Abstract:   Ions are essential in physical chemistry, colloidal science, electrochemistry, biology and many other areas of science and engineering.  While their role is commonly described in terms of screening and translational entropy, many phenomena, ranging from some classical experimental observations made many decades ago to some new systems of current interest, cannot be explained, even qualitatively, by these concepts.  A key effect that is often ignored or inadequately treated in the main literature on electrolytes and polyelectrolytes is the self-energy of the ions.  In this talk, I will discuss several self-energy effects in macromolecular and interfacial systems.   First, we show that the preferential solvation energy of the ions provides a significant driving force for phase separation.  This concept is used to develop a theory to explain the dramatic shift in the order-disorder transition temperature in PEO-PS diblock copolymers upon the addition of salt.  Second, we show that the dielectric contrast between the polymer backbone and the solvent significantly affects the conformation and charge condensation in dilute polyelectrolyte solutions.  Third, we show that the image force has qualitative effects on the double layer structure and forces, such as like charge attraction and charge inversion. Finally we present a simple model for understanding the specific ion effects in the interfacial activity for air/water and oil/water interfaces.

Bio:  Zhen-Gang Wang received his bachelor’s degree in Chemistry from Beijing (Peking) University in 1982, and his Ph. D in Chemistry from the University of Chicago where he worked with Stuart Rice and Karl Freed.  He then worked as a postdoctoral fellow first in Exxon Research and Engineering Company, with Sam Safran and Tom Witten, and then at UCLA with Bill Gelbart.  In 1991, he joined the Chemical Engineering faculty at Caltech, was promoted to Associate Professor in 1997 and became Full Professor in 2002.  Zhen-Gang Wang’s research is the theoretical and computational study of structures, phase behavior, interfacial properties and dynamics of polymers, soft materials, and biophysical systems.  His current activities revolve around three main themes: charged systems, nucleation or more generally barrier crossing, and nonlinear rheology of polymers gels and entangled polymers.