Direct Experimental Observations of Ion Distributions during Overcharging at the Muscovite‐Water Interface by Adsorption of Rb⁺ and Halides (Cl⁻, Br⁻, I⁻) at High Salinity

Abstract

Classical electric double layer (EDL) models have been widely used to describe ion distributions at charged solid‐water interfaces in dilute electrolytes. However, the chemistry of EDLs remains poorly constrained at high ionic strength where ion–ion correlations control non‐classical behavior such as overcharging, i.e., the accumulation of counter‐ions in amounts exceeding the substrate’s surface charge. Here, we provide direct experimental observations of correlated cation and anion distributions adsorbed at the muscovite (001)–aqueous electrolyte interface as a function of dissolved RbBr concentration ([RbBr] = 0.01–5.8 M) using resonant anomalous X‐ray reflectivity. Our results show alternating cation‐anion layers in the EDL when [RbBr] ≳ 100 mM, whose spatial extension (i.e., ~20 Å from the surface) far exceeds the dimension of the classical Stern layer. Comparison to RbCl and RbI electrolytes indicates that these behaviors are sensitive to the choice of co‐ion size. This new molecular‐scale understanding of the EDL structure during transition from classical to non‐classical regimes supports the development of realistic EDL models for technologies operating at high salinity such as water purification applications or modern electrochemical storage.

Ions Adsorbed at Amorphous Solid/Solution Interfaces Form Wigner Crystal-like Structures

“…20,45,67−69 At very high electrolyte concentrations (3−5 M) on mica AFM has revealed a co-ion layer, 25 and resonant anomalous X-ray reflectivity of the interface of mica with rubidium halide (RbCl) solutions revealed co-ion rich second layers. 70 The authors of the latter paper concluded that anion adsorption stabilizes the correlated cation arrangements in the Stern layer, although for a crystalline mica surface, these results are consistent with the Wigner crystal-like structures proposed in this paper for disordered substrates. As positive potential is increased, Cl − remains concentrated just above the water layer on the gold surface, followed by a Mg 2+ layer, and Mg 2+ −Cl − ion clusters also form in this this region (Figure S18a).…”

“…This suggests that Cl – is weakly associated with the Mg 2+ and displaced by the AFM tip during imaging and rapidly returns to the crystal surface as the tip passes. This is not unexpected, as co-ion adsorption cannot be detected with AFM even on strongly templating crystalline surfaces at low to moderate electrolyte concentrations. ,,− At very high electrolyte concentrations (3–5 M) on mica AFM has revealed a co-ion layer, and resonant anomalous X-ray reflectivity of the interface of mica with rubidium halide (RbCl) solutions revealed co-ion rich second layers . The authors of the latter paper concluded that anion adsorption stabilizes the correlated cation arrangements in the Stern layer, although for a crystalline mica surface, these results are consistent with the Wigner crystal-like structures proposed in this paper for disordered substrates.…”

Metal–Drug Coordination Nanoparticles and Hydrogels for Enhanced Delivery

“…[ 130 ] For example, some hydrogels composed of Fe 3+ and Al 3+ have an extremely fast self‐assembly speed (about 1 min) and good antibacterial activity against Escherichia coli and Staphylococcus aureus . [ 131 ] Therefore, based on the arrangement and combination of metal ions and drug, various metal–drug hydrogels with different applications, targets, and biological functions can be designed. This section focuses on these hybrid materials and their related applications.…”