Critical point (thermodynamics)

In thermodynamics, the critical point is a thermodynamic state of a substance which is characterized by equalizing the densities of liquid and gas phase. The differences between the two states of aggregation hear at this point to exist. In the phase diagram, the point of the upper end of the vapor pressure curve dar.

Characterization

The critical point Pc is characterized by three state variables,

• The critical temperature Tc,
• The critical density c or even the critical molar volume Vm, c.

The critical temperature is the temperature below which a gas can be liquified by pressure; above the critical temperature is no longer possible. Especially in multi-component systems are referred to in systems gases above its critical temperature, but in the presence of non-critical material, as a non- condensable components. These can be, for example, in the description of the thermodynamic absorption significant.

Since above the critical point of the liquid and gas can no longer be differentiated from each other, it is called instead of a supercritical fluid which is in a supercritical state. Another, derived from the Anglo -Saxon name is supercritical.

When approaching the critical point, the density of the gaseous state and the liquid state close to each other. The heat of vaporization decreases when approaching and disappears completely when reaching. Just below the critical point, one can observe the phenomenon of critical opalescence: Due to the extremely low heat of vaporization of the substance constantly changing parts between the liquid and gaseous state back and forth, which manifests itself by a strong streaking.

Can be atomistic behavior beyond the critical point describe clearly: If a gas is exposed to an ever increasing pressure, so the distances between the gas molecules decrease continuously. On reaching the critical pressure, the distances are then exactly the same as between the molecules in the liquid phase; there is no difference detectable.

Experimental observation

The transition from the subcritical to the supercritical state can be easily observed, since at the critical point takes place in a clearly visible change of the phases.

The substances are included in this thick-walled tubes made ​​of quartz glass under pressure. Below the critical temperature ( for example, about 304.2 K for carbon dioxide and ethane at 305.41 K ) is partially filled with the liquid substance, to the other part of the vapor of the substance, the tube. Both phases are colorless, clear, and separated by a clearly visible liquid surface ( interfacial ). Upon heating below the first critical temperature, the volume of the fluid due to thermal expansion, while the volume of the steam decreases due to the compression. Has the substance reaches the critical temperature, forms a short time a thick fog (critical opalescence ), which dissolves after a few seconds further warming. This mist can also have distinct colors. Ethane and CO2 have no staining, the fog is white. The tube is then filled with a single homogeneous, clear see-through phase, the supercritical fluid. Upon cooling, again briefly fog before dividing the substance into a liquid and a gaseous phase.

Estimation and calculation

The critical state parameters can be estimated in addition to a comparatively complex empirical measurement and from the van der Waals equation, where they will be leveraging on the definition of the reduced sizes.

In addition to these equations of state and group contribution methods such as the Lydersen method and the Joback method are often used, with which the critical values ​​are determined from the molecular structure.

Discovery

With the increasing use of steam engines in the 18th century, the study of the boiling behavior of different materials moved in the interest of science. It turned out that with increasing pressure increases the boiling point temperature. It was assumed that the coexistence of liquid and gas up to arbitrarily high pressures is possible.

This assumption was disproved in 1860 by the Irish physicist and chemist Thomas Andrews. On the basis of studies with CO2 he could show that there is a point at which the difference between gas and liquid is no longer existent, and is characterized by a certain temperature, a certain pressure and a certain density. He called this point the " critical point ". Shortly afterwards, the Dutch physicist Johannes van Diderik der Waals gave a plausible explanation (see above) for the behavior of substances in the supercritical region.

Applications

Supercritical fluids combine the high solvent power of liquids with low viscosity similar to gases. Furthermore, they disappear completely at pressure reduction ( evaporate ). Thus they are suitable as ideal solvents which have only disadvantages as the high pressure during the process. Supercritical fluids are also used to produce fine particles. Extractions with supercritical fluids are referred to as Destraktionen.

In supercritical water, SiO2 can be solved. When crystallization on seed crystals are formed of quartz. These are then cut into small pieces and used as quartz crystals in quartz watches.

Supercritical water dissolves fat out of meat. Since many different substances are deposited in fat, are extracted and detected drugs and other substances from the meat using this method.

At Textilfärbeanwendungen the good solubility of the dye can be used in the supercritical state, for receiving the dye and transfer it to the fiber. Upon completion, the supercritical fluid is expanded and the remaining dye precipitates from fixed.

An application of supercritical CO2 is the decaffeination of tea and coffee.

With supercritical CO2 to biologics can very gently dry (eg for scanning electron microscopy ). The samples are first cross-linked (usually acetone) and the acetone with supercritical CO2 discharged the water gradually replaced by a solvent. By this procedure, the structures remain largely intact and there are only a few artifacts. The method is called Critical point drying or supercritical drying.

Supercritical fluids in internal combustion engines

When a new concept for gasoline internal combustion engines the fuel before the injection process is brought into a supercritical state. In this method, the fuel is self-igniting and an external ignition ( spark plug) can be omitted. First media reports that the consumption should thus be reduced to less than 3 liters / 100 km. This process will also be used to improve the combustion process with other types of fuel.