In this blog post, we will look at the physical characteristics of supercritical carbon dioxide in the crystallization process and the potential applications and particle control that can be achieved using it.
Solubility is the amount of solute that can be dissolved in a fixed amount of solvent at a fixed temperature to the maximum extent, and is usually the mass of solute that can be dissolved in 100 grams of solvent. Solubility varies depending on the physicochemical properties and can change depending on temperature or pressure. For example, the solubility of salt increases as the temperature of the water increases, while the solubility of gas decreases as the temperature increases. These properties play an important role in various experiments and processes.
The supersaturated state of a mixture is a state in which the solute is dissolved in excess of its solubility, and a mixture in the supersaturated state tends to return to the saturated state. During this process, the solute is precipitated to form solid particles. This phenomenon can be used to promote crystallization. Crystallization is the process by which a saturated mixture becomes supersaturated, causing the solute to precipitate as solid particles. This process is used in the pharmaceutical industry to increase the bioavailability of drugs. It also plays an important role in various other industries, including food, chemical, and materials science.
Supercritical fluids are often used in the crystallization process. A substance exists in a supercritical state above the critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can exist as a liquid, and the critical pressure is the maximum pressure at which a substance can exist as a gas. When the temperature and pressure are above the critical temperature and critical pressure, the substance exists in a supercritical state that is neither liquid nor gas. In the supercritical state, the intermolecular distance of a substance is closer than when the substance is a gas, but not as close as when it is a liquid. Solutes and solvents can move more freely in the supercritical state or when the substance is a gas than when the substance is a liquid. In addition, increasing the pressure applied to supercritical fluid increases the density, allowing more solute to be dissolved, and thus the crystallization process using supercritical fluid can control the particle size of solid particles.
In the gasification process, supercritical carbon dioxide is often used as a cosolvent to precipitate the solute dissolved in the mixture as a solid of small particle size. A cosolvent is a substance that does not dissolve the solute but mixes well with the solvent. When a cosolvent is added to the mixture, the cosolvent mixes with the solvent and the solute precipitates as solid particles. In the gasification process, the substance to be crystallized is dissolved in a liquid solvent to create a mixture, which is then filled into a container and sealed. After that, the temperature and pressure of the container are adjusted to be between the critical temperature and critical pressure of carbon dioxide and the liquid solvent, and supercritical carbon dioxide is injected into the container. The mixture then becomes supersaturated, and the dissolved solute precipitates as solid particles. The amount of solute that can be saturated by mixing a non-solvent with a solvent is reduced. The amount of solute precipitated is determined by its concentration, provided that the initial amount of the mixture is the same.
When solid particles are precipitated in the crystallization process, a crystal nucleus must first be generated by a certain number of solute molecules gathering together to form a collective. The higher the concentration of the mixture, the more solute molecules that can form a crystal nucleus, resulting in more crystal nuclei. When a large number of crystal nuclei are generated, the number of solute molecules that can gather in a single crystal nucleus decreases, resulting in smaller solid particles.
On the other hand, there is also a crystallization process that uses supercritical carbon dioxide as a solvent. In the RESS process, a mixture of the material to be crystallized and supercritical carbon dioxide is sprayed from a high-pressure vessel into a vessel that maintains atmospheric pressure. Immediately after spraying, the pressure of the supercritical carbon dioxide drops rapidly and the solute precipitates as solid particles during the process of changing into a gas. At this time, crystal nuclei are generated in the mixture, and the principle of determining the particle size of the precipitated solid particles is the same as the GAS process.
Carbon dioxide is mainly used in crystallization processes such as the GAS and RESS processes. This is because carbon dioxide has a critical temperature that is not much different from room temperature, so it can be easily made into a supercritical state by slightly raising the temperature and increasing the pressure. Using supercritical carbon dioxide not only allows the particle size of the precipitated solid particles to be reduced by adjusting the pressure, but it is also non-toxic and therefore free from safety concerns. In addition, carbon dioxide is cost-effective. Therefore, it is widely used in various industries thanks to these advantages.
In addition, the solid particles obtained through the crystallization process can be used in various applications. For example, nanometer-sized particles can be used as high-performance materials or catalysts, and are also important in fields that require fine particle size control. As such, the crystallization process is an important technology that continues to be researched and developed due to its potential applications.
