Interfacial tension of a salty droplet: Monte Carlo study

The interfacial tension of an aqueous droplet with 1:1 strong electrolytes is investigated by Monte Carlo (MC) simulations. The electrostatic interactions among ions are calculated directly by solving the Poisson equation based on the restricted primitive model. The surface excess and chemical potential are evaluated from MC. The latter can be reasonably described by the mean spherical model. The interfacial tension is then obtained through Gibbs adsorption isotherm. The addition of a strong electrolyte leads to the increase of the interfacial tension and the MC result accounts reasonably well for the observed interfacial tensions up to relatively high salt concentrations (∼1.0 M). For very dilute solutions, our results agree with the Onsager–Samaras (OS) limiting law. However, the OS theory underestimates the surface excess and the interfacial tension for high salt concentrations. It is found that hydration around ions plays a very important role in interfacial tension increment.