DETERMINATION OF THE STRUCTURE OF THE MONOLAYER OF HEXADECYLTRIMETHYLAMMONIUM 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 refle ction technique. The reflectivity profiles have been analysed using bo th a layer model with the optical matrix method and a recently develop ed approach based on the kinematic approximation. Assuming a uniform l ayer distribution for the fragments comprising the layer, the total th ickness of the monolayer is found to be 21 +/- 2 angstrom, of which 10 angstrom is immersed in water. If a Gaussian distribution is assumed the total layer thickness at the height of 1/e is about 18 +/- 2 angst rom, some 3 angstrom shorter than the value from the uniform layer mod el. The Gaussian thicknesses for alkyl chain and head distributions ar e found to be 16.5 +/- 2 and 14 +/- 3 angstrom. The number of water mo lecules associated with each surfactant molecule is about 6. The shape s and widths of the distributions of the alkyl chain, head and water a re compared with those from a recent computer simulation of hexadecylt rimethyl ammonium chloride (C16TACl). Good agreement is obtained for t he distances between the centres of the three distributions.