WOemulsies

WOemulsies, also called water-in-oil emulsions, are emulsions in which water droplets are dispersed in a continuous oil phase. The oil phase is typically a nonpolar or slightly polar liquid such as mineral oil, silicone oil, or natural esters; water is the dispersed phase. Droplet sizes commonly range from submicron to a few micrometers and can influence texture and rheology. These emulsions form when lipophilic surfactants reduce interfacial tension and align at the water–oil interface with a low hydrophilic-lipophilic balance (HLB). Common stabilizers include Span series (sorbitan esters) and other oil-soluble emulsifiers, often combined with thickeners or waxes to improve physical stability and viscosity.

Preparation typically involves high-energy methods (high-shear mixing, rotor-stator homogenization, ultrasonic emulsification) or low-energy approaches (phase-inversion methods) where the water phase is gradually introduced into the oil phase under controlled temperature. Phase inversion temperature and the ratio of phases govern the final emulsion type; W/O may invert to O/W if composition or conditions shift.

Stability concerns include coalescence of water droplets, Ostwald ripening, and phase inversion after dilution or heating. Formulations may include thickeners, conditioners, or occlusive agents to enhance skin feel and barrier protection. W/O emulsions are commonly used in cosmetics and personal care products where occlusion, emolliency, and hydration retention are desired; examples include creams and some sun-care products. They are typically more oil-rich, greasier, and more water-resistant than O/W emulsions, but may be less washable and more sensitive to processing conditions.