water-in-oil emulsion

Butter and many rich moisturisers are the everyday picture: droplets of water held inside a continuous fatty or oily medium. A water-in-oil emulsion, written w/o, is the mirror image of milk — water is the dispersed phase, broken into droplets, and oil is the continuous phase surrounding them.

Because the outside is oil, a w/o emulsion feels greasy and emollient, resists being washed off by water, and forms an occlusive layer that traps moisture against the skin. This makes w/o systems valuable for protective and emollient skin products such as cold cream and barrier ointments, where staying power and skin hydration matter more than a light, washable feel.

Water-in-oil emulsions are favoured by low-HLB emulsifiers (roughly 3 to 6) that are more oil-soluble, such as sorbitan esters (Spans), cholesterol, or wool fat. At the bench they behave opposite to o/w: a w/o emulsion does not dilute freely with water, mixes readily with oil, and conducts electricity poorly because its continuous oil phase carries few ions.