Ouzo effect: Insights from Gibbsian surface thermodynamics

Abstract

The Ouzo effect, significant in cosmetics, health products, and pharmaceuticals, involves the formation of droplets through solvent exchange which leads to the supersaturation of solutes. This research investigates the thermodynamics of the Ouzo effect in the water-cyclohexane-ethanol system, taking into account the effect of the curved interface of the droplet for better understanding and control of droplet formation and its characteristics. Phase diagrams were plotted considering both flat and curved interfaces, and free energy calculations for the droplet-liquid system were performed based on the droplet radius. We investigated the effect of various factors, including feed composition, supersaturation, initial mole amount, and interfacial tension, to determine the energy barrier and droplet size in stable and unstable states. Our calculations confirm the experimental observations that the presence of a curved interface during droplet formation shifts the equilibrium composition towards the spinodal curve. Results also demonstrated that a stable cyclohexane-rich or water-rich droplet can form, after passing an energy barrier (unstable equilibrium), provided that adequate supersaturation exists in the confined solution. Increased supersaturation lowers the energy barrier and reduces the droplet radius in unstable states, facilitating droplet formation. Conversely, higher supersaturation results in the formation of more stable droplets with larger sizes. Verification against existing experimental data qualitatively validates the presented model.