Everything about Perfluorocarbon totally explained
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 from 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 °C),
hexafluoroethane (bp −78.2 °C),
octafluoropropane (perfluoropropane) (bp −36.5 °C), perfluoro-n-butane (bp −2.2 °C) and perfluoro-iso-butane (bp −1 °C). Virtually all the other commercially available perfluorocarbons are liquids (the exception being perfluorocyclohexane, which
sublimes at 51 °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're 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 isn't 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 CoF
2 + F
2 → 2 CoF
3
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.
» C
6H
14 + 28 CoF
3 → C
6F
14 + 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 isn't 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(C
6H
13)
3 + 90 HF → 6 C
6F
14 + 2 NF
3 + 45 H
2
The perfluorocarbon amine will also be produced:
» N(C
6H
13)
3 + 42 HF → 2 N(C
6F
13)
3 + 21H
2
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's completely expelled. Air travel or other environments involving changes in
pressure should be avoided. Use of
nitrous oxide as an
anaesthetic can be disastrous ; dissolved nitrous oxide from the blood accumulates in the bubble, increasing intraocular pressure to the point that blood flow to the retina is cut off and the retina dies.
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 isn't as common. Usually MRI is set up to detect hydrogen nuclei, but it's also possible to use MRI for 19-fluorine nuclei. As there's no fluorine in the human body naturally, it's 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°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.
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:
| Ingredient |
w/v% |
| Perfluorodecalin |
25.0 |
| Yolk phospholids |
3.6 |
| Fatty acid (emulsion stabilizer) |
trace |
| D-Sorbitol (emulsion stabilizer) |
3.5 |
| NaCl |
0.204 |
| KCl |
0.010 |
| MgCl2 |
0.007 |
| Sodium lactate |
0.105 |
Treatment of Decompression Sickness
Perfluorocarbons accelerate nitrogen washout after venous gas emboli. Success in the treatment of
decompression sickness has been shown in
rat,
swine,
hamster models. This treatment shows great potential as a future adjunctive therapy for decompression sickness in humans.
Non-medical Applications
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. Perfluorocarbons are also used in high end racing ski waxes due to their hydrophobic nature, which is responsible for reduced friction in wet snow conditions.
In
fluorous biphase catalysis a perfluorocarbon is used to dissolve a catalyst with a perfluoroalkyl group, while the substrate is dissolved in an organic solvent. At elevated temperature, the perfluorocarbon and organic solvent become miscible, and so the mixture becomes homogenesis, faciliating the reaction. Upon cooling, the two phases separate, allowing the catalyst to be recovered from the perfluorocarbon, and the product from the organic solvent.
The environment
PFCs are extremely potent greenhouse gases, and they're 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.
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.
Further Information
Get more info on 'Perfluorocarbon'.
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