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Phase Diagram

Phase Diagram

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 Diagrams

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. These diagrams indicate the physical states that exist under specific conditions of pressure and temperature and also provide the pressure dependence of the phase-transition temperatures (melting points, sublimation points, boiling points). Regions or areas labeled solid, liquid, and gas represent single phases, while lines or curves represent...

Phase Transitions: Melting and Freezing

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...

Phase Transitions

Phase Transitions

Whether solid, liquid, or gas, a substance's state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the solid pack closely together, generally in a pattern. The particles vibrate about their fixed positions but do not move or squeeze past their neighbors. In liquids, although the particles are closely spaced, they are randomly arranged. The position of the particles are not fixed—that is, they are free to move past their neighbors to...

Phase Transitions: Vaporization and Condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize (convert into vapor). The process is called vaporization—a surface phenomenon. Vaporization occurs when the thermal motion of the molecules overcome the intermolecular forces, and the molecules (at the surface) escape into the gaseous state. When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase...

Third Law of Thermodynamics

Third Law of Thermodynamics

A pure, perfectly crystalline solid possessing no kinetic energy (that is, at a temperature of absolute zero, 0 K) may be described by a single microstate, as its purity, perfect crystallinity,and complete lack of motion means there is but one possible location for each identical atom or molecule comprising the crystal (W = 1). According to the Boltzmann equation, the entropy of this system is zero.

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Related Experiment Video

Updated: May 27, 2025

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

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

Published on: May 15, 2017

Exploring Liquid Crystal Properties through the Two-Phase Thermodynamic Model: Structural, Dynamic, and Thermodynamic

Juan M Hümöller^1,2, Oscar A Oviedo^1,2

¹Universidad Nacional de Córdoba, Facultad de Ciencias Químicas, Departamento de Química Teórica y Computacional, X5000HUA Córdoba, Argentina.

Journal of Chemical Theory and Computation

|February 20, 2025

Summary

This study analyzes liquid crystal properties during phase transitions. Translational diffusion becomes anisotropic, while rotational diffusion decreases, with varying contributions to free energy across transitions.

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Related Experiment Videos

Last Updated: May 27, 2025

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

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

Published on: May 15, 2017

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

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Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Published on: August 15, 2018

Area of Science:

Materials Science
Physical Chemistry
Computational Physics

Background:

Liquid crystals (LCs) exhibit complex structural, dynamic, and thermodynamic behaviors.
Understanding phase transitions in LCs is crucial for materials science and device applications.

Purpose of the Study:

To comprehensively analyze the structural, dynamic, and thermodynamic properties of liquid crystals.
To investigate the evolution of these properties through isotropic-nematic-smectic-solid phase transitions.

Main Methods:

Utilized molecular dynamics simulations with NPT and NVT ensembles.
Employed a model of purely repulsive semiflexible spherocylinders.
Applied a two-phase thermodynamic model to determine density of states and thermodynamic parameters.

Main Results:

Observed anisotropic translational diffusion coefficients during phase transitions, with significant increases along the director vector in the nematic phase.
Reported a general decrease in rotational diffusion, with an increase in rotation around the principal axis during the solid transition.
Identified the mechanical term (PV) as the primary contributor to Gibbs free energy.

Conclusions:

Phase transitions in liquid crystals significantly alter translational and rotational dynamics.
The dominant contribution to Helmholtz free energy shifts from rotational to translational motion across transitions.
Detailed analysis of diffusion coefficients and free energy components provides insights into liquid crystal behavior.