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Our group has developed a novel density functional theory for multi-scale modeling of complex fluid assemblies.  To allow the wide use of the density functional theory, our group has worked with Sandia National Laboratory to include our free energy functionals in Sandia’s open source Tramonto package.  Tramonto is a 3 dimensional density functional theory in a variety of geometries.  In this project we are studying the interfacial behavior of amphiphilic molecules.  This research builds on our polymer density functional theory (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) remains 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 structure and tension.  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.  The “oil-water” interaction is only 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 simulation results for the interfacial tension and the surfactant structure in the interface.  We also find that at a given bulk surfactant concentration linear surfactants produce lower interfacial tension than branched surfactants with the same number of beads.  Since interfacial tension is affected by the area density of surfactant molecules at the interface, this result indicates that linear surfactant molecules more readily partition to the interface.  Results also show that branched surfactants with non-symmetric tail lengths produce greater reductions in interfacial tension than 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 the structure of the oil phase.  We will also consider how interfacial tension changes with concentration in mixed surfactant systems.