Engineering properties of fluid interfaces

In colloidal multiphase systems the ratio of the total surface area to the volume of materials is large enough for the surface properties of the dispersed phase to affect the overall bulk properties. Therefore, controlling interfacial properties is of great importance to many industries. Surfactants and polymers are commonly used to modify properties of fluid/fluid interfaces, mainly in reducing the interfacial tension, increasing dilatational elasticity, and changing wetting properties.

Adsorption of polyelectrolyte-surfactant aggregates (PES) of poly (cetyltrimethylammonium vinyl benzoate) (pCTVB) to the oil/water interface is studied and compared to the results at the air/water interface. The surfactant drives adsorption to the interface and the polymer forms an elastic layer at the interface. The critical aggregation concentration is lowered due to the limited but non-negligible partitioning of the oil (isopar-M) to the water phase. The surfactant molecules pack more efficiently at the oil/water than the air/water interface. The aggregates are more strongly adsorbed to the oil/water interfaces, which is attributed to the hydrophobic tails of the surfactant molecules being solubilized in the oil phase and anchoring the aggregates to the interface. Finally, pCTVB aggregates solubilize hydrophobic species (Nile Red) in the aqueous phase and facilitate transport across the oil/water interface. This creates a potential new application for a PES system that stabilizes oil/water interfaces for long enough to allow for efficient transport of solubilized species that could be useful in pharmaceutical, food, personal care, coatings, and other industries.

Binary copolymers are often used as surfactants due to their amphiphilic nature, however the impact of the monomer sequence on interfacial properties is poorly understood. Two sets of polypeptoids of identical chemical composition but four different monomer sequences are used to study the effect of monomer sequence. Interactions between the polypeptoid and solvent are utilized to drive the polymer to the air/water interface which leads to a consistent decrease in interfacial tension and low dilatational elasticity. Once polymer is adsorbed, a coil-to-globule collapse is induced which leads to significant increase in dilatational elasticity with a first order effect of the monomer sequence. These results indicate that with a properly dialed chemical composition and sequence surface activity and mechanical properties of interfaces can be controlled independently. This work is a foundational step towards developing polymeric surfactants with reversibly switchable properties for selective emulsion stabilization.

Emulsion stability is difficult to predict a priori due to the poorly understood complex interactions between water and oil phases mediated by the emulsifiers. Stability depends on the properties of the oils and emulsifiers but the relationship is unclear. Formulation often relies on a trial-and-error approach which requires a lot of experimental work. A machine learning based methodology is developed to address this limitation. A moderate throughput experimental screening method is developed to generate a training set. The model is trained on a small sample size (256 samples) of emulsions to predict emulsion stability based on the physical properties of the oils and emulsifiers for an industrially relevant water phase with a high accuracy (MAE = 0.137) when emulsion type is used as feature and MAE of 0.206 for the most rigorous modeling scheme. The results of this work enable rapid prediction of formulations likely to yield stable emulsions with a limited need of experimental work.