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 deposite d from a receding solution and hydrated in precursing thin films. The deposited monolayers are dry and have their head groups adjacent to th e substrate. Despite the solubility of the surfactant, the deposition process is similar to a Langmuir-Blodgett deposition. X-ray reflectivi ty measurements of the dry monolayer show the density of the tail regi ons to be similar to that observed for liquid hydrocarbons. Further, w e detect a lower density outer tail region which is compatible with mo lecular 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 ma de up of a water core and a surfactant monolayer at the liquid/vapor i nterface of the film. We monitor the structural evolution of the monol ayers as the films equilibrate over days. The concentration of the bul k surfactant solution influences the self-assembly of the surfactant m onolayer 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 redu ction in the screening of the intralayer head group repulsion.