Transport across the surfactant covered interface of oil-in-water microemulsions

University Of Florida, Gainesville FL

Investigators

Abstract

Intellectual Merit of the Proposed Activity Microemulsions have received considerable attention due to the numerous applications in a wide variety of areas such as separations, reactions, drug delivery, and detoxification. In all the applications listed above, the process of mass transfer across the surfactant-covered interface plays a key role. In spite of the importance of the transport across the microemulsion surface, the detailed mechanisms of this process are not clearly understood. The goal of the proposed effort will be to utilize a multiscale simulation technique based on coarse grained molecular dynamics and kinetic Monte Carlo simulations to elucidate the solute transport mechanisms and their dependence on such parameters as surfactant length, surface coverage and curvature, and solute size and shape. The following two competing mechanism are anticipated: the transient channel formation and the solubility-diffusion mechanism. In the first case, a transient channel in the surfactant-covered interface is formed, connecting the oil and water phases, and the solute diffuses through this channel. In the second case, the solute first solubilizes within the surfactant layer at the interface and then undergoes diffusion, which takes place either through hopping between voids in the microstructure of the interface or through a bulk-like Brownian diffusion process. The first stage of the theoretical modeling will be aimed at detailed understanding of the internal structure and dynamics of the microemulsion interface, as characterized by void sizes, shapes, lifetimes, and dynamics of void and channel formation. Dynamics of relatively small voids will be investigated using coarse-grained molecular dynamics (MD) simulations. However, formation of larger voids and channels and the solute transport between the voids are expected to occur at timescales that are out of reach of direct MD simulations. Therefore, the proposed study will use the umbrella sampling technique to investigate these slow dynamics. The second stage of the proposed work will utilize the obtained void distributions and the rates of formation and destruction of voids to develop a kinetic Monte Carlo scheme to predict the solute transport rates. The combined MD and kinetic Monte Carlo simulations will be used to probe the dependency of the transport rates on various surfactant and solute parameters. The parameters of the coarse-grained molecular model will be optimized by matching the simulation results to experimental data for surface tension isotherms, bulk diffusivities, and phase transitions. Effects of the interfacial curvature will be investigated by varying the size of microemulsion droplets and by considering flat oil-water-surfactant interfaces. Theoretical predictions for the dependence of the solute transport rates on the microemulsion properties will be compared with experimental data. Broader Impacts of the Proposed Activity Microemulsions are commonly used in a wide variety of areas and a detailed understanding of the transport of molecules across the surface will help in development of better products, such as more efficient drug delivery vehicles, that could have a large societal impact. Since this project is a combination of fundamental and applied studies, it will provide a good learning experience for the undergraduate and the graduate students that will work on this project. Also the PIs propose to incorporate various aspects of this project into graduate and undergraduate classes.