In this blog post, we will examine the physical characteristics of supercritical carbon dioxide in crystallization processes and its applications in particle control and other areas.
Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a constant temperature, typically expressed as the mass of solute that can dissolve in 100 g of solvent. Solubility varies depending on physical and chemical properties and can change with temperature or pressure. For example, the solubility of salt increases as the temperature of water rises, while the solubility of gases decreases as temperature increases. These characteristics play an important role in various experiments and processes.
A supersaturated mixture is a mixture in which the solute is dissolved beyond its solubility, and such a mixture tends to return to a saturated state. During this process, the solute precipitates to form solid particles. This phenomenon can be utilized for crystallization. Crystallization is the process by which a saturated mixture becomes supersaturated, causing the solute to precipitate as solid particles. Through crystallization, small solid particles can be obtained. This crystallization process is used in fields such as pharmaceuticals, where it is necessary to increase the bioavailability of drugs. It also plays an important role in various industries such as food, chemistry, and materials science.
Supercritical fluids are often used in crystallization processes. A substance exists in a supercritical state when its temperature and pressure exceed its 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 exceed the critical temperature and critical pressure, the substance exists in a supercritical state, which is neither a liquid nor a gas. In the supercritical state, the distance between molecules of a substance is closer than when the substance is in the gas state but not as close as when it is in the liquid state. Solutes or solvents can move more freely in the supercritical state or gas state than when the substance is in the liquid state.
Additionally, increasing the pressure applied to a supercritical fluid increases its density, allowing it to dissolve more solute. This property is utilized in supercritical fluid-based crystallization processes to control the particle size of solid particles. In GAS processes, supercritical carbon dioxide is often used as a co-solvent to precipitate the solute dissolved in a mixture into small solid particles.
A co-solvent is a substance that does not dissolve the solute but mixes well with the solvent. When added to a mixture, the co-solvent mixes with the solvent, and the solute precipitates as solid particles. In the GAS process, the material to be crystallized is dissolved in a liquid solvent to form a mixture, which is then filled into a container in an appropriate amount and sealed. The temperature and pressure of the container are then adjusted to the critical temperature and critical pressure of carbon dioxide and the liquid solvent, and supercritical carbon dioxide is injected into the container. This causes the mixture to become supersaturated, and the dissolved solute precipitates as solid particles. The amount of solute that can be dissolved in the solvent decreases as the cosolvent mixes with the solvent.
The amount of solute precipitated is determined by its concentration, provided that the initial amount of the mixture is the same. In the crystallization process, when solid particles are precipitated, a certain number of solute molecules must first aggregate to form aggregates, creating crystal nuclei. The higher the concentration of the mixture, the more solute molecules available to form crystal nuclei, resulting in more crystal nuclei.
When many crystal nuclei are formed, the number of solute molecules that can gather around a single crystal nucleus decreases, resulting in smaller solid particles.
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 container into a container maintained at atmospheric pressure.
Immediately after injection, the supercritical carbon dioxide rapidly decreases in pressure and transforms into a gas, causing the solute to precipitate as solid particles. At this point, crystal nuclei form in the mixture, and the principle governing the particle size of the precipitating solid particles is the same as in the GAS process. Carbon dioxide is primarily used in crystallization processes such as the GAS process and the RESS process.
This is because carbon dioxide has a critical temperature that is not significantly different from room temperature, allowing it to be easily converted to a supercritical state by slightly increasing the temperature and pressure. Using supercritical carbon dioxide enables the particle size of the precipitated solid particles to be reduced by adjusting the pressure, and it is non-toxic, making it safe for use. Additionally, carbon dioxide is economically advantageous in terms of cost. Therefore, thanks to these advantages, it is widely used in various industries.
Furthermore, solid particles obtained through the crystallization process can be utilized in various applications. For example, particles of nanometer size can be used as high-performance materials or catalysts, and they 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.
