|Other names||propylene glycol, 1,2-propanediol, 1,2-Dihydroxypropane, methyl ethyl glycol (MEG), methylethylene glycol, PG, Sirlene, Dowfrost|
|Molar mass||76.09 g/mol|
|Solubility in water||fully miscible|
|Solubility in ethanol||fully miscible|
|Solubility in diethyl ether||fully miscible|
|Solubility in acetone||fully miscible|
|Solubility in chloroform||fully miscible|
|Thermal conductivity||0.34 W/m-K (50% H2O @ 90°C)|
|Related glycols||Ethylene glycol|
|Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Propylene glycol, also known by the systematic name propane-1,2-diol, is an organic compound with the chemical formula C3H8O2. Under standard conditions (temperature of 25°C and pressure of 100 kPa), it is a colorless, odorless, viscous liquid.
This chemical has many applications. It is used as a moisturizer in food, medications, and various personal care products. It is a good coolant in liquid cooling systems, a solvent for food colorings and flavorings, and a solvent for photographic chemicals. It is a carrier in fragrance oils and the main ingredient in deodorant sticks. It is used for de-icing aircraft and producing artificial smoke. It is also a working fluid in hydraulic presses.
Propylene glycol has negligible toxicity if taken orally through food and medicines. In addition, it does not irritate the skin even on prolonged contact. However, high intake of the substance over a relatively short period of time can have toxic effects.
As noted above, the systematic name for propylene glycol is propane-1,2-diol, as endorsed by the International Union of Pure and Applied Chemistry (IUPAC). In addition, it is referred to as PG, 1,2-propanediol, 1,2-dihydroxypropane, methyl ethyl glycol (MEG), or methylethylene glycol.
Propylene glycol is miscible with water and various organic solvents such as ethanol, ether, acetone, and chloroform. Moreover, it is hygroscopic, that is, it readily absorbs water from the surrounding air. It has a boiling point of 188.2°C and a melting point of -59°C.
Each molecule of propylene glycol consists of a chain of three carbon atoms (as in propane), with two hydroxyl (or "alcohol," OH) functional groups attached to two adjacent carbon atoms. It is therefore classified as a "diol." The chemical formula may be written out as CH2OH-CHOH-CH3.
In each molecule of propylene glycol, the central carbon is attached to four different groups of atoms, and it is therefore an asymmetric atom. Consequently, the molecule exists in two forms that are known as stereoisomers. The commercial product is a mixture of both forms, called a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.
Industrially, propylene glycol is produced by the hydration of propylene oxide. Manufacturers may use a noncatalytic, high-temperature process at 200-220°C, or a catalytic route that proceeds at 150-180°C in the presence of an ion exchange resin or small amounts of sulfuric acid or alkali. The final products contain 20 percent 1,2-propanediol, 1.5 percent dipropylene glycol, and small amounts of other polypropylene glycols. Propylene glycol can also be converted from glycerol, a biodiesel by-product.
Propylene glycol has a variety of applications. It is used:
The oral toxicity of propylene glycol is very low. In one study, rats were provided with feed containing as much as 5 percent PG over a period of 104 weeks and they showed no apparent ill effects. However, there have been cases of propylene glycol poisoning, resulting from either inappropriate intravenous use or accidental ingestion by children.
Serious toxicity occurs only at extremely high intakes over a relatively short period of time, when its concentration in blood plasma rises above 4 g/L. Such levels of ingestion would not be possible when consuming reasonable amounts of a food product or dietary supplement containing at most 1 g/kg propylene glycol.
The U.S. Food and Drug Administration (FDA) has categorized propylene glycol as "generally recognized as safe" for use in food, cosmetics, and medicines. Like ethylene glycol, propylene glycol affects the body's chemistry by increasing the amount of acid. Propylene glycol is metabolized into pyruvic acid, which is a normal metabolite in the breakdown of glucose, while ethylene glycol is metabolized into oxalic acid, which is toxic.
However, propylene glycol is not approved for use in cat food. The U.S. Food and Drug Administration has determined that propylene glycol in or on cat food has not been shown by adequate scientific data to be safe for use. Use of propylene glycol in or on cat food causes the feed to be adulterated and in violation of the Federal Food, Drug, and Cosmetic Act. 21CFR589.1001
Prolonged contact with propylene glycol is essentially non-irritating to the skin. Undiluted propylene glycol is minimally irritating to the eye, and can produce slight transient conjunctivitis (the eye recovers after the exposure is removed). Exposure to mists may cause eye irritation, as well as upper respiratory tract irritation. Inhalation of the propylene glycol vapors appears to present no significant hazard in ordinary applications. However, limited human experience indicates that inhalation of propylene glycol mists could be irritating to some individuals. Therefore, inhalation exposure to mists of these materials should be avoided. Some research has suggested that propylene glycol not be used in applications where inhalation exposure or human eye contact with the spray mists of these materials is likely, such as fogs for theatrical productions or antifreeze solutions for emergency eye wash stations.
Recent Clinical Journal of Medicine article states two cases of adult men experiencing psychosis from use of propylene glycol used in phenytoin injection USP. Both patients had to be switched to Cerebyx (Fosphenytoin Sodium) in order to avoid propylene glycol co-solvent.
Research has suggested that individuals who cannot tolerate propylene glycol probably experience a special form of irritation, but that they only rarely develop allergic contact dermatitis. Other investigators believe that the incidence of allergic contact dermatitis to propylene glycol may be greater than 2 percent in patients with eczema.
Patients with vulvodynia and interstitial cystitis may be especially sensitive to propylene glycol. Women struggling with yeast infections may also notice that some OTC creams can cause intense burning. Post-menopausal women who require the use of an estrogen cream may notice that brand name creams made with propylene glycol often create extreme, uncomfortable burning along the vulva and perianal area. In these cases, patients can request that a local compounding pharmacy make a "propylene glycol free" cream.
All links retrieved June 15, 2019.
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