Diffusio-osmosis of surfactant solutions: Molecular modeling and interfacial physics

Diffusio-osmosis is an important transport mechanism at the micro/nanoscale, with applications such as chemical injection-based enhanced oil recovery. Here, we investigate the diffusio-osmosis of surfactant solutions, a class of diffusio-osmosis where solutes are mainly contact adsorbed on solid surfaces. We compute the diffusio-osmosis using molecular dynamics simulations and delineate the limitations of classical continuum models in predicting such transport. We adopt model surfactants whose tail groups exhibit a stronger affinity to solid walls than their head groups. For dilute solutions of such surfactants, the strength of diffusio-osmosis is independent of the surfactant concentration and is generally enhanced when tail groups interact more strongly with solid walls or when head groups become smaller. As the surfactant concentration increases beyond a critical value, the strength of diffusio-osmosis decreases, initially rapidly and then slowly, with increasing surfactant concentration. The classical continuum model for diffusio-osmosis can predict these trends, but quantitative discrepancies are also evident. These discrepancies arise mainly from two interfacial effects neglected in those theories: the loss of flow driving force due to the intermittent adsorption of surfactants on walls and the deviation of the viscosity of interfacial fluids from bulk values due to the formation of surfactant nanostructures near walls. The insights from the present study are relevant to the broad class of diffusio-osmosis where the contact adsorption of solutes is important.