Molecular dynamics simulation of adsorption from microemulsions and surfactant micellar solutions at solid-liquid and liquid-liquid interfaces
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In enhanced oil recovery, the production is influenced by the ability of injected surfactants to adsorb and to modify interfacial tension at different interfaces. It has previously been demonstrated that surfactant and microemulsion additives enhance the production to a different extent but the mechanisms behind the differences in their action are not fully understood. Recent molecular dynamics simulation shows that the adsorption at a solid surface differs for these two systems. Furthermore, it shows that the solvent solubilized in a microemulsion is transported to the solid surface together with the surfactant, and hence the surface modification can be controlled by designing the chemistry and composition of the self-assembled structure. Here, we report salient new results on the adsorption from microemulsion at graphite-liquid, and liquid-liquid interfaces. The simulation involves an aqueous solution in the presence of an oil (heptane) phase. The solution consists of nonionic surfactant dodecylhepta(oxy-ethylene)ether or C12E7, and a solubilized terpene solvent. We found that the presence of solvent inside the micelles causes the mechanism of adsorption behavior to deviate from those expected for adsorption from micellar surfactant solutions. In the case of solubilized terpene, the swollen micelles adsorb on the surface as one entity. The delivery of a surfactant to the interface and the associated reduction of the interfacial tension is influenced by the change in interaction potential between the surface and surfactant aggregate, and it is controlled by the solvent concentration. Molecular dynamics simulation also reveals the complex distribution of fluids at the capillary wall. The terpene swollen micelle merges with the thin film of oil on the wall. The surfactant deposits on the interface between the aqueous phase and the oil, thereby reducing its interfacial tension. The solvent originally solubilized in a microemulsion droplet, penetrates the thin film of oil. The resulting mixture of oil and solvent has different properties from the oil alone, indicating a primary difference between the mechanism of action between surfactant and a combination of surfactant and solvent. For comparison, the same simulation conditions were applied to the case of C12E7micelles without solvent. As expected, the whole micelle did not adsorb at the interface. The results are important for our understanding of microemulsion behavior under confinement and its application to organic rich shale oil recovery.
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Technical Proceedings of the 2014 NSTI Nanotechnology Conference and Expo, NSTI-Nanotech 2014
