Grant: Discovery Grants Program - Individual

Chemists and many other scientists use techniques such as magnetic resonance spectroscopy to investigate molecular motions in solutions at uniform composition. The proposed diffusion research is different. We study gradient diffusion processes driven by changes in chemical composition. This kind of diffusion is important in a wide range of practical applications because it provides mass transport and mixing on the molecular level, and therefore controls the rates of many physical and chemical processes, such as chemical reactions, crystal growth and dissolution, gas absorption, detergency, chemical signaling, and the spread of pollutants. We measure diffusion by monitoring the dispersion of solution samples injected into flowing carrier solutions of different composition. This proven technique will be used to study multicomponent solutions of polymers, electrolytes, proteins, detergents, microemulsions and other materials of scientific and technological interest. Diffusion in these systems is strongly coupled. A diffusing protein molecule, for example, can transport hundreds or even thousands of salt ions. Protein diffusion will be measured at different pH values to provide new information about transport in biological systems, such as proton-coupled transport. To help understand why diffusion is strongly coupled, osmotic pressure measurements will be used to determine the fundamental thermodynamic driving forces for diffusion. Another goal is to accurately describe propagating chemical waves by including migration in the electric field generated by the transport of ions of different mobilities. Laser light scattering is frequently used to characterize polymers, proteins and other colloids by analyzing microscopic concentration fluctuations produced by diffusion. Our work will establish the correct interpretation of popular light-scattering measurements by comparing light-scattering and gradient diffusion data for a series of well defined multicomponent solutions. To solve the longstanding problem of measuring gradient diffusion in many-component systems, infrared spectroscopy will be used for solute-specific detection across diffusion profiles, which has considerable advantages over conventional bulk refractive-index profiling.