Atomistic Simulations of Electrolyte Solutions and Hydrogels with Explicit Solvent Models

Two of the most challenging tasks in molecular simulation consist in capturing the properties of systems with long-range interactions (e.g. electrolyte solutions), and of systems containing large molecules such as hydrogels. These tasks become particularly demanding when explicit solvent models are used. Therefore, massively parallel supercomputers are needed for both tasks. For the development and optimization of molecular force fields and models, a large number of simulation runs have to be evaluated to obtain the sensitivity of thermodynamic properties with respect to the model parameters. This requires both an efficient work flow and, obviously, even more computational resources. The present work discusses the force field development for electrolytes regarding thermodynamic properties of their solutions. Furthermore, simulation results for the volume transition of hydrogels in solution containing electrolytes are presented. Both applications are of interest for engineering. The present work proves that alkali and halogen ions can be reliably modeled by a LJ sphere with a superimposed charge located at the center of mass. The developed force fields allow predicting structural properties like the radial distribution function and the hydration number as well as thermodynamic properties like the density. The self-diffusion coefficient of pure water was determined using various well known molecular models of water. The agreement with experimental data is often poor. Furthermore, ion self-diffusion coefficients were determined in simulations using these different water models. No correlation was observed between the accuracy of the self-diffusion coefficients of pure water and the accuracy of the self-diffusion coefficients of the ions. Further research effort in developing a new water model is needed to gain accurate predictions of transport properties in aqueous electrolyte systems. In addition, the study shows that the self-diffusion coefficient of ions in aqueous solutions is almost independent of the LJ energy parameter ϵ of the ion. With the developed electrolyte models, it was possible to predict the volume transition of hydrogels in electrolyte solutions qualitatively and in some cases even quantitatively. The results also reproduce the effect of the Hofmeister series on the swelling of hydrogels.