Pair correlation functions and phase separation in a two component point Yukawa fluid

TL;DR

This study explores how the structure and phase behavior of a binary Yukawa fluid change with density and mixing rules, revealing a transition from oscillatory to monotonic decay in correlations and identifying phase separation conditions.

Contribution

It demonstrates the impact of ideal versus non-ideal mixing rules on correlation decay and phase separation in binary Yukawa fluids, using hyper-netted chain closure analysis.

Findings

As density increases, correlation decay shifts from monotonic to oscillatory.

Non-ideal mixing induces fluid-fluid phase separation for all compositions.

The random phase approximation accurately predicts phase separation and Lifshitz line.

Abstract

We investigate the structure of a binary mixture of particles interacting via purely repulsive (point) Yukawa pair potentials with a common inverse screening length $\lambda$. Using the hyper-netted chain closure to the Ornstein-Zernike equations, we find that for a system with `ideal' (Berthelot mixing rule) pair potential parameters for the interaction between unlike species, the asymptotic decay of the total correlation functions crosses over from monotonic to damped oscillatory on increasing the fluid total density at fixed composition. This gives rise to a Kirkwood line in the phase diagram. We also consider a `non-ideal' system, in which the Berthelot mixing rule is multiplied by a factor $(1+\delta)$. For any $\delta>0$ the system exhibits fluid-fluid phase separation and remarkably the ultimate decay of the correlation functions is now monotonic for all (mixture) state points. Only in the limit of vanishing concentration of either species does one find oscillatory decay extending to $r = \infty$. In the non-ideal case the simple random phase approximation provides a good description of the phase separation and the accompanying Lifshitz line.