Determination of Equilibrium Adsorbed Morphologies of Surfactants at Metal-Water Interfaces Using a Modified Umbrella Sampling-Based Methodology br

Surfactants adsorb to metal-water interfaces in variousmorphologies, including self-assembled monolayers (SAMs), cylindricaland spherical micelles, or hemimicelles. Current molecular simulationmethods are unable to efficiently sample the formation of thesemorphologies because of the large diffusive/energetic barriers. Weintroduce a modified umbrella sampling-based methodology that allowssampling of these morphologies from any initial configuration and providesfree energy differences between them. Using this methodology, we havestudied adsorption behavior of cationic [quaternary ammonium (quat) of 4and 12 carbon long alkyl tails], uncharged [decanethiol], and anionic[phosphate monoester] surfactants and their mixtures at a gold-waterinterface. Wefind that while Coulombic repulsion between the chargedhead groups of quat-4 limits their adsorption to a sparse layer, strongerhydrophobic interactions between the alkyl tails of quat-12 promote adsorption resulting in a morphology with adsorbedhemispherical micelles sitting atop a monolayer. Decanethiol molecules adsorb in a densely packed bilayer with the moleculesstanding-up on the surface in thefirst layer and lying parallel to the surface in the second layer. Cationic and anionic surfactantmixtures display a synergistic adsorption behavior. These results elucidate the role of molecular characteristics in dictating the natureof adsorbed morphologies of surfactants at metal-water interfaces.