Propylene glycol is a chemical compound derived from glycerin and is used as a lubricant by the cosmetic and pharmaceutical industries.

It appears as a viscous colourless transparent liquid with a slightly sweet taste.

What it is used for and where

Propylene Glycol is known for its ability to absorb water, making it an excellent humectant that helps to maintain moisture in the skin and hair. It is colorless, nearly odorless, and soluble in both water and alcohol, making it versatile for a wide range of applications. It is used to stabilize and enhance the efficacy of water-soluble actives in products such as lotions, creams, gels, and sprays. Additionally, it facilitates the penetration of active ingredients into the skin, improving product functionality. Propylene Glycol is also employed as a carrier in medications, foods, and flavorings.

• serves as a solvent
• stabilizes insoluble fluids (as an emulsifier)
• helps to bind and carry other substances (excipient)
• attracts / retains water / moisture (hygroscopic)
• reduces the freezing point
• increases boiling point

Propylene Glycol is also used in:

• food and drink
• feed
• cosmetics
• electronic cigarettes (1)
• pharmaceutical applications to dissolve flavorings in beverages
• feed to keep them moist
• oily and aqueous elements in creams, sprays and liquids as an emulsifier
• active ingredients in drugs as a solvent

Food

Labelled with the number E1520 in the list of European food additives as a humectant.

Industry

In industry it is used to create other chemicals, such as unsaturated polyester resins, which are used in the bathroom and dishes, tubes. It is also used for the manufacture of plastics, resins, paints and coatings and plays an important role as an ingredient in liquid detergents and anti-ice liquids. 

Cosmetics

In cosmetic products such as creams and ointments it has a humectant, emollient and emulsifying function. In topical preparations, it acts as a co-solvent to improve drug permeation through the skin.

INCI functions:

• Fragrance. It plays a decisive and important role in the formulation of cosmetic products as it provides the possibility of enhancing, masking or adding fragrance to the final product, increasing its marketability. It is able to create a perceptible pleasant odour, masking a bad smell. The consumer always expects to find a pleasant or distinctive scent in a cosmetic product. 
• Humectant. Hygroscopic compound used to minimise water loss in the skin and to prevent it from drying out by facilitating faster and greater absorption of water into the stratum corneum of the epidermis.  The epidermis is the most superficial of the three layers that make up human skin (epidermis, dermis and hypodermis) and is the layer that maintains hydration in all three layers. In turn, the epidermis is composed of five layers: horny, the most superficial, granular, spinous, shiny, and basal. Humectants have the ability to retain the water they attract from the air in the stratum corneum and have the function of moisturising the skin. They are best used before emollients, which are oil-based.
• Skin conditioning agent -  Humectant. Humectants are hygroscopic substances used to minimise water loss in the skin and to prevent it from drying out by facilitating faster and greater absorption of water into the stratum corneum of the epidermis. The epidermis is the most superficial of the three layers that make up the human skin (epidermis, dermis and hypodermis) and is the layer that maintains hydration in all three layers. In turn, the epidermis is composed of five layers: corneum, the most superficial, lucidum, granulosum, spinosum and basale. Humectants have the ability to retain in the stratum corneum the water they attract from the air and have the function of moisturising the skin. It is better to use them before emollients that are oil-based.
• Skin conditioning agent - Miscellaneous.  This ingredient has the task of modifying the condition of the skin when it is damaged or dry by reducing its flakiness and restoring its elasticity.
• Solvent. It is the substance for dissolving or dispersing surfactants, oils, dyes, flavourings, bactericidal preservatives in solution.In fact, it dissolves other components present in a cosmetic formulation. Solvents are generally liquid (aqueous and non-aqueous).
• Viscosity control agent. It controls and adapts viscosity to the required level for optimal chemical and physical stability of the product and dosage in gels, suspensions, emulsions, solutions. 

Other uses

• Pharmaceuticals. Solvent in pharmaceutical products, including oral, injectable and topical formulations.
• Chemical industry. Common component of polyester resins and propylene glycol alginate production, which is a thickener, stabiliser.
• Antifreeze and de-icing. It is used as an antifreeze due to its low freezing point in applications such as anti-icing fluid for aircraft.
• Electronic cigarettes. Acts as a carrier for nicotine and flavours.

Safety:

• It is a product studied for over 50 years
• In the human body it quickly turns into lactic acid
• It has a very low degree of toxicity
• Easily biodegradable
• However, it can cause contact dermatitis

The most relevant studies on this chemical compound have been selected with a summary of their contents:

    Propylene glycol studies

• Molecular Formula: C₃H₈O₂
• Linear Formula   CH₃CHOHCH₂OH
• Molecular Weight: 76.095 g/mol
• CAS: 57-55-6  25322-68-3  63625-56-9
• EC Number: 200-338-0  613-293-0
• FEMA Number: 2940
• DSSTox Substance ID  DTXSID0021206
• IUPAC  propane-1,2-diol
• InChI=1S/C3H8O2/c1-3(5)2-4/h3-5H,2H2,1H3 
• InChl Key      DNIAPMSPPWPWGF-UHFFFAOYSA-N
• SMILES   CC(CO)O
• MDL number: MFCD00064272
• Beilstein Registry Number:1340498
• ICSC   0321
• NSC   69860    35749    35748    35747
• RTECS    TY2000000
• JECFA    925

Synonyms:

• 1,2-propanediol
• 1,2-Propylene glycol
• 2,3-Propanediol
• 1,2-dihydroxypropane
• 1,2-propane diol
• Monopropylene glycol
• 2-Hydroxypropanol
• Trimethyl glycol
• Methylethyl glycol
• Methylethylene glycol
• Solargard P
• Isopropylene glycol
• alpha-Propyleneglycol

References____________________________________________________________________

(1) Hartmann-Boyce J, McRobbie H, Bullen C, Begh R, Stead LF, Hajek P. Electronic cigarettes for smoking cessation. Cochrane Database Syst Rev. 2016 Sep