Impact of Specific Counterion Binding on Surfactant Aggregates and Polyelectrolytes: Beyond Electrostatic Screening Effects

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Linda Magid of the University of Tennessee Knoxville is supported by the Experimental Physical Chemistry Program for research that focuses on how changes in counterion structure change the three-dimensional structure (shape and conformation) of surfactant aggregates, and how these changes affect the macroscopic properties (such as viscosity, detergency, and wetting) of their solutions. Surfactants self-assemble in aqueous solution to form aggregates with a variety of molecular shapes, including spheres, extended wormlike structures, branched networks of worms and vesicles, and bilayer structures. Ionic surfactant aggregates carry a net electrical charge and have a variety of similarities to conventional polyelectrolytes. At the core of this research is a technique for tuning the aggregates' structures by changing the chemical structure of the surfactant ions. Special emphasis is placed on understanding which structural features lead to aggregate branching, and which lead to shrinking of wormlike structures back to spheres. Scattering methods (light, x-rays, and neutrons), microscopy, and nuclear magnetic resonance spectroscopy will be used to investigate both the microstructure and dynamics of aggregates. Deuterated surfactants and polyelectrolytes will be synthesized for neutron scattering experiments. Teaching and training will be promoted through two curricular development activities benefiting high school students and undergraduates. Understanding the properties of surfactant aggregates is of significant fundamental as well as practical interest. Surfactant aggregates are used widely in personal care products, drug delivery, and a variety of other applications. A long-term goal of this research is to establish a "chemical dictionary" that allows surfactant scientists to understand how bulk properties like viscocity can be manipulated by simple changes in counterion content. In addition, this research has important connections to understanding the impact of counterions on the flexibility of biological molecules such as DNA.