STRUCTURE OF PRECURSING THIN-FILMS OF AN ANIONIC SURFACTANT ON A SILICON-OXIDE SILICON SURFACE

We use X-ray and neutron reflectivity to probe sodium dodecyl sulfate surfactant monolayers on the silicon oxide/silicon surface as deposited from a receding solution and hydrated in precursing thin films. The deposited monolayers are dry and have their head groups adjacent to the substrate. Despite the solubility of the surfactant, the deposition process is similar to a Langmuir-Blodgett deposition. X-ray reflectivity measurements of the dry monolayer show the density of the tail regions to be similar to that observed for liquid hydrocarbons. Further, we detect a lower density outer tail region which is compatible with molecular dynamics calculations. Placing the monolayers in contact with bulk surfactant solutions in a vapor-sealed environment causes them to hydrate and to become precursing films. These precursing films are made up of a water core and a surfactant monolayer at the liquid/vapor interface of the film. We monitor the structural evolution of the monolayers as the films equilibrate over days. The concentration of the bulk surfactant solution influences the self-assembly of the surfactant monolayer at the liquid/vapor interface in the film. Further, the area per surfactant molecule in the monolayer increases with film thickness due to decreasing compression of the counterion distribution and reduction in the screening of the intralayer head group repulsion.