Neutron reflection and surface tension measurements have been used to study the composition and structure of a mixed (alkane + surfactant) monolayer at the air-water surface, formed either by adsorption from vapor or by spreading dodecane from small lenses placed on the surface of an aqueous solution of tetradecyltrimethylammonium bromide (C14TAB). The highest concentration of dodecane in the mixed monolayer occurs when the solution concentration of surfactant is about one-third of the critical micelle concentration (cmc), at which point the area per surfactant molecule is about 60 Å2, some 20% higher than its value at the cmc but similar to its value in the absence of dodecane. At lower bulk surfactant concentrations (and hence lower surface concentrations), the amount of dodecane in the mixed layer also decreases. At a surfactant concentration of 2.6 mM (cmc = 3.7 mM) the surfactant chain is found to be slightly more extended (20 Å when modeled as a uniform layer) in the presence of dodecane than in its absence, while the dodecane forms a layer significantly thicker (22 Å) than its fully extended chain length (17 Å). The separation of the centers of the surfactant chain and dodecane distributions at this concentration was found to be 5 ± 2 Å, which together with the separation of each component from the water subphase, 7.5 and 13 Å, respectively, indicates that the oil is completely out of the water and that the surface of the mixture is very similar to that of pure dodecane. © 1992 American Chemical Society.
