Direct determination by neutron reflection of the penetration of water into surfactant layers at the air/water interface

Neutron reflectivity with isotopic substitution has been used to determine directly the mean separation between the center of the distribution of a hydrophobic chain of a surfactant adsorbed at the air/water interface and the mean position of the aqueous surface. The method is described and applied to the study of three surfactant layers, sodium dodecyl sulfate, tetradecyltrimethylammonium bromide, and triethylene glycol monododecyl ether, all adsorbed from solution at concentrations close to their critical micelle concentrations. The separations are found to be, respectively, 7.5, 8.0, and 10.5 +/- 1 angstrom. This result is shown to be independent of detailed assumptions about the structure of the layer. Comparison of these values with the width of the chain and solvent distributions shows that the hydrocarbon chain is significantly immersed (congruent-to 30%) in the water.