away from the surface normal. They also show that the overall chain distribution is well represented by a Gaussian distribution, consistent with the assumptions of earlier measurements. Combination of the fragment separations with the roughness of the layer has been used to predict the widths of various parts of the chain. The excellent agreement of these with the observed widths shows that the roughness is characteristic of the layer as a whole, rather than varying throughout the layer, and indicates that roughness in the water layer is coupled to that of the surfactant. © 1995 American Chemical Society