The objective of this project is to determine the influence of water chemistry (i.e. potential of hydrogen [pH], salt concentration), temperature, and particulate and polymeric additives on the stability of model oil-in-water emulsions. The research team will achieve this object through a combination of interfacial rheometry, zeta-potential and dynamic light scattering measurements, and in situ flow visualization microscopy. While it is largely understood how simple oil-in-water emulsions are stabilized, there are fundamental knowledge gaps with respect to the formation and long-term stability of complex oil-in-water emulsions that arise from shipboard operations. These complex emulsions, which contain a range of oil-based moieties, are stabilized by a mixture of common industrial and natural surfactants. As a result, the breaking and separation of these emulsions is difficult and costly. Advancing the knowledge of physicochemical factors that impact emulsion stability will enable new technology to be developed for advanced water treatment of stable oil-in-water emulsions. This project intends to apply advanced measurement tools to characterize the physicochemical processes which drive the formation and breaking of surfactant stabilized oil-in-water emulsions.

The long-term goal of this project is to characterize the droplet-droplet interactions and coalescence for complex oil-in-water emulsions. Specifically, researchers intend to quantify the fundamental structure-property relationships between the viscoelasticity of surfactant-laden immiscible fluid interfaces and the coalescence behavior observed between concentrated oil-in-water droplets. As a step toward this goal, the objective of the project is to understand how the physical and chemical attributes of surfactants and emulsions directly impact the kinetic stability at the droplet level.

The central hypothesis is that some polymer or surfactant additives are able to dynamically disrupt the chemical stabilizing shell surrounding small oil droplets and specifically, the coalescence of emulsion oil droplets in aqueous solution will strongly depend on the viscoelastic properties exhibited by the surfactant-stabilized oil-water interface under controlled system and environmental conditions. By measuring the rheological response of the surfactant-laden interface as a function of chemical composition of the interface and surrounding media, fundamental structure-property relationships can be developed to describe emulsion stability. To test this hypothesis through deliberate experimentation, the following specific aims will be pursued:

1. Quantify the effect of water pH, salinity, and temperature on the rheology (dilatational viscoelasticity) of model oil-water interfaces for nanoscale and microscale droplets containing adsorbed surfactant molecules with known physical and chemical structures and controlled intermolecular associations.
2. Correlate the measured interfacial viscoelasticity of surfactant-stabilized, oil-water interfaces with the observed rate of droplet coalescence between close-packed oil droplets in model two- and three-dimensional, concentrated emulsions.
3. Develop predictive models to relate the rheology and physicochemical properties of oil-water interfaces with the kinetic stability of dispersed oil-in-water emulsion droplets across a range of droplet diameters.

At the conclusion of this work, the research team will have a fully characterized dataset and predictive stability model for assessing contaminated bilgewater and shipboard emulsions in real-world marine environments. The outcomes of this work can be used by Department of Defense (DoD) operators to rapidly assess the physicochemical nature of specific shipboard emulsions within a vessel’s bilgewater as well as provide guidance on the best practices to destabilize the emulsions. Additionally, the outcomes of this work will provide improved guidance to DoD operations managers for optimizing their detergent formulations and/or cleaning procedures in order to facilitate end-of-use OW separations prior to bilgewater discharge that are effective but less time and energy intensive.