UC Riverside

Charge regulation of monomers, polymers, and surfaces have attracted great interest in recent years because of its fundamental and practical importance to many technological applications such as bioadhesion and drug delivery. The ability to tune the properties of these ionizable species opens avenues to “smart” systems which can respond to changes in pH, salt concentration, and other environmental factors. A better understanding of when and why the electrostatic charge of these species emerges and fluctuates will allow for the more rational design to meet targeted needs. Unfortunately, the chemistry at the microscopic scale is difficult to discern and an adequate theoretical tool is needed to provide insights into the mechanisms governing the charge regulation of these species.

The purpose of this dissertation is to advance the knowledge of the charge regulation through the innovation of new theoretical methods to characterize the properties of ionizable systems. In particular, we emphasize the applications of our new theoretical methods to describe the interfacial phenomena. These new tools pave the way towards realistic models to understand bioadhesion, thermal energy storage, drug delivery, waste-water treatment and many more applications. We first developed a molecular thermodynamic model that is able to describe the charge regulation and activity coefficients of amino acids in excellent agreement with experimental data. Next, we extended the thermodynamic model using classical density functional theory to predict the interfacial behavior of amino acids at inorganic surfaces. Different from conventional methods, our model accounts for key correlation effects that dictate the charge behavior of amino acids near an interface. We also developed a coarse-grained model that can be used to predict the ionization of weak polyelectrolytes in a solution with physically realistic parameters. In order to describe weak polyelectrolytes in non-uniform fluids such as near a surface, we developed a theoretical tool known as the Ising density functional theory (iDFT), which bridges the gap between the site-binding model and polymer density functional theory. We demonstrated that iDFT is able to capture the adsorption isotherms of polypeptides near ionizable inorganic surfaces in good agreement with experimental data. The last two chapters of this dissertation are focused on theoretical developments for incorporating long-range intrachain correlations that are neglected in conventional treatments of polymers in a bulk solution or near an interface. The inclusion of long-range interactions provides an accurate description of the coupling of polymer charge and conformation in weak polyelectrolyte systems. Lastly, we combined iDFT with the single-chain-in-mean-field algorithm to capture the long-range two-body correlation effects in non-uniform fluids. The computational framework developed in this dissertation opens up new opportunities for engineering design of ionizable molecular systems.