A phase diagram is a graphical tool used to show how a substance changes its physical state based on variations in temperature and pressure. The carbon dioxide phase diagram is particularly interesting because its behavior at standard atmospheric pressure is very different from common substances like water. At everyday pressure, carbon dioxide skips the liquid phase entirely, transitioning directly from a solid to a gas. This unusual characteristic is represented by specific lines and points on the diagram.
Mapping the Fundamental Phases and Boundaries
The carbon dioxide phase diagram plots pressure on the vertical axis and temperature on the horizontal axis. This representation is divided into three primary areas corresponding to the solid, liquid, and gas phases, indicating the stable state of CO2 under specific temperature and pressure combinations.
The boundaries between these regions are represented by three distinct lines, where two phases can coexist in equilibrium. The sublimation curve separates the solid and gas phases. The fusion curve delineates the boundary between the solid and liquid states. Finally, the vaporization curve marks the equilibrium between the liquid and gas phases.
Defining the Triple and Critical Points
The phase diagram defines two specific coordinates: the triple point and the critical point. The triple point is the unique intersection where the sublimation, fusion, and vaporization curves meet. This point represents the only combination of temperature and pressure where carbon dioxide can exist simultaneously in all three phases—solid, liquid, and gas—in thermodynamic equilibrium. For carbon dioxide, this occurs at approximately 5.11 atmospheres and -56.4 degrees Celsius.
The critical point is located at the upper end of the vaporization curve. This point marks the temperature and pressure above which the liquid and gas phases become indistinguishable, existing as a single, homogenous fluid. The critical point for carbon dioxide is found at approximately 72.8 atmospheres of pressure and 31.1 degrees Celsius.
Understanding Supercritical Carbon Dioxide
The region beyond the critical point is where carbon dioxide enters the state known as a supercritical fluid (sCO2). In this state, the substance exhibits properties midway between a gas and a liquid. Supercritical carbon dioxide has a density comparable to a liquid, providing it with strong solvent capabilities. It also maintains the low viscosity and high diffusivity of a gas, allowing it to penetrate materials with ease.
These combined properties make sCO2 an effective and tunable solvent for industrial processes. Because its solvent power can be fine-tuned by small changes to pressure or temperature, engineers can selectively dissolve specific compounds from a mixture. Upon reducing the pressure, the sCO2 returns to its gaseous state, leaving behind no solvent residue.
Practical Applications of CO2 Phase Changes
The unique properties detailed in the phase diagram translate into several important real-world applications, starting with solid carbon dioxide, commonly known as dry ice. Since the triple point pressure is significantly higher than standard atmospheric pressure, solid carbon dioxide cannot melt into a liquid at 1 atmosphere. Instead, it sublimates directly into a gas at about -78.5 degrees Celsius. This clean, residue-free phase change makes dry ice an excellent coolant for transporting perishable goods or medical supplies without exposure to water.
The supercritical state is leveraged in modern industrial extraction processes. One prominent example is the decaffeination of coffee beans, where sCO2 is pumped through the green beans. The fluid selectively dissolves the caffeine molecules without removing the compounds responsible for the coffee’s flavor and aroma. The fluid is also used in specialized industrial cleaning and the extraction of natural oils and flavors, offering a non-toxic and environmentally sound alternative to traditional organic solvents.