Strong competition between adsorption and aggregation of surfactant in nanoscale systems

Abstract

HypothesisThe size-dependent behavior including surface tension, surface density (Γ), and critical micelle concentration (CMC) of a nanoscale liquid film containing surfactant has not been investigated until now. ExperimentsStrong competition between surface adsorption and bulk aggregation of surfactant in nanoscale systems was explored by Many-body Dissipative Particle Dynamics simulations. FindingsIn nanoscale systems, as the surfactant concentration increases, Γ continues rising even after CMC is exceeded. The saturation level of Γ is achieved only when the surfactant bulk concentration is over ten times of CMC. Moreover, both surface micelles formed by adsorbed surfactant and the sublayer below the adsorbed layer are clearly identified. The former can reduce the contacts of adsorbed surfactant with water, while the latter has the surfactant concentration significantly higher than that in bulk. The strong coupling between adsorption and micellization is attributed to large surface-to-volume ratio compared to macroscopic systems, and can be simply realized by the fact that the ratio of the numbers of surfactant distributed in bulk (nbulk) and at interface (nads) is always less than unity (nbulk/nads < 1) in nanoscale systems.