Linking excess entropy and acentric factor in spherical fluids

Published

September 9, 2024

Author(s)

Tae Jun Yoon, Ian Bell

Abstract

Introduced by Kenneth Pitzer in 1955, the acentric factor (ω) became a standard quantity in chemical thermodynamics, representing a molecule's "shape" effect on the deviation from the corresponding state behavior. Nevertheless, its interpretation remains elusive from a physical point of view. This work aims to clarify the physical significance of the centricity (ω = 0) from the excess-entropy perspective. Our investigation begins by noting that the excess entropy per particle of centric fluids (ω = 0) becomes −kB at its gas-liquid critical point. We prove that this equality means the hard-sphere and attractive interaction contributions to the excess entropy become equal at the gas-liquid critical point of a centric fluid. Leveraging this finding, we systematically study the relationship by performing molecular simulations of spherical particles. Central to our findings is that centricity arises not from the spherical symmetry of a molecule but from the delicate balance between attractive and repulsive forces. The balance is explained in terms of the structuring ability of a liquid-like system that maximizes the multi-particle excess entropy in centric liquids.

Citation

Journal of Chemical Physics

Volume

161

Pub Type

Journals

Download Paper

https://doi.org/10.1063/5.0216126

Local Download

Keywords

acentric factor, excess entropy, critical point, liquid

Citation

Yoon, T. and Bell, I. (2024), Linking excess entropy and acentric factor in spherical fluids, Journal of Chemical Physics, [online], https://doi.org/10.1063/5.0216126, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=956540 (Accessed October 14, 2025)