Spontaneous Emulsification: Elucidation of the Local Processes.

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.

Tubulation of Supported Lipid Bilayer Membranes Induced by Photosensitized Lipid Oxidation.

We show that photosensitized phospholipid oxidation, initiated by the lipid-conjugated fluorophore TopFluor-PC, causes defects, namely, membrane tubes and vesicle-like structures, in supported lipid bilayers (SLBs). Lipid oxidation is detrimental to the integrity of the lipid molecules; when oxidized, they undergo a conformational expansion, which causes membrane tubes to protrude from the SLB. Lipid oxidation is verified by FT-IR spectroscopy, and area expansion is observed in Langmuir trough experiments. Upon growing to a critical length, the membrane tubes arising from SLBs rapidly undergo transition to vesicle-like structures. We find a correlation between the maximum tube length and the diameter of the resulting vesicle, suggesting the conservation of the surface area between these features. We use geometric modeling and the measured tube length and vesicle radius to calculate the tube radius; our calculated mean tube diameter of 243 nm is comparable to other groups' experimental findings. In the presence of fluid flow, membrane tubes can be extended to tens to hundreds of microns in length. SLBs composed of saturated lipids resist light-induced tubulation, and the inclusion of the lipophilic antioxidant α-tocopherol attenuates the tubulation process and increases the light intensity threshold for tubulation.

Thermodynamic and Kinetic Pathways to Agitated and Spontaneous Emulsification.

Emulsification of an oil (dodecane and diesel fuel) in salinized water was studied under turbulent and agitation-free conditions in the presence of a mixture of an ionic and a nonionic surfactant. The properties of the air-water and the oil-water interfaces were investigated using the methods of du-Nouy ring, drop resonance vibrometry, and Langmuir film balance that allowed pinpointing the relevance of certain interfacial properties in emulsification. Estimation of the droplet size and its distribution from the nanometer-to-micrometer range was carried out with optical microscopy, acoustic attenuation spectroscopy, and continuous hydrodynamic flow fractionation. These measurements provided the platform for the comparison of the emulsion droplet size with those predicted from the fluctuation of the dynamic stress in the turbulent water via a capillary hydrodynamic model. While such a comparison was reasonably meaningful for micron size emulsion droplets, production of nanometer size droplets was beyond such a rudimentary expectation. We thus carried out systematic investigations into other factors that contribute to emulsification under both agitated and agitation-free conditions. An important finding of these studies is that the infusion of air bubbles that profoundly enhance the hydrodynamic fluctuation produces mainly submicroscopic emulsion droplets, while a fluctuation inhibiting water-soluble polymer has the opposite effect. Furthermore, while a hydrophilic polymer dissolved in water enhances the ripening of the droplets with time, hydrophobic polymer in oil thwarts aging, plausibly by osmotic backpressure and interfacial stiffening, which, upon compression, acts against surface tension, thereby decreasing the chemical potential of the trapped oil molecules inside the droplet. These effects are similarly observed in spontaneous emulsifications, that is, when a layer of oil containing the additives is deposited upon the surface of the aqueous phase in the absence of any external work input.