PEG-75 stearate is a polyethylene glycol ester of stearic acid, a saturated fatty acid that can be derived from both animal and vegetable fats and oils. The long hydrocarbon chain of the stearic acid is lipophilic, which means that it is attracted to oils and fats. One way to make stearic acid more hydrophilic (water-soluble) is by adding multiple units of ethylene glycol to sites along the polymer chain. The oxygen atoms add polarity to the fatty acid and are readily available for association with water molecules. This process is called ethoxylation or polyethylene glycol (PEG)-modification. The number associated with the ingredient name (i.e., PEG-75 stearate) corresponds to the number of repeat units of ethylene glycol.
Since PEG-75 stearate has a structure with one lipophilic end and one hydrophilic end, this ingredient functions as a surfactant in cosmetics and personal care products. Upon application to the skin, the lipophilic end attracts oil and dirt while the hydrophilic end allows these impurities to be washed away with water. Due to these properties, PEG-75 stearate can be found in many different cleansers, body washes, and hair care products.