Spontaneous Emulsification: Elucidation of the Local Processes

Abstract

While various attempts have been made to understand the mechanism of spontaneous emulsification within the scopes of equilibrium and nonequilibrium thermodynamics, the local processes underlying spontaneous emulsification still remain elusive. In this research, we investigate the local processes that involve the transfer of surfactants as well as water from an aqueous phase to oil, which results in the formation of a water-in-oil emulsion in the organic phase. Subsequently, these emulsions invert into the oil-in-water emulsion rather abruptly as they cross the phase boundary. Studies based on ultraviolet (UV) spectroscopy and nonequilibrium molecular dynamics simulations indicate that the crossing of the phase boundary may involve catastrophic explosions and subsequent assembly of the resulting fragments to other organized structures. These processes lead to the fluctuation of the component concentrations below the interface that also becomes evident in the fast (athermal) diffusion of the emulsion droplets from the interfacial region farther into bulk water. Spontaneous emulsification is found to be amplified in mixed solvents, but it can be arrested with additives that reduce solubility or inhibit the transfer of water and surfactants in the organic phase.