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.