Steareth-2 is part of the group of steareth ingredients, which are polyethylene glycol ethers of stearic acid. Steareth ingredients are created through a process known as ethoxylation, a chemical reaction in which ethylene oxide is added to a substrate. In regards to the steareth ingredients, the substrate is stearic acid, a saturated fatty acid with an 18-carbon chain. The number associated with the ingredient (i.e. steareth-2) indicates the average number of repeating ethylene oxide units in the molecule.
The stearic acid portion of this ingredient in lipophilic (attracted to oil) while the polyethylene glycol portion is hydrophilic (attracted to water). This structure enables steareth-2 to function as an emulsifier. An emulsifier is needed for products that contain both water and oil components. Mixing water and oil together creates a dispersion of oil droplets in water (and vice versa). However, these two phases can separate if the product is left to settle. To address this problem, an emulsifier can be added to the system to help the droplets remain dispersed. With an HLB of 4.9, steareth-2 is soluble in oil and gives rise to water-in-oil emulsions.