(189i) Capturing Non-Ideal Surfactant/Nanoparticle Interfacial Structure with Variable Coverage Molecular Simulations

Surfactants adsorbed to hard nanoparticle interfaces are poorly understood despite their importance in nanotechnologies because of their complex non-ideal behavior, the non-deformable nanoparticle interface, and the similar length scales of surfactant molecules and surfaces. We develop a molecular thermodynamic approach to directly simulate isotherms of surfactants at interfaces with arbitrary coverages and to describe surfactants that are not well described with simple molecular thermodynamic theories. All-atom molecular simulations are used to estimate the chemical potential of non-ideal surfactant structures by calculating the free energy required to concentrate a fixed number of adsorbed surfactants at low density and under ideal conditions onto a small subsection of the interface with high local density. Standard adsorption calculations are used to reference the low density state to the bulk liquid surfactant potential, thus providing equilibrium coverage estimates. This approach captures non-ideal effects such as specific surfactant-surfactant interactions, amphiphilic surfactant structure, and more than one monolayer of adsorbates. We show that this new approach reproduces analytical results for hard spheres at the interface and can resolve qualitative changes in surfactant structure at between low and high loadings.