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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...

Polymer Classification: Crystallinity

Polymer Classification: Crystallinity

Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals...

Phase Transitions: Sublimation and Deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and standard pressure, a piece of dry ice (solid CO2) sublimes, appearing to gradually disappear without ever forming any liquid. Snow and ice sublimate at temperatures below the melting point of water, a slow process that may be accelerated by winds and the reduced atmospheric pressures at high altitudes. When solid iodine is warmed, the solid sublimes...

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 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 Changes

Phase Changes

Phase transitions play an important theoretical and practical role in the study of heat flow. In melting or fusion, a solid turns into a liquid; the opposite process is freezing. In evaporation, a liquid turns into a gas; the opposite process is condensation.
A substance melts or freezes at a temperature called its melting point and boils or condenses at its boiling point. These temperatures depend on pressure. High pressure favors the denser form of the substance, so typically, high pressure...

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Related Experiment Video

Updated: Aug 16, 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

Solid-solid Phase Transitions between Crystalline Polymorphs of Organic Materials.

Ivo B Rietveld^1,2

¹Laboratoire SMS - UR 3233, Université Rouen Normandie, Mont Saint Aignan, F 76821, France.

Current Pharmaceutical Design

|December 22, 2022

Summary

This summary is machine-generated.

This review explores solid-solid phase transitions in organic crystalline solids, including active pharmaceutical ingredients (APIs). Predicting these transitions for specific molecules remains challenging despite a general understanding of polymorphism.

Keywords:

Pharmaceutical ingredient crystal crystalline materials phase behavior phase diagram transition mechanism

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Last Updated: Aug 16, 2025

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

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Published on: May 15, 2017

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

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

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Area of Science:

Solid-state chemistry and materials science, focusing on crystalline organic materials.

Background:

Polymorphism and solid-solid phase transitions are crucial for organic molecules, including active pharmaceutical ingredients (APIs).
Understanding these transitions is key to controlling material properties and applications like drug product bioavailability.

Approach:

This review synthesizes experimental methods for analyzing phase transitions, complemented by computational and theoretical approaches for mechanism visualization.
It covers thermodynamics, transition mechanisms (displacive and concerted), and experimental techniques.

Key Points:

Thermodynamics, including Gibbs free energy, temperature, and pressure, governs phase transitions between polymorphs.
Key transition mechanisms include displacive and concerted processes within organic crystals.
Applications such as heat storage and controlling drug product bioavailability through metastable phases are discussed.

Conclusions:

While the general theory of polymorphism is established, predicting phase transition behavior for specific organic molecules remains complex.
Further research is needed to refine predictive models for solid-state transformations.