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Phase Diagram

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The phase of a given substance depends on the pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region provide considerable insights into the thermal properties of substances. Such plots are known as phase diagrams. For instance, in the phase diagram for water (Figure 1), the solid curve boundaries between the phases indicate phase transitions (i.e., temperatures and pressures at which the phases coexist).

Phase Diagram

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A phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure. It shows the boundaries between solid, liquid, and gas phases and the conditions at which these phases coexist in equilibrium. An area in a phase diagram represents a single phase, whereas lines or phase boundaries represent the equilibrium between two phases.In the phase diagram of water, the boundary line between the solid and liquid states illustrates...

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Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

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Published on: May 15, 2017

Thermodynamic geometry, phase transitions, and the Widom line.

G Ruppeiner¹, A Sahay, T Sarkar

¹Division of Natural Sciences, New College of Florida, 5800 Bay Shore Road, Sarasota, Florida 34243-2109, USA. ruppeiner@ncf.edu

Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics

|December 11, 2012

Summary

A new method using thermodynamic curvature (R) characterizes liquid-gas phase transitions. This approach accurately determines coexistence curves and the Widom line without problematic constructions.

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Area of Science:

Thermodynamics
Phase Transitions
Statistical Mechanics

Background:

First-order liquid-gas phase transitions are fundamental in physical chemistry.
The Maxwell equal area construction is conventionally used but has limitations.
Characterizing the supercritical regime, including the Widom line, remains an active research area.

Purpose of the Study:

To propose a novel microscopic characterization for first-order liquid-gas phase transitions.
To develop a method for determining liquid-gas coexistence curves without the Maxwell construction.
To enable direct determination of the Widom line in the supercritical regime.

Main Methods:

Utilizing thermodynamic curvature (R) as a microscopic characterization parameter.
Proposing that R is equal in coexisting phases near the critical point.
Relating R to the correlation volume (ξ(3)) near the critical point.

Main Results:

A new method for determining liquid-gas coexistence curves is presented, avoiding the Maxwell equal area construction.
The relationship |R| ~ ξ(3) allows for direct determination of the Widom line in the supercritical region.
The proposed method is illustrated using the van der Waals model and NIST Chemistry WebBook data.

Conclusions:

Thermodynamic curvature offers a robust framework for analyzing liquid-gas phase transitions.
The proposed method provides a more accurate and direct approach to phase transition characterization.
This work advances the understanding of critical phenomena and supercritical fluid behavior.