Hints of an anomalously large decay length in the force between charged surfaces in electrolytic solutions have recently been observed in experiments. Using molecular dynamics simulations and classical density functional theory, Cats et al. aimed to explain the origin of this anomaly if it turns out to be real.

“If this slow decay is indeed true, then we and other researchers missed one or more fundamental ingredients in describing electrolytes,” said author Peter Cats.

The researchers used the basic Restrictive Primitive Model (RPM) of an electrolyte, in which the solvent is treated as a dielectric medium and the ions as charged hard spheres. Focusing on far-field decay, they applied three different electrostatic free-energy functionals to a variety of ionic concentrations in their calculations, representing various combinations of the Coulomb potential and bulk direct correlation functions.

They found excellent agreement between the theory and their computer simulations, confirming the accuracy of the functionals. They concluded that the RPM alone cannot adequately explain the large decay length, which suggests that the physical factors at play in the experiments are more complicated than current research efforts had anticipated.

The group speculates on the deficiencies of the primitive model for real electrolytes and the need for improvements, posing the question, “Do we really understand the chemical physics of concentrated electrolytes?”

“It still is a two-way problem,” Cats said, emphasizing the need for further experimental tests, which could be useful to clarify if the long-range decay is a genuine physical feature of electrolytes and not due to any undesired artifact in the measurements.

Source: “Primitive model electrolytes in the near and far field: Decay lengths from DFT and simulations,” by P. Cats, R. Evans, A. Härtel, and R. van Roij, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0039619.