The propylene glycol is an alcohol whose chemical formula is C3H8ORtwo or CH3CHOHCHtwoOH. It consists of a colorless, odorless, tasteless, viscous liquid, similar to syrup. It has great solubility in water, which gives it the property of being hygroscopic and moisturizing.
Furthermore, propylene glycol is a relatively stable and chemically inert compound, which has allowed its use as a diluent in the intravenous administration of numerous drugs and in the topical and oral treatment of various diseases, for example ichthyosis..
Its structural formula is shown above, which allows a clear glimpse of how its molecule should be. Note that it has two hydroxyl groups on adjacent carbons, and that its carbon skeleton is derived from the hydrocarbon propane; that is, it has three carbon atoms.
Other names for this alcohol are 1,2-propanediol (which the IUPAC recommends), and 1,2-dihydroxypropane, apart from some not so common.
Propylene glycol is used industrially in the production of various foods and beverages, fulfilling the functions of humectant, stabilizer, antioxidant, antimicrobial, enhancer of the action of other additives, etc..
In the industrial field it has numerous uses, among others: antifreeze, solvent, production of resins and plastics, polyester fabrics, colorants and dyes.
Although it is considered a safe compound, under certain conditions it can be toxic and harmful to health, which can lead to cardiac arrest. However, it is more environmentally friendly compared to ethylene glycol, replacing the latter as car antifreeze..
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In the first image the structural formula of propylene glycol was shown; Above, instead, you have its molecular structure represented with a spheres and bars model. The red spheres correspond to the oxygen atoms of the two OH groups.
All bonds that can be observed in the structure have the ability to rotate on their own axis, which gives flexibility to the molecule; and in turn, affects their vibrational modes.
From the point of view of their intermolecular interactions, thanks to the two OHs so close to each other, they allow propylene glycol to form several hydrogen bonds. As a consequence of these bridges, this alcohol exhibits a boiling point higher than that of water (187.6ºC), despite being heavier..
In the molecular structure, the carbon atom in the center (the Ctwo), is asymmetric, since it has four different substituents: OH, H, CH3 and CHtwoOh.
In the image above you can also see the above. On the left is the structural formula, and on its right the two stereoisomers of propylene glycol:
Note that said pair of stereoisomers are mirror images, that is, they cannot be superimposed; and therefore, they are enantiomers.
If the stereoisomer of the extreme right (of configuration R) is “flipped”, it will have the same formula as its partner (of configuration S); with the difference, that your H atom will point out of the plane, towards the reader, and not behind the plane.
-Propylene glycol or α-propylene glycol
-1,2-propanediol (preferred by IUPAC)
-1,2-dihydroxypropane
-Methyl ethyl glycol.
76.095 g / mol
-60 ºC.
187.6 ºC.
-104 ºC.
-99 ºC (closed cup).
106 mg / L at 20 ° C; that is, practically miscible in any proportion.
Soluble in ethanol, acetone and acetone. In addition to water, it is also miscible with acetone and chloroform. Soluble in ether.
1.0361 g / cm3 at 20 ºC.
2.62 (in relation to air taken as 1).
0.13 mmHg at 25 ºC.
At low temperatures it is stable in tightly closed containers. But, in open containers and at high temperatures, it tends to oxidize, producing propionaldehyde, lactic acid, pyruvic acid and acetic acid..
Propylene Glycol is chemically stable when mixed with 95% ethanol, glycine, or water.
0.581 cPoise at 20 ° C.
431 kcal / mol.
168.6 cal / g (at boiling point).
0.590 cal / g at 20 ºC.
-116.1 kcal / mol at 25 ºC.
1,431 - 1,433 at 20 ºC.
14.9 at 25 ºC.
Industrially, propylene glycol is produced from propylene oxide (which is an epoxide). Some manufacturers do not use catalysts by this method, and carry out the reaction at high temperatures, between 200 and 220 ºC..
Other manufacturers, on the other hand, use metals as catalysts and the temperatures used are somewhat lower than the previous ones, between 150ºC and 180ºC, with the presence of an ion exchange resin and small amounts of sulfuric acid or alkali..
The reaction is shown in the following chemical equation, where practically the carbon in the upper vertex of the triangle is hydrated in an acid medium:
The final product contains 20% propylene glycol, being able to additionally purify up to 99.5%.
The synthesis of propylene glycol can also be carried out from glycerol by using catalysts; such as Raney's Ni catalyst.
Glycerol has three OH groups, so it must get rid of one, at the same time that hydrogen is needed to allow this transformation and water is released in a hydrogenolysis reaction.
Although the use of propylene glycol in food is considered safe by the US Food and Drug Administration (1982), there are reports of adverse effects. Among them, hyperosmolality, hemolysis, cardiac arrhythmia, and lactic acidosis have occurred..
A 15-month-old boy who received large doses of vitamin C, suspended in propylene glycol, has had episodes of lack of response to stimulation, tachypnea, tachycardia, sweating, and hypoglycemia..
The presence of attacks, related to the intake of vitamin D, using propylene glycol as a vehicle of administration has been reported. Likewise, a condition of hyperosmolality has been reported in young children who consumed multivitamins containing propylene glycol.
In adults, the intravenous administration of large doses of propylene glycol can cause kidney failure and liver dysfunction. Additionally, toxic effects include hyperosmolality, metabolic acidosis (lactic acidosis), and septic syndrome..
Rapid intravenous injection of drugs containing propylene glycol has been associated with nervous depression, hypotension, stroke, arrhythmia, unconsciousness, and eventually cardiac arrest..
The WHO (1974) recommended a maximum dose of propylene glycol of 25 m / kg / day in food. Some studies found toxic effects of propylene glycol when its serum concentration exceeds 25 mg / dL.
Some artificial tear preparations, such as Systane, use propylene glycol as an ingredient.
It is used as a diluent for the intravenous administration of numerous medications, including: diazepam, digoxin, lorazepam, ferritoin, etomidate, nitroglycerin, sodium phenobarbital, etc..
Propylene glycol has been used in alcohol-based hand sanitizers, with a moisturizing action to prevent dry hands..
Propylene glycol is added to respiratory inhalants in order to reduce the viscosity of bronchial secretions, as is the case in asthmatic disease..
It is used as an antiseptic with an action similar to ethanol; but being a little less effective than ethanol. Topical application of propylene glycol at an aqueous dilution of 40 -60%.
It has been used in the treatment of patients with X-linked ichthyosis and ichthyosis vulgaris. These diseases are characterized by a process of skin spots and scales.
Propylene glycol, and other nonalcoholic gels, cause an increase in the keratolytic action of salicylic acid. This combination of chemical compounds can be useful in the treatment of ichthyosis..
The Ointment in whose composition has 70% propylene glycol, is used with good results in the treatment of corneal edema.
.Propylene Glycol is a very useful humectant in foods. In addition, it works as a preservative agent that can act on pathogens. Additionally, it is a low reactive compound, so it does not react with other components of food..
-Its hygroscopicity ensures the absorption of water and other substances in optimal conditions. In this way, a control of the humidity of the food is maintained to avoid the dryness of the same and an acceleration of its deterioration..
-It is an antioxidant that prolongs the half-life of foods, protecting them from the damage that oxygen can produce..
-It works by dissolving other additives that may be present in food processing, such as: colorants, flavorings and antioxidants.
-It is used in the preparation of liquid sweeteners, creamy ice creams, whipped milk, etc..
It fulfills a function similar to ethylene glycol. But, the use of propylene glycol has been preferred because of its low toxicity. Ethylene glycol can cause calcium oxalate crystals, which can damage the heart, lungs, and kidneys..
Propylene glycol is able to block the formation of ice crystals, preventing freezing. In addition, it lowers the freezing point of the water, hindering its occurrence. It is also used in the deicing of aircraft wings.
It is used to maintain the moisture of the hair. In addition, it helps to soften the skin and hair and controls bacterial growth..
Propylene glycol is used in the manufacture of numerous personal care products, such as: deodorants, sunscreens, facial creams, body lotions and excipient to stabilize the foam used in shaving cream.
It is also used as a moisturizer to prevent dry skin and hands..
Propylene glycol is used in the manufacture of adhesive agents, sealants and coatings.
It is used in thermal heat transfer fluids and in hydraulic and brake fluids. Additionally, it is used in the suppression of dust; in colorants and dyes: lubricants, natural gas dehydration, plasticizers, surfactants and waxes.
It is used in the textile industry for the manufacture of polyester fibers. It is used in paints and coatings for the protection of the elements; as a solvent in printing inks and in the manufacture of compliant plastics.
The US Army uses it to produce a smoke that serves as a curtain or screen to hide the movement of troops on the battlefield..
It is also used in scale models of trains and ships to simulate the emission of smoke, by means of small heaters..
Propylene glycol, along with glycerol, is used in the liquid cartridges of electronic cigarettes, which reduce the toxic effects of the consumption of the use of conventional cigarettes.
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