(203b) Accurate Determination of Osmotic Pressure for Monovalent Ions Using Osmotic Force Balance and Chemical Potentials

Concentrated aqueous electrolyte solutions are commonly found in numerous biological and environmental scenarios of significance. Whether in physiological processes within living organisms or in the dynamics of ecosystems, understanding the behavior of these solutions is of high importance. In this context, the development of precise force field models that accurately describe the interactions among dissolved species is indispensable. These models serve as the cornerstone for simulating complex systems involving concentrated salt solutions, facilitating a molecular-level comprehension of the underlying processes, particularly in saline environments where the interactions between ions and water molecules play a crucial role. At the heart of understanding the behavior of aqueous electrolyte solutions the concept of osmotic pressure lies - a fundamental thermodynamic property. Osmotic pressure provides a direct link between experimental observations and computational simulations, offering insights into the associative properties of ions in solution. By further refining existing methodologies for determining osmotic pressure, researchers aim to enhance the accuracy of their simulations and deepen their understanding of electrolyte solutions. In this study, we have further developed Milner’s approach based on which a restraint potential is exclusively applied to the ions within the solution. This approach, named the osmotic force balance, entails determining the pressure required to counterbalance this potential. Using molecular dynamics simulations, concentrations of the various components present in the simulated system are determined. Leveraging this data alongside a modified Debye-Hückel equation, we are able to fit the chemical potentials of the dissolved salts as functions of concentration. This allows for the determination of osmotic pressures not only for common salts like NaCl but also for less studied compounds such as CsBr, RbI, and LiCl in aqueous environments over a wide range of concentrations, up to ~ 3.5 M (corresponding to ~ 4 molal). To accurately replicate experimental osmotic pressures observed for these salts, we undertake the optimization of cross interactions between cations and anions within the solution. Our computational approach is of high efficiency as it requires only a single simulation run to estimate osmotic pressures across a wide range of salt concentrations. Additionally, the modified Debye-Hückel has proven to effectively describe salt concentrations and their osmotic pressures. Moreover, the optimized Lennard – Jones ion potentials from this work hold significant value for researchers, as they eliminate the need for additional computational time typically required to determine osmotic pressure for these salts. Furthermore, our approach opens avenues for exploring more complex systems, such as the behavior of hydrophobic molecules in aqueous environments.