Phase behaviour and structure of a superionic liquid in nonpolarized nanoconfinement

Abstract

The ion-ion interactions become exponentially screened for ions confined in ultranarrow metallic pores. To study the phase behaviour of an assembly of such ions, called a superionic liquid, we develop a statistical theory formulated on bipartite lattices, which allows an analytical solution within the Bethe-lattice approach. Our solution predicts the existence of ordered and disordered phases in which ions form a crystal-like structure and a homogeneous mixture, respectively. The transition between these two phases can potentially be first or second order, depending on the ion diameter, degree of confinement and pore ionophobicity. We supplement our analytical results by three-dimensional off-lattice Monte Carlo simulations of an ionic liquid in slit nanopores. The simulations predict formation of ionic clusters and ordered snake-like patterns, leading to characteristic close-standing peaks in the cation-cation and anion-anion radial distribution functions.

Superionic liquids in conducting nanoslits: A variety of phase transitions and ensuing charging behavior

“…Similarly as in the case of zero voltage [43], there are two stable thermodynamic phases for u = 0, namely:…”

“…At zero applied potential, u = 0, the ordered phase consists of an equal amount of cations and anions (possibly mixed with voids, depending on the transfer energy w, see Fig. 1c for the case βw 1); in the corresponding continuous system, this likely corresponds to in-plane crystal-like ordering of ions [43]. At a non zero potential and in the ordered phase, the charge on one sublattice exceeds that on the other, so that the pore is overall charged.…”

“…where a is the lattice constant. In this case, the value of I varies in the range from a few k B T to about 15k B T , depending on the ion size (≈ lattice constant), pore width and dielectric constant [43].…”

“…Average densities of ± ions on the root (central) site of the Cayley tree can be obtained via the variables x N and y N as follows [43] ρ 0,+ = z + x q…”

“…In our previous work [43], we have applied this model to analyse the phase behavior of ions in non-polarized slit nanoconfinement. We demonstrated the emergence of ordered and disordered phases, and first and second-order phase transitions, which could be induced by changing temperature and slit width.…”