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Quantify Mg2+ density peak height within 3 Å of the illite surface

Determine the relative height of the small Mg2+ number-density peaks within 3 Å of the illite basal surface in the electric double layer for the simulated aqueous MgCl2 system, which could not be resolved within the 30 ns molecular dynamics simulations due to the slow escape of Mg2+ from the near-surface region.

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Background

The paper analyzes ion density profiles near the negatively charged illite basal surface to understand how different salts affect nanoscale friction. Monovalent cations (Na+, K+, Cs+) and divalent cations (Mg2+, Ca2+) exhibit distinct spatial distributions and mobilities in the electric double layer (EDL).

While clear peaks are observed for several ions, Mg2+ shows very slow dynamics near the surface. Lifetime histograms indicate that Mg2+ ions within ~3 Å of the surface do not escape over the 30 ns simulation, preventing adequate sampling of near-surface configurations. Consequently, the authors were unable to quantify the height of the small Mg2+ density peaks in this region, leaving the precise near-surface Mg2+ peak heights unresolved.

References

Mg2+ close to the surface is not able to escape on the time scales of our simulation (see Figure 1(a) below), and we cannot therefore draw any conclusions about the height of the small Mg2+ peaks within 3 Å from the surface.

— Molecular Dynamics Simulations of Nanoscale Friction on Illite Clay: Effects of Solvent Salt Ions and Electric Double Layer (2507.14158 - Li et al., 4 Jul 2025) in Section 3.3 (Electric double layer outside illite clay surface), discussion around Figures 5(c)–(d) and 6(a)