PEG-9, also known as Polyethylene Glycol 9, is a non-ionic surfactant and polymer that is widely used in cosmetics and personal care products. It is derived from ethylene oxide and is characterized by its average molecular weight corresponding to the addition of nine ethylene oxide units. PEG-9 is valued for its ability to act as an emulsifier, solubilizer, and skin-conditioning agent.

Chemical Composition and Structure

The chemical composition of PEG-9 includes:

• Polyethylene Glycol (PEG): A polymer formed from the polymerization of ethylene oxide, characterized by a repeating unit of -[O-CH2-CH2]n-.
• Molecular Weight: The average molecular weight of PEG-9 corresponds to approximately 400 g/mol (based on 9 ethylene oxide units).
• Hydroxyl Groups: The presence of hydroxyl groups (-OH) at both ends of the polymer chain contributes to its solubility and emulsifying properties.

Structurally, PEG-9 is a linear polymer with hydrophilic segments (ethylene oxide) that enhance its compatibility with water-based formulations.

Physical Properties

• Appearance: Typically a clear to slightly yellow viscous liquid.

• Solubility: Soluble in water and many organic solvents; exhibits good compatibility with a variety of cosmetic ingredients.

• pH: Generally neutral.

• Odor: Mild or odorless.

• Stability: Stable under normal conditions; may be sensitive to extreme temperatures.

Production Process

• Polymerization: PEG-9 is produced by the polymerization of ethylene oxide under controlled conditions, where the average degree of polymerization corresponds to nine ethylene oxide units.

• Purification: The resulting polymer is purified to remove unreacted monomers and other impurities.

• Formulation: PEG-9 is incorporated into various cosmetic and personal care products, enhancing their texture and stability.

Applications

• Medical: Occasionally used in pharmaceutical formulations as a carrier for active ingredients.

• Cosmetics: Commonly found in creams, lotions, and hair products for its emulsifying and skin-conditioning properties.

• Food: Not typically used in food products, but may be found in food-related applications as a processing aid.

• Industrial Uses: Used in industrial formulations as a lubricant and surfactant.

Environmental and Safety Considerations

Cosmetic Safety

PEG (Polyethylene glycol) polymerize condensed ethylene oxide and water and are referred to as polyethylene glycols, but they are actually complex chemical components, polymers bonded together. For example, plastic is polyethylene and has a hard consistency, while polyethylene aggregated with glycol forms a liquid. PEGylation is produced not only as heterification but also as transesterification, which is the transformation of an alcohol by an ester. 

The number that appears after the abbreviation PEG represents the molecular weight, and the higher this number is, the less it penetrates the skin.

The term 'eth' refers to the ethoxylation reaction with ethylene oxide after which residues of ethylene oxide and 1,4-dioxane, chemical compounds considered carcinogenic, may remain. The degree of safety therefore depends on the degree of purity of the compound obtained. No manufacturer appears to provide this information on the label, at least as of the date of this review.

Kim MC, Park SY, Kwon SY, Kim YK, Kim YI, Seo YS, Cho SM, Shin EC, Mok JH, Lee YB. Application of Static Headspace GC-MS Method for Selective 1,4-Dioxane Detection in Food Additives. Foods. 2023 Sep 2;12(17):3299. doi: 10.3390/foods12173299. 

Abstract. "Efficient detection methods must be developed for 1,4-dioxane due to its suspected status as a human carcinogen, which is highly mobile in food and environmental resources. In this regard, this experiment has been conducted to develop reliable and selective detection and measurement methods by using static headspace (SH) isolation, followed by gas chromatography-mass spectrometry (GC-MS). A new method was developed for determining the spiked 1,4-dioxane contents in a polyethylene glycol 600 (PEG 600). The optimal condition for SH-GC-MS was discussed. The representative ions of 1,4-dioxane and 1,4-dioxane-d8 in the SIM mode of MS are 88 and 96, respectively, and the peaks of the SIM mode were separated and confirmed. The linear range for the method covers 0.25 to 100 mg/L with a coefficient of determination (R2) ≥ 0.999. The method applicability was demonstrated by spike recovery across a variety of food additives (i.e., chlorine bitartrate, choline chloride, polysorbate 20 and 60, and PEG 1000). All spike recovery from the tested samples was in the range of 89.50-102.68% with a precision of 0.44-11.22%. These findings suggest a new analytical method for food safety inspection, and could be applicable for ensuring the safety of foods and environmental and public health on a broad scale."

 Responsible sourcing and formulation practices are essential to ensure that the ingredient is free from harmful contaminants and produced sustainably.