Van der Waals-Like Isotherms in a Confined Electrolyte by Spherical and Cylindrical Nanopores

Abstract

Electrolytes confined by spherical, cylindrical, and slit-like charged nanopores are studied. Results for ionic distribution profiles, pressures of the confined fluid, and absorption isotherms are obtained through the hypernetted chain/mean spherical approximation (HNC/MSA) integral equations theory. In spherical and cylindrical geometries, an inward, non-monotonic behavior of the pressure is found as confinement increases, implying a negative compressibility. The pressure vs volume isotherms resemble liquid-vapor van der Waals-like phase transition diagrams. This effect is correlated with a charge separation inside a spherical pore previously reported (Phys. Rev. Lett., 79, 3656, 1997). Here, the mechanism of charge separation and negative compressibility are explored in detail. When compared with the slit-like pore pressure, important qualitative differences are found.

Metamaterials with negative compressibility — a novel concept with a long history

“…To generate this kind of transition, one should de-stabilize a stable state (like A or D) with a "force" sufficiently high to overcome the energy barrier for the heterogeneous or homogeneous nucleation or reach the stability limit (spinodal) [7,17]. Here we should mention that there are several indications where this kind of loop -together with the negative compressibility part -can be realized experimentally in confined systems (nanopores) [19].…”

Electrokinetic properties of monovalent electrolytes confined in charged nanopores: Effect of geometry and ionic short-range correlations

“…(14) for the slit pore, and Eqs. (21) and (22) for the cylindrical nanopore). Thus, if ψ(x) is expressed in terms of σ , the HNC/MSA equations (Eqs.…”

“…(14) and (15) for the slitlike pore, by Eqs. (21), (22), and (23) for the cylindrical pore, and by Eqs. (26) and (27) for a single wall.…”

“…The hypernetted-chain/mean spherical approximation (HNC/ MSA) has been successfully used to compute the structure of a confined restricted primitive model (RPM) electrolyte [17][18][19][20][21]. Therefore, it has been applied to study electrokinetic phenomena in a slitlike nanopore [22][23][24].…”

“…Further descriptions of the structure of an electrolyte confined by a cylindrical pore have been given through approximate solutions to the PB equation [28][29][30][31] and by means of Monte Carlo simulation [32,33]. In the same way, theoretical descriptions beyond the PB approximation, which incorporate ionic short-range correlations, have been given through integral equation theories [20,21,32]. To our best knowledge, a theoretical study of electrokinetic phenomena considering curvature and ionic size effects is still missing.…”