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 non-ionic surfactant hexaethylene glycol monododecyl ether. 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 concentrations below the critical micelle concentration the layer thickened, and when the anionic surfactant concentration reached the critical micelle concentration the cationic layer desorped completely from the mica surface.