Structure of hydrocarbon chains in surfactant monolayers at the air/water interface: Neutron reflection from dodecyl trimethylammonium bromide

We have determined the structure of a monolayer of dodecyl trimethylammonium bromide (C(12)TAB) adsorbed at the air/water interface at its critical micelle concentration (c.m.c., 14 mmol dm(-3)) at a level of resolution of blocks of four methylene groups. This has been done by labelling the surfactant molecule in different positions with blocks of four atoms (C4D8 or C4D9). We have made an independent measurement of the roughness of the layer by measuring the width of the labelled fragment of the layer from reflection profiles of the series of compounds OC(12-2N)dC(2N)dTAB (0 indicates null reflecting and d, deuteriated) in null reflecting water (NRW) as a function of N. The width tends to a limiting value of 12 Angstrom as N 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 labelled fragments of the chain, and between the fragments and the solvent, were determined using a previously developed model-independent method. When the C(12)TAB is divided into three C-4 fragments, the layer is characterized fully by three separations within the surfactant molecule and one between any part of the chain and the water. We have made 15 independent determinations of these fragment separations, from which a least-squares fit gave the five 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. The mean structure of the C-12 chain in the C(12)TAB layer is identical with that of the first 12 methylene groups in C(16)TAB, at a comparable area per molecule. It is shown that measurement of the overall width of the chain distribution may give a misleading impression of the average conformation of the chains in a layer because of the large contribution due to the roughness.