Determination of the structure of the monolayer of hexadecyltrimethyl ammonium bromide adsorbed at the air-water interface

We have studied the adsorption of hexadecyltrimethyl ammonium bromide (C₁₆TAB) at the air-water interface at its CMC using the neutron reflection technique. The reflectivity profiles have been analysed using both a layer model with the optical matrix method and a recently developed approach based on the kinematic approximation. Assuming a uniform layer distribution for the fragments comprising the layer, the total thickness of the monolayer is found to be 21 ± 2 Å, of which 10 Å is immersed in water. If a Gaussian distribution is assumed the total layer thickness at the height of 1/e is about 18 ± 2 Å, some 3 Å shorter than the value from the uniform layer model. The Gaussian thicknes for alkyl chain and head distributions are found to be 16.5 ± 2 and 14 ± 3 Å. The number of water molecules associated with each surfactant molecule is about 6. The shapes and widths of the distributions of the alkyl chain, head and water are compared with those from a recent computer simulation of hexadecyltrimethyl ammonium chloride (C₁₆TAC1). Good agreement is obtained for the distances between the centres of the three distributions.