polyethylene glycol

polyethylene glycol (PEG), synthetic chemical compound derived from petroleum that is widely used as a laxative for the treatment of constipation and that has various other uses, including as a moisture carrier, solvent, and thickener. Polyethylene glycol is a hydrophilic (“water-loving”) polymer that is made by reacting ethylene oxide with ethylene glycol (the simplest member of the glycol family) or with ethylene glycol oligomers (molecules that contain repeating units) or water. Its formula is H(OCH₂CH₂)_nOH, in which n denotes the number of individual ethylene oxide units.

Polyethylene glycol was first produced in 1859 by Portuguese chemist A.V. Lourenço, who heated ethylene glycol with ethylene dibromide and then isolated the resulting ethylene glycol oligomers by fractional distillation. French chemist Charles Adolphe Wurtz independently synthesized the compound about the same time. One of the first commercial products based on polyethylene glycol was CARBOWAX, a water-soluble wax introduced in 1940 by American manufacturer Dow Chemical Company.

Polyethylene glycol is commonly used in the treatment of chronic constipation and fecal impaction, with polyethylene glycol powder being mixed with a liquid and taken by mouth. As an osmotic laxative, the compound works by forming hydrogen bonds with water molecules, which prevents the reabsorption of water in the gastrointestinal tract, causing water retention in the stool and increasing osmotic pressure. Water retention softens the stool, making it easier to pass; the number of bowel movements also increases. Potential side effects include nausea, bloating, and intestinal cramping; more serious side effects include diarrhea and, in sensitive individuals, hives. Polyethylene glycol is sometimes also used in colonoscopy preparation as a way to cleanse the bowel to help provide a clear image; it is typically mixed with two litres of a sports drink the day before the procedure.

Polyethylene glycol–based gels, or hydrogels, are widely used for the delivery of therapeutic biomolecules, owing to their inert characteristics, their high biocompatibility, and their ability to be chemically modified to control material properties. Coatings made of polyethylene glycol are also applied to implants to minimize the risk of immune reaction.

Nonmedical applications of polyethylene glycol include as a surfactant, emulsifier, cleaning agent, humectant, and skin conditioner in cosmetic products. Because it is hydrophilic, it may be used to passivate glass microscope slides to prevent nonspecific sticking of proteins in single-molecule fluorescent studies. Polyethylene glycol is also used to preserve waterlogged wood and other organic artifacts salvaged in water; it replaces water in wood, improving the stability of the wood and preventing shrinkage as it dries.

Polyethylene glycol has also been used to preserve colours on the terra-cotta army, the terra-cotta sculptures of warriors discovered in Shaanxi province in northwest China in 1974. The primitive paint and lacquer used to colour the statues flaked off almost immediately after the statues were removed from the damp earth and exposed to the region’s dry air. Polyethylene glycol replaced most of the moisture in the lacquer, though it sometimes darkened the terra-cotta, and cracks appeared in the lacquer when the moisture evaporated faster than the polyethylene glycol solution was applied.