In this proposal, we request beamtime to study the unexpected behaviour of a strongly bound bilayer of the cationic surfactant didodecyldimethylammonium bromide on the anionic mica surface in the presence of the anionic surfactant sodium dodecyl sulfate. This builds on previous work at ISIS where a cationic surfactant layer was found to be persistent to an extended wash with water and salt solutions, but upon exposure to sodium dodecyl sulfate at the critical micelle concentration the layer completely desorped from the surface. At concentrations below the critical micelle concentration however, the adsorbed layer was observed to thicken and it is the structure of the modified layer that is the focus of this proposal.