Sorbitan sesquiisostearate is produced by reacting sorbitol with a mixture of mono- and diesters of isostearic acid (a fatty acid). In cosmetics and personal care products, sorbitan sesquiisostearate functions as an emulsifier. Emulsifiers are needed in formulations that contain both water and oil components. Mixing water and oil together creates a dispersion of oil droplets in water (and vice versa). These two phases can separate if the product is left to settle. To address this problem, an emulsifier like sorbitan sesquiisostearate can be added to help the droplets remain dispersed. With an HLB value of 4.2, sorbitan sesquiisostearate is soluble in oil and gives rise to water-in-oil emulsions.