Colloid and Surfactant Science

Interfacial Energetics of Dynamically Reconfigurable Complex Emulsions

Emulsification is a powerful age-old technique for mixing and dispersing immiscible components within a continuous liquid phase.  Consequently, emulsions are central components of medicine, food, and performance materials.  Complex emulsions, including multiple emulsions and Janus droplets, are of increasing importance in pharmaceuticals and medical diagnostics, in the fabrication of microparticles and capsules for food, in chemical separations, for cosmetics, and for dynamic optics.  As complex emulsion properties and functions are related to the droplet geometry and composition, the development of rapid and facile fabrication approaches allowing precise control over the droplets’ physical and chemical characteristics is critical.  Significant advances in the fabrication of complex emulsions have been accomplished by a number of procedures, ranging from large-scale less precise techniques that give compositional heterogeneity using high-shear mixers and membranes to small-volume microfluidic methods.  However, such approaches have yet to create droplet morphologies that can be controllably altered after emulsification. Reconfigurable complex liquids potentially have greatly expanded utility as dynamically tunable materials. 

Figure 1: Temperature-controlled phase separation of hydrocarbon and fluorocarbon liquids can be used to create complex emulsions. a, Schematic of complex emulsion fabrication. b, Above Tc, hexane and perfluorohexane are miscible and emulsified in 0.1% Zonyl (top left). Below Tc, hexane and perfluorohexane phase separate to create a hexane-in-perfluorohexane-in-water (H/F/W) double emulsion (bottom right). Hexane is dyed red. Scale = 200 µm. c, Emulsions of uniform composition made by bulk emulsification (such as shaking). Scale = 100 µm. d, XY confocal cross-section of H/F/W double emulsion droplets. Hydrocarbon-soluble Nile Red dye (green) selectively extracts into hexane. Rhodamine B dyes the aqueous phase (red). Scale = 100 µm. Monodisperse droplets in b and d were made using a micro-capillary device.

Using theories of interfacial energetics, we have modeled the interplay between interfacial tensions during the one-step fabrication of three- and four-phase complex emulsions displaying highly controllable and reconfigurable morphologies. The fabrication makes use of the temperature-sensitive miscibility of hydrocarbon, silicone, and fluorocarbon liquids and is applied to both microfluidic and scalable batch production of complex droplets.  We demonstrate that droplet geometries can be alternated between encapsulated and Janus configurations via variations in interfacial tensions as controlled with hydrogenated and fluorinated surfactants including stimuli-responsive and cleavable surfactants. Therefore, we have discovered a generalizable strategy for the fabrication of multiphase emulsions with controllably reconfigurable morphologies to create a diversity of responsive materials.

Figure 2: (Top) Hexane-perfluorohexane droplets reconfigure in response to variation in the concentration of Zonyl as it diffuses through 0.1% SDS. Scale  =  100 µm.  (Bottom) Chemical structure of the light-responsive surfactant which reversibly isomerizes under UV and blue light between the more effective trans form of the surfactant (left) and the less effective cis form (right). Aligned beneath are optical micrographs of hexane-perfluorohexane emulsions that are tuned to undergo specific morphological transitions in response to light. Hexane is dyed red, and the aqueous phase consists of Zonyl and the light-responsive surfactant pictured.

Predicting interfacial tension by combining molecular dynamics simulations with molecular-thermodynamic theory

The reduction in interfacial tension by surfactants underlies several natural phenomena in multi-phase systems including emulsions such as paints, cosmetics, and yogurt as well as foams. This effect is also important for many industrial processes such as spray painting, emulsion polymerization, distillation in packed bed columns, and froth flotation.

For systems where interfacial tension values cannot be readily determined experimentally, estimates can be obtained by using one of the several adsorption isotherms available in the published literature. All of these adsorption isotherms, however, contain several empirical parameters that can only be determined by fitting the adsorption isotherms to experimental data. With this in mind, we propose a modeling methodology that can reliably predict the interfacial tension for different surfactants, and their mixtures, solely from the surfactant molecular structures and the solution conditions, without the need for experiments. Using such predictions, one can use the existing models for foam and emulsion stability, particle size distributions, and wettability, to predict the performance of novel surfactants, in industrial applications such as foaming, wetting, or emulsification, even before these surfactants are synthesized.

Figure 1: By combining free energy values determined from simulations of surfactant molecules at interfaces with molecular thermodynamic (MT) theories for these molecules in the bulk, we can predict interfacial tensions as a function of the system composition.

Selecting an optimal surfactant formulation for the extraction of phosphate from the mixture of phosphates (apatite), silicates, and carbonates (e.g. calcite) that comprise phosphate ore is just one of the many industrially relevant challenges that can be addressed using this predictive modeling approach.

Figure 2: Phosphate separation technologies such as froth flotation are controlled by the relative adsorption affinities of surfactant molecules for silica/calcite/apatite surfaces.