Spontaneous Curvature of Compressed Surfactant Monolayer Induces Capillary Pressure

Using molecular dynamics simulations, we show that as surfactant coverage at a vapor-liquid interface increases, a surfactant monolayer forms and then compresses, causing a three-stage change in the surface tension. After saturated surfactant adsorption is reached, a further increase in the coverage of insoluble surfactant would compress the monolayer, and the resulting monolayer rigidity would not only lower the surface tension to ultralow value but also impact the mechanical equilibrium between neighboring phases. The simulation results indicate that the equilibrium pressure of coexisting phases may break down, although the interface with the compressed surfactant monolayer is roughly flat. The simulation results along with the thermodynamic analysis reveal that, different from the conventional capillary pressure that is induced by the interface curvature, the new type of capillary pressure (pressure difference) identified here is caused by the nonzero spontaneous curvature of the compressed surfactant monolayer. This nonzero spontaneous curvature-induced capillary pressure also shows a size effect, i.e., the capillary pressure decreases with the increase of monolayer size.