In the dyeing of textiles, surfactants help the dye penetrate the fabric evenly. They are used to disperse aqueous suspensions of insoluble dyes and perfumes. Surfactants function by breaking down the interface between water and oils (and/or dirt). They also hold these oils (and/or dirt) in suspension, and so allow their removal. They are able to act in this way because they contain both a hydrophilic (water loving) group, such as an acid anion, (-CO2- or SO3-) and a hydrophobic (water hating) group, such as an alkyl chain. Molecules of water tend to congregate near the former and molecules of the water-insoluble material congregate near the latter. Surfactants are produced in special reactors where ethylene oxide is added to a substrate called "starter" (ethoxylation).