New theoretical tool allows for direct and simple measurement of bulk electrolyte solutions

Understanding the fundamental physical behavior of electrolytes in water is crucial for a broad range of scientific applications, such as biology, geoscience and energy storage materials. Molecular simulations therefore have become an increasingly important tool thanks to the microscopic insight they offer in understanding the behavior of electrolytes in solutions. However, many molecular simulation methods encounter challenges posed by Coulombic interactions, especially when used in periodic boundary conditions—a typical simulation setup for the finite system sizes used in computer models.

In a recent paper, Cox and Sprik applied a new theoretical method called the finite field formalism to investigate behaviors of different concentrations of bulk aqueous electrolyte solutions via computer simulations. Originally developed for the study of ferroelectric capacitors, the finite field formalism has recently been shown to be an effective tool for dealing with finite size effects.

Using this method, the researchers have studied the response of bulk sodium chloride solutions to a variety of electric or electric displacement fields, showing the new approach offers great conceptual simplifications over existing theoretical frameworks. Particularly, the researchers were able to derive the linear response formula used for evaluating ionic conductivity in a much simpler form than before. Conductivities at infinite dilution of a simple point charge model were within 15% of the experimental values.

According to the researchers, the finite field formalism also provides a theoretical framework that can be coded into molecular dynamics simulations, allowing the researchers to directly measure important quantities such as the solvent’s dielectric constant from its polarization response, which decreases as ionic strength increases. This new approach offers a simplifying perspective and an intuitive physical understanding of the observed anticorrelations between ionic and solvent polarizations.

Source: “Finite field formalism for bulk electrolyte solutions,” by Stephen J. Cox and Michiel Sprik, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5099207 .