(355j) Informing the Design of New Oil Dispersants through Studies of Dispersant-Oil Mixtures' Dynamic Interfacial Tension and Microstructure

Oil dispersants are mixtures of surfactants and solvents which are commonly applied to marine crude oil spills around the world in order to facilitate their dispersion, dilution, and biodegradation in the ocean. The relatively high surfactant content of typical dispersant-treated oil slicks (1-5 wt%) raises the question of whether dispersants’ surfactants form water-in-oil microstructures in crude oil slicks, but the native surfactants and aggregates found in crude oils obfuscate direct, in situ investigation of this possibility. A “model crude oil” composed of nonpolar solvents similar to those found in South Louisiana crude oil has therefore been developed to facilitate the study of water-in-oil microstructures formed by oil spill dispersants in crude oil slicks. Various blends of the surfactants Tween 80, Span 80, and dioctyl sodium sulfosuccinate (DOSS) which have previously been shown to be effective oil dispersants were dissolved in this model oil at overall surfactant concentrations of 2 wt% (~25 mM), typical for dispersant-treated oil slicks and greater than concentrations at which reverse micelles have previously been reported for DOSS (~1 mM) and for Tween 80/Span 80 mixtures (~10 mM) in nonpolar solvents. Synthetic seawater was added to the surfactant-oil mixtures at various seawater:surfactant ratios, ranging from the minimum amount required to solubilize all of the surfactants into the model oil up to the maximum amount soluble in each surfactant blend. Initial and dynamic interfacial tensions measured between synthetic seawater and these surfactant-seawater-model oil solutions were similar to those observed between synthetic seawater and solutions of the same surfactant blends in crude oil, including (and most notably) at those surfactant mixture compositions which produce ultralow (<10^-3 mN/m) interfacial tensions. Dynamic light scattering and cryogenic transmission electron microscopy revealed that the surfactants and seawater form monodisperse spherical water-in-oil microstructures in the model oil; the dependence of the microstructures’ diameters on surfactant blend composition and on seawater:surfactant ratio was explored. Such microstructures likely play a key role in solubilizing hydrophilic components of dispersants, such as Tween 80, into dispersant-treated oil slicks, and point to the importance of incorporating surfactants which readily form reverse micelles, such as DOSS, into dispersants.