Electroneutrality Breakdown and Specific Ion Effects in Nanoconfined Aqueous Electrolytes Observed by NMR

TL;DR

This study uses NMR to reveal significant electroneutrality breakdown and ion-specific effects in nanoconfined aqueous electrolytes, challenging traditional assumptions and highlighting the importance of ion identity and surface charge in confined systems.

Contribution

It provides the first experimental validation of electroneutrality breakdown in nanoconfined electrolytes and links ion-specific responses to the Hofmeister series.

Findings

Electroneutrality breakdown observed in nanoconfined electrolytes.

Asymmetric ion responses depend on ion type and surface charge.

Ion-specific effects follow Hofmeister series ranking.

Abstract

Ion distribution in aqueous electrolytes near the interface plays critical roles in electrochemical, biological and colloidal systems and is expected to be particularly significant inside nanoconfined regions. Electroneutrality of the total charge inside nanoconfined regions is commonly assumed a priori in solving ion distribution of aqueous electrolytes nanoconfined by uncharged hydrophobic surfaces with no direct experimental validation. Here, we use a quantitative nuclear magnetic resonance approach to investigate the properties of aqueous electrolytes nanoconfined in graphitic-like nanoporous carbon. Substantial electroneutrality breakdown in nanoconfined regions and very asymmetric responses of cations and anions to the charging of nanoconfining surfaces are observed. The electroneutrality breakdown is shown to depend strongly on the propensity of anions toward the water-carbon interface and such ion-specific response follows generally the anion ranking of the Hofmeister series. The experimental observations are further supported by numerical evaluation using the generalized Poisson-Boltzmann equation