The adsorption of surfactants on solid surfaces plays an important role in a variety of processes both established (e.g., detergency, lubrication, and oil recovery) and exotic promising ones (e.g., the production of sensors, structural nanometerials, and nano-batteries). Although several studies investigated the adsorption of surfactants from aqueous solutions to solid surfaces in terms of adsorption isotherms, in many applications adsorption occurs in the presence of low-molecular-weight compounds. Here, for the first time, we assess the effect of the presence of monomers on the adsorption of two surfactants on a solid surface.

The surfactants considered are hexadecyltrimethylammonium bromide (CTAB, cationic) and hexaethylene glycol monododecyl ether (C12E6, non-ionic). The surface is gold (XPS analysis reveals the presence of a large number of O atoms, rendering the surface somewhat hydrophilic). The solvent is pure water in which we dissolve either toluene (hydrophobic) or phenol (partially hydrophilic). The adsorption isotherms are measured using a quartz crystal microbalance with dissipation monitoring (QCM-D). The equilibrium adsorption isotherms will be compared to those measured for both surfactants from pure water. For example, when toluene is present we observe more CTAB adsorbed on the solid surface compared to when the pure water is used as solvent. On the contrary, less C12E6 adsorbed when toluene is present than when pure water is used. The results will be interpreted with a phenomenological model aided by recent molecular dynamics simulations.