Nanofoam-like structure of surfactants in oil-water mixtures

[Display omitted] •Computer simulations demonstrate stability of various self-assembled morphologies in aqueous solutions of surfactants.•Small fractions of oil in the solution are absorbed in the aggregates’ interior.•Oil absorption by the worm-like micelles leads to their decomposition into the spherical micelles.•Vesicles and lamellae transform into nanofoam-like structure (oil- and water-rich voids) upon addition of oil. Addition of small portions of oil in aqueous solutions of surfactants self-assembled into different morphologies is studied with computer simulations. Uptake of oil by the spherical micelles leads to their swelling, decrease in the number of the micelles which keep their spherical shape. On the contrary, initial self-similar (keeping elongated shape) swelling of cylindrical micelles takes turn by their decomposition into the spherical micelles caused by the Rayleigh instability upon increase of the oil concentration. As a result, the number of the micelles in the system non-monotonously changes with oil concentration: initial growth in the number is followed by the diminishing. Vesicular and lamellar solutions are shown to reveal quite different behavior. At first, the wall thickness of the vesicles and lamellae increases due to the up-taken oil which leads to enlargement of the vesicles in the solution. Then, the vesicles aggregate forming nanofoam-like structure: nanobubbles are glued in the foam by means of oil-rich domains. Similar behavior is observed for the lamellar structure, when some of the lamellae bend and bind with neighbor ones creating isolated water- and oil-rich domains. We can expect that in contrast to the cylindrical micelles, whose viscosity dramatically decreases with the increase of the oil concentration, vesicular and lamellar solutions behave oppositely increasing viscosity upon addition of oil.