Electrically charged polymers, called polyelectrolytes, are common in biology and include DNA, RNA, proteins, and mucous layers. They are increasingly used for applications ranging from drug delivery to membranes for sensing or batteries. Membranes of polyelectrolytes for these applications can be built up, layer by layer, by sequentially dipping a surface into a negatively charged polyelectrolyte followed by dipping into a positively charged one. Additionally, gels swollen with water, known as "coacervates", can be made by mixing two oppositely charged polyelectrolytes. Despite the importance of these materials in biology and advanced applications, their behavior is poorly known, and neither the properties of coacervates nor the rate of growth of layer-by-layer membranes is understood at present. While promising theories for such polyelectrolyte materials have recently been developed, there is little systematic experimental data to test and confirm these theories so as to help design advanced materials. To provide such tests, this project will use both simple and advanced experimental methods to measure the composition of coacervates, and the growth rate and thickness of layer-by-layer films. These measurements will be carried out systematically with varying salt concentration and pH to establish quantitative trends needed to test and confirm newly developed theory and provide a firm base for design of advanced materials made from polyelectrolytes. Such knowledge is also relevant in biological systems, including the interactions of positively charged proteins with negatively charged DNA, which controls the structure and function of chromosomes. Beyond the research, broader impacts of this project will include the education of graduate and undergraduate students, outreach, and development of specialized computational codes relevant to this topic.
To understand and test theories for the equilibrium and dynamics of assemblies of oppositely charged polyelectrolytes, several phases of study will be performed. First, the phase behavior including compositions of individual polyions in both coacervate and supernatant phases for four common polyelectrolyte mixtures will be measured by high pressure liquid chromatography, proton NMR, and other methods. The results will be compared to the predictions of new theories, and used to test and improve these theories. Titrations with acid and base will be used to determine ion pairing equilibrium constants, ionization equilibrium, and thermodynamic "chi" parameters. In addition, Layer-by-Layer (LbL) growth rates for these polyelectrolytes will be measured and the thermodynamic information determined by theory and phase behavior will be used to predict these growth rates. The diffusivities of polyions through the polyelectrolyte multilayer needed for predictions of LbL growth will be inferred from strengths of ion pairing obtained from atomistic molecular dynamics (MD) simulations of a polyanion and polycation in water and salt.