Dehydration-Driven Ion Aggregation and the Onset of Gelation in ZnCl$_2$ Solution

Abstract

A minimal model of ionic aggregation in concentrated ZnCl$_2$ is developed, guided by molecular dynamics simulations with a machine-learned potential. It explicitly incorporates solvent-site depletion, correlated chloride binding, and allows for loops within Zn-Cl clusters. Dehydration is shown to drive ion binding through two sharp transitions set by the Zn coordination number $Z$: a crossover at $Z=2$ from isolated ions to Cl-bridged clusters, and gelation near $Z\approx 3$. The model agrees quantitatively with MD results, and the critical exponent of the cluster-size distribution matches percolation theory.

Figures (5)

• Figure 1: MD simulation snapshots taken at different salt concentrations. A: salt-to-water molecular ratio $n_s=1:53$ (molality $1.05$ m); B: $n_s=1:8.5$ ($6.53$ m); C: $n_s=1:3$ ($18.5$ m); D: $n_s=1:1.85$ ($30$ m).
• Figure 2: Schematic representation of the key processes in the model. A: Bridging reaction. B: Hydration-dependent association of Zn$^{2+}$ and Cl$^{-}$ ions.
• Figure 3: Theoretical relationships between $p(Z)$ (dashed) and $\alpha(Z)$ (solid) compared with MD data (symbols) for $K=0.0005$.
• Figure 4: Comparison of theoretical dependencies of the mean Zn-Cl coordination number $Z$ (panel A), the mean bridging probability $p$, and the Cl$^-$ dissociation fraction $\alpha$ with MD simulations (panel B).
• Figure 5: Cluster population distributions from MD simulations for Zn-Cl aggregates at different concentrations, plotted on log-log scale. The markers show probability density function (pdf) vs. the number of Zn$^{2+}$ ions per cluster. In the 30 m electrolyte (Zn:O = 1:1.85), the distribution follow the power-law behavior $f\sim n_{Zn}^{-\tau}$, with expected critical exponent of $\tau=2.19$ for percolation theory in 3D. This behavior is distinctly different from loop-free result of FS theory, $\tau=5/2$