The investigation of buried interfaces, such as the mineral/aqueous solution interface, is experimentally challenging. Neutron reflectometry is a well-established and non-invasive technique that allows behaviour at these interfaces to be measured, and a technique has been developed within the group to allow the surface of the mineral muscovite mica to be measured. This surface shows atomic flatness due to perfect cleavage along the basal plane, and is negatively charged in solution.
It had previously been shown that the cationic surfactant didodecyldimethylammonium bromide (DDAB) adsorbs as a strongly bound bilayer from solution onto the mica surface. This bilayer could not be removed by extended water washing, nor by 10mM NaCl or CaCl2 solutions. In fact a UV/O3 treatment was required to remove the layer and return the reflectivity profile to that of the bare mica. It has subsequently been discovered that exposing the DDAB bilayer to a solution of the anionic surfactant sodium dodecyl sulfate (SDS) at the critical micelle concentration (CMC) or above resulted in complete desorption of the cationic layer from the surface. By contrast when the bound bilayer was exposed to SDS at concentrations below the CMC a thickening of the layer was observed as SDS was incorporated into the bilayer, but on reaching the CMC the layer completely desorbed as before. This new insight into surfactant interactions has implications ranging from detergency to enhanced oil recovery.
Allen, F. J.; Truscott, C. L.; Welbourn, R. J. L.; Clarke, S. M., Appl. Clay Sci. 2018, 160, 276–281.