Adsorption of Surfactants at the Solid-Liquid Interface: Effect of Surface Topological Variations

Adsorption of surfactants at the solid-liquid interface is key to many applications, including the removal of particulates from laundry or hard surfaces, the removal of oil from underground, deinking of plastics and the separation of minerals, e.g. copper and lead from other species. Our group has studied in detail the effect of topological variations on surfactant adsorption using both experimental and molecular dynamic (MD) simulations. On a surface that is not molecularly smooth but where variations in topology are random, we have shown that with an increase in variation, the amount of surfactant adsorption decreases on a total surface area basis, and, in some cases, even on a nominal surface area basis (e.g. the surface area assuming the surface is molecularly smooth). For a regular topology, we explored adsorption on top of tens of nanometer size pillars. Adsorbed surfactant thickness was higher near the edge of the pillar and decayed exponentially with distance from the edge while the amount adsorbed was less near the edge and increased exponentially with a slightly larger decay-length constant. These decay length constants of ~ 10 nm are much larger than expected given the ~2 nm size of a surfactant molecule and are also much larger than MD simulations. Surfactant adsorption is driven by entropy and known to be highly cooperative, and these large decay constants as well as the decrease in total amount adsorbed on a rough surface are a manifestation of the cooperativity of surfactant adsorption which in turn is related to the ability of the adsorbed structures to exclude water from the hydrophobic regions.