Singular Coexistence-curve Diameters: Experiments and Simulations

TL;DR

This study investigates the behavior of coexistence-curve diameters in specific fluids and electrolytes, revealing distinct singularities and deviations from classical models through experiments and simulations.

Contribution

It provides detailed experimental and simulation data on the singular behavior of coexistence-curve diameters in HCSW fluids and RPM electrolytes, highlighting differences from rectilinear expectations.

Findings

HCSW diameter shows a sharp $|t|^{1- alpha}$ singularity near criticality

RPM diameter varies slowly over a broad temperature range

Leading singular term amplitudes are estimated but not directly measurable

Abstract

Precise calculations of the coexistence-curve diameters of a hard-core square-we ll (HCSW) fluid and the restricted primitive model (RPM) electrolyte exhibit mar ked deviations from rectilinear behavior. The HCSW diameter displays a $|t|^{1- alpha}$ singularity that sets in sharply for $|t|\equiv |T-T_c|/T_c\lesssim 10^{-3}$; this compares favorably with extensive data for ${SF}_6$, also reflec ted in C$_2$H$_6$, N$_2$, etc. By contrast, the curvature of the RPM diameter va ries slowly over a wide range $|t|\lesssim 0.1$; this behavior mirrors observati ons for liquid alkali metals, specifically Rb and Cs. Amplitudes for the leading singular terms can be estimated numerically but their values cannot be taken li terally.