THE DIRECT DETERMINATION OF THE MEAN SEPARATION OF A TETHERED CHAIN FROM ITS ANCHOR

Neutron reflectivity with isotopic substitution has been used to determine the mean separation between a tethered chain and its anchoring point. The method is described and applied to the study of the structure of a layer of the surfactant tetradecyl trimethylammonium bromide adsorbed at the air-solution interface. At a concentration just below the critical micelle concentration the area per molecule in the adsorbed layer is 48 ± 2 A and the midpoint of the distribution of the tetradecyl chains is found to be 7 ± 0.5 A from the centre of the trimethylammonium headgroup distribution. This result is independent of the widths of the two individual distributions, the only assumption being that both distributions are even functions about their centres