Mixture of neutral and dipolar-hard spheres confined in a slit pore: field-induced population inversion and demixing

TL;DR

This study uses Monte Carlo simulations to explore how a mixture of neutral and dipolar hard spheres behaves in a slit pore under an external field, revealing complex interactions between population inversion and demixing.

Contribution

It provides new insights into the combined effects of confinement, external fields, and electrostatic interactions on phase behavior of binary mixtures.

Findings

Normal field favors mixed state, reducing population inversion effects.

Parallel field induces complex demixing and population inversion scenarios.

Demixing lines are characterized in bulk and confined geometries.

Abstract

We study by Monte Carlo simulation a binary mixture of neutral and dipolar hard-spheres with non-additive diameters. With a view to understanding the interplay between population inversion for an open pore and the demixing phase transitions, the mixture is considered in the bulk and confined between two parallel hard-walls modeling a slit pore. A uniform field is applied in the pore in order to control its composition as shown previously. The demixing lines in the bulk and in the pore are studied by the Gibbs Ensemble Monte Carlo method. The open pore-bulk mixture equilibrium is studied by a combination of canonical/grand canonical simulations. A moderate electrostatic coupling is considered for remaining close to the conditions in which a jump in the adsorption of the minority species has been observed at zero field. Demixing lines are given in the bulk and for two different pore widths in parallel and normal fields, together with population inversion paths. Similarly to the effect of geometrical confinement, a normal field is found to favor the mixed state so that the population inversion does not interfere with demixing. A parallel field leads to more complex scenarios. Some indications for future work are briefly discussed.