(19b) Hidden Ion Structures and Underscreening in Concentrated Electrolytes

The study of concentrated electrolytes has garnered considerable attention in recent decades due to their salient role in diverse applications, ranging from colloidal self-assembly [1, 2] and biological processes [3] to supercapacitors and batteries [4]. The balance between delicate long-range electrostatic interactions and steric repulsion in concentrated electrolytes presents a physically complex problem. The structure of ions in bulk and near interfaces is important in dictating properties such as capacitance and the effective forces between colloids in solution. The structuring of counter-ions and ions in solution is a collective and many-bodied process that determines the effective behavior of the electrolyte.

By assuming that ions behave as point charges and that charges surrounding a central ion can be represented by an averaged cloud of continuous charge density, Debye and Huckel showed that the interactions between charges decay exponentially with the inter-particle distance. Debye-Huckel’s (DH) theory also posits that the spatial decay of charge-charge correlations is dictated by the Debye screening length, a correlation length defined to be inversely proportional to the ions’ concentration. In reality, ions are finite in size and, as their concentration increases, excluded volume interactions compete with electrostatic forces in establishing the solution microstructure, rendering the DH approximation inadequate in describing the charge density profile of concentrated electrolytes. Furthermore, recent experiments [5, 6] have uncovered anomalous long-range forces in concentrated electrolytes arising from a non-monotonic dependency of the screening length on the ion concentration. This phenomenon is known as underscreening and is believed to be electrostatic in origin.

Here we use Brownian Dynamics coupled with Poisson’s equation to investigate the structures of ions in bulk at high concentrations using the Restrictive Primitive Model. We extract the correlation length and frequency of charge oscillations from the charge-charge correlation function and charge structure factor. The analysis is performed over a large range of ionic concentrations, extending from the very dilute regime, where the Debye-Huckel theory is applicable, up to the regime where ionic hard-sphere packing dominates. We compare the concentration dependency of the correlation screening length with the anomalously long forces found by recent experiments.[5, 6] Importantly, we show that at high concentrations, charge oscillations are controlled by packing effects and not electrostatic interactions, even at high ionic strengths. These packing effects are so influential as to cause ions of equal charge in sign to become positively correlated in space, and lead to many-body hidden structures, which have not been previously studied. We believe these newly identified structures play an important role in underscreening.

References

[1] W. B. Russel, D. A. Saville and W. R. Schowalte, Colloidal dispersions, Cambridge Monographs on Mechanics.

[2] J. N. Israelachvili, Intermolecular and surface forces, 2nd ed. (Academic Press London; San Diego, 1991).

[3] B. Eisenberg, "Interacting ions in biophysics: real is not ideal", Biophysical Journal 104, 1849-1866 (2013).

[4] Y. Wang, X. Meng, J. Sun, Y. Liu, and L. Hou, "Recent progress in "water-in-salt" electrolytes toward non-lithium based rechargeable batteries", Frontiers in Chemistry 8, 595 (2020).

[5] A. M. Smith, A. A. Lee, and S. Perkin, "The electrostatic screening length in concentrated electrolytes increases with concentration", The Journal of Physical Chemistry Letters 7, 2157-2163 (2016).

[6] M. A. Gebbie, A. M. Smith, H. A. Dobbs, A. A. Lee, G. G. Warr, X. Banquy, M. Valtiner, M. W. Rutland, J. N. Israelachvili, S. Perkin, and et al., "Long range electrostatic forces in ionic liquids", Chemical Communications 53, 1214-1224 (2017).