SUPERCRITICAL FLUIDS An overview

What is a supercritical fluid? By definitions, a supercritical fluid is any substance that is above its critical temperature (T c ), where T c is a unique temperature for every substance. In other words, when a substance is above T c, it exists in a single phase which is neither liquid nor gas; this is a supercritical fluid. Another way to look at it is that T c is the highest temperature that a gas can be liquefied by only a change in pressure.

- They have a combination of vapor and liquid properties. They have densities and viscosities less than liquids. Their diffusivities are gas-like. They transfer mass very rapidly. They are compressible and homogeneous. Continuity of States: The substance changes from gas- like to liquid-like in the supercritical region without ever changing phases. Their solvating strength can range from ideal gas to almost pure liquid, depending on slight variations of pressure or temperature.

This figure shows isotherms and typical behavior of a real gas as it is subjected to different pressures and temperatures. Note that there are no phase transitions above Tc These isotherms are smooth; they have no tie lines. Tie lines are the horizontal portions of the isotherms, though they are really not really part of the isotherms.

The figure below shows three photos of the same system. From left to right, the temperature is increasing. In the left photo, there are two phases present, liquid and gas, and the distinction between them is obvious. The center photo is near the critical temperature, so the separation of the two phases is becoming obscured. In the photo on the right, there is no phase distinction, so this is above the critical temperature and is a supercritical fluid.

Above THE CRITICAL POINT TEMPERATURE, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.

In addition, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary. By changing the pressure and temperature of the fluid, the properties can be “tuned” to be more liquid- or more gas-like. One of the most important properties is the solubility of material in the fluid. Solubility in a supercritical fluid tends to increase with density of the fluid (at constant temperature). Since density increases with pressure, solubility tends to increase with pressure. The relationship with temperature is a little more complicated. At constant density, solubility will increase with temperature. However, close to the critical point, the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again. [2]surface tension [2]

A supercritical fluid has densities similar to that of liquids, while the viscosities and diffusivities are closer to that of gases. Thus, a supercritical fluid can diffuse faster in a solid matrix than a liquid, yet possess a solvent strength to extract the solute from the solid matrix.

CO 2 304K and 75Pc (atm) H 2 O 647K and 224 Pc C 2 H 6 305K and 50 Pc C 2 H 4 282K and 51Pc C 3 H K and 44 Pc Xe 290 K and 59 Pc NH 3 406K and 116 Pc N 2 O 310K and 73 Pc. The critical parameters of many useful substances are given. One can see that there is a wide range in critical parameters for different substances.

All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. NB: Carbon dioxide is not a very good solvent for high molecular weight and polar compounds. To increase the solubility of such compounds in supercritical carbon dioxide, small amounts (ranging from 0 to 20 mol %) of polar or non-polar cosolvents called modifiers may be added. The cosolvent interacts strongly with the solute and significantly increases the solubility. For example, addition of a small amount (3.5 mol%) of methanol to carbon dioxide increases the solubility of cholesterol by an order of magnitude.

Compressed gases and fluids have the ability to dissolve in and expand organic liquid solvents at high pressures (50 to 100 bar). This expansion usually decreases the solvent strength of the liquid. Eventually the mixture solvent strength is comparable to that of the pure compressed fluid.

Figure 1. Carbon dioxide pressure-temperature phase diagram

Applications of supercritical fluids There are many applications of supercritical fluids, but among the most important are industrial extraction and purification. The almost liquid-like density promotes solubility, and the gas- like viscosity and diffusitivity make extraction and purification faster in supercritical fluids than in conventional solvents. The most commonly used supercritical fluid used in industry is carbon dioxide, due to it’s convenient critical parameters, low cost, easy and non-toxic disposal, and safety. The most well known application of supercritical fluids is Supercritical Fluid Extraction (SFE).

Supercritical extraction has been applied to a large number of solid matrices. The desired product can be either the extract or the extracted solid itself. The advantage of using supercritical fluids in extraction is the ease of separation of the extracted solute from the supercritical fluid solvent by simple expansion. In addition, supercritical fluids have liquid like densities but superior mass transfer characteristics compared to liquid solvents due to their high diffusion and very low surface tension that enables easy penetration into the porous structure of the solid matrix to release the solute.

SFE is commonly used to extract chemicals or flavors from products such as coffee, tea, hops, herbs, and spices. As can be seen in the schematic, the process begins with CO 2 in vapor form (lower right). It is then compressed into a liquid before becoming supercritical (upper left). While supercritical, the extraction takes place. Afterwards, the CO 2 is depressurized/cooled down and the gaseous CO 2 can then be separated from the desired extract (upper right). Then the CO 2 can either be harmlessly released into the atmosphere (almost harmlessly, depending on who you ask...) or it can be recycled and used again for extraction.

The extraction of vitamin E from soybean oil and a purification method for vitamin E has been well studied. The latter process avoids the step of vacuum distillation, which usually results in the thermal degradation of the product. Solubilities and recrystallization of various drugs has been demonstrated in supercritical fluids.

Nanoparticles Traditional means of making specifically sized material involves several techniques. These include: Milling Grinding or Crushing. Unfortunately, each of these traditional techniques has problems such as thermal and chemical degradation. Crystallization by adjusting supersaturation, using anti-solvents, or employing reactions and precipitations also have shortcomings: Product contamination High energy requirements Waste solvents Low yields Non-uniform particles

Using Supercritical Fluids The use of Supercritical Fluids to make particles eliminates these shortcomings. There are number of SCF techniques to produce particles of controlled size and morphology for organic molecules in the sub micron range: RESS-Rapid Expansion of a Supercritical Solution With RESS, material is dissolved in the SCF and then depressurized though a nozzle. GAS- Gas Anti-Solvent Here the compound is dissolved in an organic solvent, a supercritical fluid is introduced, expanding the volume and lowering the solvents solvent strength causing the compound to precipitate under controlled conditions of particle formation. PCA- Precipitation by Compressed Fluid Anti-Solvent With PCA, the compound dissolved in an organic solvent is sprayed into a SCF, casuing supersaturation and solute precipitation. SEDS -Solution Enhanced Dispersion of Supercritical Fluids Using SEDS, the compound is dissolved in an aqueous solution and the simultaneously sprayed through a coaxial nozzle with an organic solvent into the supercritical fluid. The water is dissolved into the solvent and SCF causing supersaturation and precipitation