Span (sorbitan fatty acid ester series) is typically an amber to dark brown viscous oily liquid or waxy solid at room temperature (the specific form depends on the type of fatty acid attached). It is poorly soluble in water but readily soluble in most organic solvents. As a non-ionic surfactant, its chemical properties are stable, with resistance to acids, alkalis, and hard water. However, as a polyol ester, it can undergo hydrolysis under strongly acidic or strongly alkaline conditions. Its molecular structure contains a polyol hydrophilic group and a long-chain hydrocarbon hydrophobic tail, resulting in overall strong lipophilicity, classifying it as an oil-soluble emulsifier with a low Hydrophile-Lipophile Balance (HLB) value. Based on its excellent emulsifying (particularly stabilizing the oil phase), dispersing, and wetting properties, the primary application of Span is as a lipophilic base emulsifier. It is widely used in food (e.g., ice cream), cosmetics (creams and ointments), pharmaceuticals (creams and ointments), and industrial fields (e.g., textile lubricants, metalworking fluids) for formulating oil-in-water (O/W) or water-in-oil (W/O) emulsions. It is also frequently used in combination with the Tween series to precisely adjust the HLB value of emulsification systems.