(742f) Phase Behavior of the Primitive Model of Ionic Liquid in the Slit Pore: A Density Functional Approach with the Association Concept

Ionic liquids confined in porous materials are of great interest from both scientific and practical points of view. Such systems are very promising candidates for new types of electrolytes in energy storage devices such as fuel cells, supercapacitors, solar cells, or batteries. A porous medium, or a confinement, strongly affects the phase behavior of a liquid compared to its unconfined (bulk) state. However, it is still a challenge to provide a quantitative or, in some cases, a qualitative description of the phase behavior, i.e. the vapor-liquid phase-equilibrium, of ionic fluids, even in the bulk. The original Debye-Hückel theory and the mean spherical approximation (MSA) fail to capture the trends of critical properties of bulk fluids observed in simulation, and an understanding on confined phase behavior is still lacking. In this work, we have applied the classical Density Functional Theory (cDFT) to study the phase behavior of the confined ionic liquids. Within the framework of the cDFT, our theoretical approach combines the electrostatic functional with the reference fluid density determined using the method from Gillespie et al. [D.Gillespie et al., J.Phys.: Condens. Matter 14, 12129 (2002)] and the associative MSA incorporating the concept of ion association. A size-asymmetric primitive model (PM) was employed for ionic liquids, and the effect of size asymmetry on the phase diagram was investigated. Based on this associative model for ionic liquid, our approach provides a qualitatively correct prediction of the critical properties in the bulk and describes the confined phase diagram of ionic liquids. The effects of confinement on the phase behavior of ionic liquids will be the focus of this presentation.