Coadsorption of an anionic and a non-ionic surfactant on calcium fluoride in water

Isotherms have been determined for the adsorption of mixtures of sodium dodecyl sulfate (SDS) and the non-ionic surfactant TX-100 on calcium fluoride immersed in water. SDS was adsorbed via its anionic head groups directly on the solid surface but TX-100 could be adsorbed only through chain–chain interactions involving surfactant molecules already in the interfacial region. A synergistic effect caused mutual promotion of coadsorption. At low SDS and high TX-100 concentrations sub-monolayer coverage of the surface by SDS nucleated adsorbed aggregates of TX-100. At high SDS and low TX-100 concentrations a tendency towards bilayer formation led to both TX-100 and SDS being adsorbed in the second layer. However, a preference for TX-100 adsorption gradually diminished with an increasing proportion of SDS in the system.