Metastable Zone Width (MSZW) in Crystallization

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What Is Metastable Zone?

The metastable zone refers to a specific region in the phase diagram of a substance where a solution or melt can exist temporarily in a state that is thermodynamically unstable. In this zone, the substance remains in a supersaturated or supercooled state, meaning it contains a higher concentration of solute or is at a lower temperature than its equilibrium state would typically allow.

Within the metastable zone, the solution or melt is in a state of kinetic stability rather than thermodynamic stability. This means that although the system is not in its most energetically favorable state, it can persist in this state due to the absence of nucleation or crystallization triggers.

The metastable zone is important in processes such as crystallization, where controlling the formation and growth of crystals is desired. By operating within the metastable zone, it is possible to induce controlled nucleation and subsequent crystal growth, leading to the formation of desired crystal structures with specific properties.

However, it is crucial to carefully navigate the metastable zone since any disturbances or external factors can trigger nucleation and rapid crystal formation, leading to an undesired outcome. Therefore, understanding the boundaries of the metastable zone and implementing appropriate control strategies are essential in optimizing processes such as crystallization, precipitation, or supercooling.

metastable zone graphs

How Do You Determine the Metastable Zone?

One way to determine the metastable zone is by utilizing a probe-based optical instrument like ParticleTrack. This instrument monitors the changes in particle size and counts during a process. By accurately identifying the point of dissolution on the solubility curve and the point of nucleation within the metastable zone at different solute concentrations, ParticleTrack enables the measurement of both the solubility curve and the metastable zone width (MSZW).

In a study conducted by Barrett and Glennon (Trans ICHemE, vol. 80, 2002, pp. 799-805), an unsaturated solution is gradually cooled at a consistent rate. Using ParticleTrack with FBRM, the point of nucleation within the metastable zone is determined, indicating a specific position within the MSZW. Subsequently, the solution is slowly heated until the point of dissolution is measured, marking a point on the solubility curve. This process is repeated by adding solvent to reduce the concentration, allowing for a swift measurement of the solubility curve and MSZW over a wide range of temperatures.

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