Nanometer-Scale Organization of Ethylene Oxide Surfactants on Graphite, Hydrophilic Silica, and Hydrophobic Silica

Abstract

To better understand the role of the interactions between surfactant, solvent, and a solid substrate on surface aggregation, we have studied the adsorption of a series of alkylpoly(ethylene oxide) CnEm surfactants on three different substrates: graphite, hydrophilic silica, and hydrophobic silica. Using atomic force microscopy (AFM), we find that adsorption to hydrophilic silica, with two exceptions, results in the formation of globular structures that are similar to bulk micelles. On silica that has been made hydrophobic by reaction with organosilane, adsorption results in a surface layer that is laterally homogeneous and is probably a monolayer with ethylene oxide groups in contact with the solution. A number of surfactants with ionic and zwitterionic headgroups were also observed to form monolayers on hydrophobic silica. This large perturbation from the solution-aggregate structure on the hydrophobic surface is driven by a minimization of the area of contact between water and the hydrophobic silica substrate. On graphite, the surface layer is either long, thin aggregates (consistent with a hemicylindrical structure) or a laterally homogeneous layer (consistent with a monolayer with the headgroups facing the solution). The nonionic C12 and C14 surfactants form hemicylinders, and the C10 surfactants with the same headgroups form a laterally homogeneous layer. This suggests that above a critical alkyl chain length the interaction between the graphite and the surfactant tail is sufficient to orient a layer of alkyl tail groups parallel to the graphite surface, which then templates further adsorption. Below the critical alkyl length, the arrangement on graphite is similar to that on hydrophobic silica and is probably driven by a minimization of the water−graphite interfacial area. The critical alkyl length for (zwitterionic) sulfobeteine surfactants is not the same as for the nonionic poly(ethylene oxide) surfactants. This shows that the headgroup also plays an important role in determining the adsorbed structure. All measurements were performed in equilibrium with bulk micelles at or above the critical micelle concentration and at approximately 25 °C.