By Efrat Ohayon, Staff Writer
Solid, liquid and gas: these are the three phases of matter that we are taught exist from a very early age. But what if I told you that two of these phases could almost merge into one? Supercritical fluids (SCFs) are defined as highly compressed fluids that have the diffusivity of gases but the density of liquids. As a result, they can move through solids with the properties of a gas while dissolving substances like liquid. These properties make SCFs good solvents.
The most common states of matter are also solid, liquid and gas. Solids are defined by having a hard physical appearance. Liquids are free-flowing but have a defined volume. Gases have no fixed shape or volume. These states of matter are defined by a phase diagram, which shows the effects of changing temperature and pressure on a substance. Pressure is measured on the ordinate axis, while temperature is on the abscissa. Generally, the left side of the graph indicates matter that is solid, the middle section liquid and the right side a gas.
Key points on the phase diagram include the triple point and critical point. When a substance is at its triple point, it is generally at low temperature and pressure, where the substance exists in the forms of solid, liquid and gas at the same time. The critical point is defined by the highest temperature and pressure at which a substance can exist while keeping the identity of liquid and gas separately. A substance that can pass beyond the critical point is considered a supercritical fluid. At this point, the substance can no longer be distinguished as either liquid and gas.
One of the most common compounds that has a use in its supercritical phase is CO2, otherwise known as carbon dioxide. Carbon dioxide is used in its supercritical phase due to several factors. First, supercritical CO2 has high solubility and quick diffusion rates, making it ideal for creating microcellular, uniform pores. Second, it has a low critical point, meaning that to reach the supercritical point, the intensity of the temperature and pressure is relatively easier to reach in comparison to other substances. The critical temperature and pressure of CO2 is 88 degrees Fahrenheit and 72.8 atm, respectively. This temperature is achieved naturally in some environments. It is the critical pressure that poses more difficulties to reach, requiring specialized equipment, as the pressure required is 73 times the pressure that we feel at sea level. These parameters still mean that making supercritical CO2 is much more energy efficient in comparison to other substances.
Supercritical fluids are most commonly used in industrial, pharmaceutical and food industries. Within the food industry, supercritical fluids are used to decaffeinate coffee, as well as in the sterilization of milk.
Interestingly, the density of solutions at supercritical conditions fluctuates. This means that there are regions where the supercritical fluid is more or less dense, and the density of the solution is not uniform. This lack of uniformity leads to challenges when harnessing the benefits of these fluids, as they do not perform as one state. Therefore, even a small change in temperature or pressure can lead to non-linear changes in density, which may pose challenges for the use of supercritical fluids.
Supercritical fluids are one of many fascinating phenomena in science that is not commonly spoken about. The boundaries between solid, liquid and gas are not defined, and there is another state of matter that exists beyond these. Supercritical fluids remind us that there is so much left to discover, and the future application of such discoveries is brimming with possibilities.
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