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Our group has developed a novel density functional theory (DFT) for multi-scale modeling of complex fluid assemblies.  To allow for the wide use of the DFT, our group has worked with Sandia National Laboratory to include our free energy functionals in Sandia's open source Tramonto package. Tramonto is a program for 3 dimensional DFT calculations in a variety of geometries.  In this project we are studying the interfacial behavior of amphiphilic molecules.  This research builds on our polymer DFT that has shown great promise for predicting interfacial properties and microstructure of block copolymer solutions and blends.

Predictions of the behavior of mixed surfactant systems (e.g., oil / water / surfactant systems) remain a challenge with applications to self-assembly of templates for nanostructured materials and to enhanced oil recovery.  We have undertaken a systematic study of how surfactant structure affects interfacial tension and microstructure.  We begin by considering a simple model that was first proposed by Telo da Gama and Gubbins (Molecular Physics, 1986. 59(2): p. 227).  The "oil" and "water" molecules are represented by attracting spheres with self-affinity while the "oil-water" interaction is purely repulsive.  The surfactant molecules are represented as a chain of these same "oil" and "water" beads bonded together.  Using such a model, we are studying the effect of surfactant structure (including the number of beads of each type, bead position, and branching) on interfacial tension and the structure of the interface.  Results show excellent agreement with molecular simulations for the interfacial tension and the surfactant structure in the interface.  We also find that linear single-tailed surfactants produce lower interfacial tension than branched double-tailed surfactants with the same number of beads at a given bulk surfactant concentration.  Since interfacial tension is affected by the surface coverage of surfactant molecules on the interface, this result indicates that linear surfactant molecules more readily partition to the interface.  Studying the equilibrium microstructure also shows a tendency of the surfactant to collapse or pack tightly near the interface rather than stretch into the solvent as commonly depicted.  This effect is exaggerated in the "oil" phase when a more realistic chain is used for the "oil" solvent.  Here, the more restrictive packing effects on the "oil" side of the interface require more surfactant to be dissolved in the bulk "water" phase despite being energetically unfavorable to achieve the same partitioning to the interface and the same interfacial tension lowering. Results also show that branched surfactants with non-symmetric tail lengths produce greater reductions in interfacial tension than those with the same number of tail segments and symmetric tail lengths.  Both of these results agree with experimental observations.  Further, for a given interfacial tension, the log of the bulk concentration of the surfactant in the water phase varies linearly with increasing tail length.  This again is consistent with experimental observations.  In further calculations we will study the effect of changing the head group size and also consider how interfacial tension changes with concentration in mixed surfactant systems.