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Phase behavior and mechanical properties of catanionic surfactants. The term catanionic surfactants has been coined to refer to a mixture of anionic and cationic surfactants. For decades ionic surfactants have been used in industry, largely to reduce surface tension between immiscible phases. Surprisingly, mixtures of cationic and anionic surfactant have received little attention from researchers until recently. Earlier researched focused largely on equimolar mixtures of cationic and anionic surfactants, which tend to form precipitates and yield little interesting behavior. When mixed at ratios other than their equimolar compositions, catanionic surfactants yield a rich phase behavior. Most importantly, the microstructures arise spontaneously and persist indefinitely. Thus, they are equilibrium phases. This is in sharp contrast to many non-equilibrium surfactant phases studied in other systems. Phospholipid vesicles, for example, are currently the focus of much research as a proposed means of pharmaceutical delivery. While structurally similar to catanionic vesicles, phospholipid vesicles must be formed through physical or chemical means such a sonication or extrusion. Phospholipid vesicles are stable for long periods of time, but eventually revert to a flat lamellar equilibrium phase. Such reversion to a lamellar phase does not occur with catanionic vesicles. Catanionic surfactants have many potential uses. Already, they are used in detergents and to reduce the energy lost to turbulence when pumping fluids (such as crude oil) long distances. Catanionic vesicles have the potential to be used as microreactors, where chemical reactions may take place under highly controlled conditions. Since they closely resemble the phospholipid bilayers that form cell walls, catanionic bilayers may also be used to model membranes for biological systems. Since catanionic surfactant self-assemble with highly ordered microstructure, it is hoped they might be used as templates for novel materials. Already, hydrophobic polymer monomers have been introduced into catanionic vesicle systems. These monomers may then be cross-linked to form hollow polymer spheres on the scale of hundreds of microns in diameter. Similarly, it is hoped that surfactants might be used to form nanoporous materials and materials with novel magnetic properties. Finally, mixtures of amphiphilic polymers and surfactants can yield hydrogels. These hydrogels may be predominantly water, but still demonstrate extraordinarily high viscosities. The focus of this research project is to understand how the structure of surfactant molecules and the mixing ratios of anionic and cationic surfactants determine the mechanical properties of catanionic surfactant bilayers. Three parameters are used to describe the mechanical properties of bilayers: the natural radius of curvature, the bending modulus, and the saddle-splay deformation modulus. The natural radius of curvature is simply the favored curvature of the bilayers. The bending modulus is proportional to the energy needed to bend the bilayer away from this natural radius of curvature. The saddle-spay deformation modulus is proportional to the energy needed to form a saddle-point in the bilayers. Return to the RISE 2000 project list |
