What are supercritical fluids? What are the characteristics of supercritical fluids?

In this blog post, we’ll talk about supercritical fluids.

 

“Here comes the singularity!” is what many people exclaimed on the internet after watching the shocking Go match between Google’s AlphaGo and 9th ranked Go player Lee Sedol. The phrase was made famous as the title of a book by Google’s Director of Technology Ray Kurzweil, who describes the singularity as the point at which human-made technology transcends humans. In other words, he argues that the singularity is the point at which human technology and human capabilities are equalized, and that once we cross this point, things will happen that we never expected.
However, the word “singularity” is more commonly used in math and science to refer to a broader concept of a point of balance between competing things other than technology and humans. For example, in mathematics, the ratio of two variables in a formula determines the characterization of the formula. However, there are situations where the magnitudes of the two factors are so precisely balanced that it’s impossible to say anything about the characteristics of the formula, and this is called the singularity of the formula. If we understand the word singularity in the broader sense of a balance point, we can see that the states of all the substances around us also have their own singularities, called critical points, where the characteristics of liquids and gases are balanced. And beyond this point, they exhibit useful properties that we never imagined.
All matter can exist in three states. Consider water. When it’s cold, it exists as a solid called ice; when it gets warmer, it melts into a liquid called water; and when it gets hotter, it boils into a gas called water vapor. These three states of matter-solid, liquid, and gas-change with temperature. In addition, the state of matter depends not only on temperature, but also on pressure. The spray in a can is compressed to a very high pressure and remains in a liquid state, but when it is sprayed, it becomes an invisible gas as it flies out into the air. In this way, temperature and pressure determine whether a substance is a solid, liquid, or gas.
So how do temperature and pressure change states? First, let’s understand what temperature and pressure mean. Temperature describes how fast the molecules-the tiny particles that make up a substance-move: when the temperature is low, the molecules move slowly, and when the temperature is high, the molecules move quickly. Pressure, on the other hand, describes the distance between molecules. Higher pressure means that the material is compressed and the distance between the molecules is smaller, while lower pressure means that the molecules are farther apart. However, this adjustment of the distance between molecules by pressure has an additional effect. Molecules have a tendency to attract each other because the magnitude of the attraction force is greater when the molecules are closer together. In other words, as the pressure increases, the molecules get closer together and the force to pull them together increases, and conversely, as the pressure decreases, the force to pull the molecules together decreases.
Now let’s go back to water. When the temperature is low, the water molecules, the particles that make up water, move slowly, and these slow-moving molecules are unable to overcome the forces that pull them together and escape, so they clump together and become a solid, or ice. When ice is heated to a higher temperature and the molecules move faster to some extent, they are able to overcome some of the forces of attraction and move around, although they are still clumped together in a large mass, and this is the liquid state of water. At higher temperatures, the forces of attraction between molecules can no longer hold them together, and the molecules are free to fly around, which is the gaseous state of water vapor. To summarize, the state of matter is determined by what wins the competition between the force of attraction of the molecules and the speed of the molecules: the force of attraction of the molecules increases with pressure, and the speed of the molecules increases with temperature, so the state of matter changes with temperature and pressure.
So let’s try to turn water vapor into a liquid without lowering the temperature. Increasing the pressure brings the water molecules closer together. The force of attraction between the molecules increases. If you increase the pressure enough, the force of attraction will be strong enough to catch the molecules as they escape at high speeds, and they will become liquid again. But can you always turn a gas into a liquid by increasing the pressure?
The short answer is no. Increasing the pressure reduces the distance between the molecules and increases their attraction to each other. But there’s a limit to how much. If the molecules are compressed until they’re close together and there are no gaps, they can’t get any closer. Temperature, on the other hand, can be raised indefinitely before something goes wrong inside the molecule or it breaks down. So beyond a certain temperature, the competition between pressure and temperature ends, and no amount of pressure can create a strong enough intermolecular pull to catch the fast-moving molecules, and the gas becomes a liquid. This final point of equilibrium just before the competition between temperature and pressure breaks down is called the critical point, which can also be thought of as the singularity of matter.
However, just because a substance cannot become liquid beyond its critical temperature and pressure does not mean that the substance beyond the critical point exists as a gas. Beyond the critical point, the distance between the molecules is so close that they attract each other with a strong force, although not enough to become liquid. So even though the molecules are not clumped together as they would be in a liquid, they are not completely free to move around as they would be in a gas. When a substance crosses this critical point and becomes neither a liquid nor a gas, it is called a supercritical fluid.
Supercritical fluids exhibit properties that are not found in ordinary liquids or gases, including very low viscosity and the ability to dissolve other substances very well. Low viscosity means good penetration, which is easy to understand if you think about it: if you pour water into sand, the water can penetrate into every nook and cranny between the grains of sand and flow out the bottom, but if you pour honey, which has a higher viscosity than water, into sand, it barely flows and only soaks into the sand a little.
In short, supercritical fluids can be used as extraction solvents to penetrate any nook and cranny and dissolve any other substance you want. In the case of lignin, an antioxidant that doesn’t come out when sesame seeds are pressed to make sesame oil, extracting it with supercritical fluids can increase the amount by more than 10,000 times, and there are actually sesame oils on the market that are extracted this way. Many pharmaceutical companies are also investigating the use of supercritical fluids to extract medicinal ingredients from herbs and other substances. In addition to this, the unique properties of supercritical fluids can be used to create very small nanoparticles or as solvents for chemical reactions that would not otherwise occur.