Computational investigation of a switchable emulsion stabilized by the mixture of a surfactant and tertiary amine

Molecular dynamics simulations were performed to investigate the CO2-responsiveness of an oil-in-water (O/W) emulsion stabilized by sodium oleate (NaOA) with a tertiary amine additive, named pentamethyl diethylenetriamine (PMA). The simulated results were in accordance with the experimental observations. That is, the surfactant NaOA itself can stabilize dodecane/water emulsions in aqueous solution, while the CO2-reponsiveness was strongly related to the added PMA. The electroneutral PMA molecules preferred to be located in the core region of the droplets. Thus, under the same conditions, the size of the droplet containing PMA is predictably larger than that without PMA. The increased extent of the charged surfactant headgroups distribution can increase the electrostatic repulsion between the droplets in the emulsion solution, which is the important reason why a much more stable emulsion is obtained by adding PMA. When PMA molecules were protonated to PMA2+ by bubbling CO2, they migrated from the interior to the surface of the droplets under electrostatic attraction, forming ion pairs with OA-. The binding between PMA2+ and OA- made the distribution of the surfactants very concentrated on the droplet surface, leading to large hydrophobic areas exposed to water. Besides, the hydration interactions of OA- headgroups decreased because they were covered by PMA2+. The calculated potential of mean force (PMF) confirmed that the electrostatic repulsion between droplets was crucial for the emulsion stabilization.