Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

Abstract

Ion distribution in aqueous electrolytes near the interface plays a critical role 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 towards 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.

Charge neutrality breakdown in confined aqueous electrolytes: Theory and simulation

“…Contrary to the traditional Donnan approach -in which electroneutrality is enforced by the introduction of a potential difference across the system boundaries -in our calculations charge neutrality has not been assumed a priori. The model system can be used to describe the situation in which an electrolyte is confined in carbon nanoporous, for which experimental evidences of local electroneutrality violation has been recently reported 51 . The breakdown of electroneutrality occurs naturally when confined electrolyte is able to exchange particles with a bulk reservoir.…”

“…The possibility that electrolytes in contact with a bulk reservoir might lack charge neutrality in situations of strong confinement has been already discussed both theoretically 46,47 and observed in computer simulations [48][49][50] . However, these findings have only very recently been confirmed experimentally in the work of Luo et al 51 , who demonstrated that electrolytes confined in narrow pores do not in general exhibit electroneutrality.…”

Solid-state NMR studies of supercapacitors

“…Luo et al developed an ingenious method whereby a copper shield surrounds one of the supercapacitor electrodes. 75 Radiofrequency radiation cannot reach the shielded electrode, and the other (unshielded) other electrode is then studied in the NMR experiments. This approach was recently used to study supercapacitors with activated carbon electrodes and aqueous electrolytes.…”

Electrodiffusion phenomena in neuroscience: a neglected companion

“…On the scale smaller than the Debye length, spontaneous Brownian motion of individual molecules incessantly breaks medium electroneutrality 26 . Differential ion mobility or the presence of diffusion obstacles and non-electrostatic interactions between ions and cell walls could exacerbate such effects 22 . Such perturbations are thought to normally arise and dissipate with a time constant shorter than the Debye time.…”