Perfluorocarbon

Perfluorocarbons (PFCs) are compounds derived from hydrocarbons by replacement of hydrogen atoms by fluorine atoms. PFCs are made up of carbon and fluorine atoms only, such as octafluoropropane, perfluorohexane and perfluorodecalin.

Perfluorocarbon derivatives are perfluorocarbons with some functional group attached, for example perfluorooctanesulfonic acid. Perfluorocarbon derivatives can be very different to perfluorocarbons in their properties, applications and toxicity.

The term perfluorochemical (also abbreviated to PFC) may indicate perfluorcarbons, but is often used to include perfluorocarbon derivatives.

Properties
Perfluorocarbons are chemically inert, thermally stable and non-toxic. This is attributed to the strength of the carbon-fluorine bond and the shielding effect of the fluorine atoms. They are non-flammable.

There are five perfluorocarbon gases; tetrafluoromethane (carbon tetrafluoride) (bp -128&deg;C), hexafluoroethane (bp -78.2&deg;C), octafluoropropane (perfluoropropane) (bp -36.5&deg;C), perfluoro-n-butane (bp -2.2&deg;C) and perfluoro-iso-butane (bp -1&deg;C). Virtually all the other commercially available perfluorocarbons are liquids (the exception being perfluorocyclohexane, which sublimes at 51&deg;C

Perfluorocarbon liquids are clear and colorless. They have high density, up to over twice that of water, due to their high molecular weight. Very low intermolecular forces gives the liquids low viscosities (compared to liquids of similar boiling points), low surface tension and low heats of vaporization. They have particularly low refractive indices too.

They are not miscible with most organic solvents (eg, ethanol, acetone, ethyl acetate and chloroform), but are miscible with some hydrocarbons (eg, hexane in some cases). They have very low solubility in water, and water has a very low solubility in them (on the order of 10 ppm). However, they are relatively good solvents for gases, again because of the very low intermolecular forces.

The number of carbon atoms in the perfluorocarbon molecule largely defines most physical properties. The greater the number of carbon atoms, the higher the boiling point, density, viscosity, surface tension, critical properties, vapour pressure and refractive index. Gas solubility decreases as carbon atoms increase, while melting point is determined by other factors as well, so is not readily predicted.

Manufacture
Prior to World War 2, the only known route to perfluorocarbons was by direct reaction of fluorine with the hydrocarbon. This highly exothermic process was capable only of synthesising tetrafluoromethane, hexafluoroethane and octafluoropropane; larger hydrocarbons decomposed in the extreme conditions. The Manhattan project saw the need for some very robust chemicals, including a wider range of perfluorcarbons, requiring new manufacturing methods. The so-called "catalytic" method involved reacting fluorine and hydrocarbon on a bed of gold-plated copper turnings, the metal removing the heat of the reaction (so not really acting as a catalyst at all), allowing larger hydrocarbons to survive the process. However, it was the Fowler process that allowed the large scale manufacture of perfluorcarbons required for the Manhattan project.

The Fowler Process
The Fowler process uses cobalt fluoride to moderate the reaction. In the laboratory, this is typically done in two stages, the first stage being fluorination of cobalt difluoride to cobalt trifluoride.


 * 2 CoF2 + F2 &rarr; 2 CoF3

During the second stage, in this instance to make perfluorohexane, the hydrocarbon feed is introduced and is fluorinated by the cobalt trifluoride, which is converted back to cobalt difluoride. Both stages are performed at high temperature.


 * C6H14 + 28 CoF3 &rarr; C6F14 + 14 HF

Industrially, both steps are combined, for example in the manufacture of the Flutec range of perfluorocarbons, using a vertical stirred bed reactor, with hydrocarbon introduced at the bottom, and fluorine introduced half way up the reactor. The perfluorocarbon vapor is recovered from the top.

Electrochemical Fluorination
An alternative technique, electrochemical fluorination (ECF) (also known as the Simons' process) involves electrolysis of a substrate dissolved in hydrogen fluoride. As fluorine is itself manufactured by the electrolysis of hydrogen fluoride, this is a rather more direct route to perfluorocarbons. The process is run at low voltage (5 - 6 V) so that free fluorine is not liberated. The choice of substrate is restricted as ideally it should be soluble in hydrogen fluoride. Ethers and tertiary amines are typically employed. To make perfluorohexane, trihexylamine is used, for example:


 * 2 N(C6H13)3 + 90 HF &rarr; 6 C6F14 + 2 NF3 + 45 H2

The perfluorocarbon amine will also be produced:


 * N(C6H13)3 + 42 HF &rarr; 2 N(C6F13)3 + 21H2

Both of these products, and others, are manufactured by 3M as part of the Fluorinert range.

Medical applications
Medical applications require high purity perfluorocarbons. Impurities with nitrogen bonds can have high toxicity; hydrogen-containing compounds (which can release hydrogen fluoride) and unsaturated compounds must also be excluded. Infrared spectroscopy, nuclear magnetic resonance and cell cultures can be used to test the perfluorocarbon.

Eye surgery
Perfluorocarbons are commonly used in eye surgery as temporary replacements of the vitreous humor in retinal detachment surgery. Retinal tears following a penetrating trauma or retinal detachments associated with proliferative vitreoretinopathy can be corrected with surgery in which the dense perfluorocarbon liquid, typically perfluoro-n-octane, is injected into the eye, to push out vitreous liquid trapped behind the retina, and to aid removal of membranes (essentially scar tissue). Perfluoro-1,3-dimethylcyclohexane has been used in the removal of a lens nucleus dislocated into the vitreous cavity, the lens floating on the heavy perfluorocarbon for easy removal.

Octafluoropropane can be used almost in a reverse sense. It is injected into the eye diluted in air (typically 12% to 16%). The patient must then lie face down for about an hour. The gas bubble pushes onto the retina to perform the same task as before. The octafluorpropane may remain in the eye for up to three months after surgery before it is completely expelled. Air travel or other environments involving changes in pressure should be avoided. Use of nitrous oxide as an anaesthetic can be disasterous.

Imaging
Perfluorocarbons are also used in contrast-enhanced ultrasound to improve ultrasound signal backscatter. The perfluorocarbons used in the microbubbles are gases at body temperature (though they may be liquids at room temperature). The gas-filled microbubbles oscillate and vibrate when a sonic energy field is applied and characteristically reflect ultrasound waves. This distinguishes the microbubbles from surrounding tissues. Their stability, inertness, low diffusion rate and solubility increase the duration of contrast enhancement as compared to microbubbles containing air.

Perfluorocarbons can also be used in magnetic resonance imaging (MRI), though this is not as common. Usually MRI is set up to detect hydrogen nuclei, but it is also possible to use MRI for 19-fluorine nuclei. As there is no fluorine in the human body naturally, it is very easy to determine exactly where the sample has gone. Perfluorocarbons can be introduced into the blood in an emulsion, or neat in the lungs.

In radiographic imaging, the perfluorocarbon derivative perfluorooctyl bromide (PFOB) is employed, as this is opaque to radio waves.

Liquid breathing
Perfluorocarbons dissolve relatively high concentrations of gases, for example, 100 ml of perfluorodecalin at 25&deg;C will dissolve 49 ml of oxygen at STP. This led Leland C. Clark in 1966 to experiment with liquid breathing, resulting in the submersion of a mouse for several hours in an oxygenated perfluorocarbon.

Sadly, the mice he used later died due to trauma to their lungs. However, it seems like this was due to impurities in the perfluorocarbon. In recent years there has been new interest in liquid breathing for various procedures from lung lavage to treatment of congenital diaphragmatic hernia. Perfluorocarbon liquids (and liquids in general) are much denser and more viscous than air; rates of breathing, and therefore of gas exchange, are limited, and there are challenges still to be overcome.

Artificial blood
Clark's experiments also triggered interest in using perfluorocarbons in artificial blood (perhaps more accurately described as artificial erythrocytes, as they only serve as gas carriers). The Green Cross Corporation attempted to commercialize this technology in the 1980s under the Fluosol tradename, without success. Recently, however, there has been renewed interest in this field.

In this application, the perfluorocarbon is used as a part of an emulsion, typically using Pluronic F-68 or egg yolk phospholipids (lethicin) as surfactants, in water. For example, Fluosol-DC:

Electical and Electronic Applications
Perfluorocarbons have high dielectric strengths and high insulating properties, and so can be used in direct contact with high voltage components, either as dielectric fluids or as coolants.

Perfluorcarbon Tracers
Perfluorocarbons can be detected at extremely low levels using electron capture detectors or negative ion mass spectroscopy. They can be released at a certain point and the concentration measured in the surrounding area. Perfluorocarbon tracers (PFTs) have been used to map oil fields, study building ventilation, track pollution, detect cable oil leaks and even recover ransom money.

Cosmetics
Inspired by the medical applications, several companies incorporate perfluorocarbons in their cosmetic formulations, claiming the oxygen dissolved in the perfluorocarbon has an anti-aging effect on the skin

Other Applications
PFCs are being used in refrigerating units as CFC replacements, often in conjunction with other gases, and as "clean" fire extinguishers. They are used in plasma cleaning of silicon wafers.

The environment
PFCs are extremely potent greenhouse gases, and they are a long-term problem with a lifetime up to 50,000 years (PMID 14572085). In a 2003 study, the most abundant atmospheric PFC was tetrafluoromethane (PMID 14572085). The greenhouse warming potential (GWP) of tetrafluoromethane is 6,500 times that of carbon dioxide, and the GWP of hexafluoroethane is 9,200 times that of carbon dioxide. Several governments concerned about the properties of PFCs have already tried to implement international agreements to limit their usage before it becomes a global warming issue. PFCs are one of the classes of compounds regulated in the Kyoto Protocol.

The primary source of tetrafluoromethane in the environment is from the production of aluminium by electrolysis of alumina. Aluminium producers are taking effective steps in reducing emissions by better controlling the electrolysis process http://www.natural-resources.org/minerals/cd/docs/twb/PPAH/47_alum.pdf].

Two PFC derivatives, perfluorooctanesulfonic acid and perfluorooctanoic acid, have been found to be persistent in the environment and are detected in blood samples all over the world.