Although less popular than o/w emulsions, these systems may be desirable when greater release of a medicating agent or the perception of greater emolliency is desired. Emulsifiers having an HLB range of 2.5 to 6 are frequently selected. When multiple emulsifiers are used, the predominant one is generally lipophilic with a smaller quantity of a hydrophilic emulsifier. These emulsions typically have a total of 45 to 80% oil phase.

Nowdays, formulators have become interested in more elegant w/o emulsions. This has been achieved by formulating with new emulsifying agents, emollient such as esters, Guerbet alcohols, and silicones. Selection of a suitable emollient depends on ability of the material to spread on skin with low tack, dermal compatibility, and perceived elegance by the user. In achieving this elegance, some researchers suggest a correlation of emollient and molecular weight of the emollients. In these studies, viscosity of w/o creams has correlated with molecular weight of the emollients used in test formulations.

High–molecular-weight co-emulsifiers formulated with high–molecular-weight emollients gave more stable w/o emulsions. The polarity of the emollients used was found to be important as well.

Emollients or mixtures of emollients with medium polarity gave test lotions the most desirable stability results. Anionic emulsifiers are generally inefficient w/o emulsion stabilizers, because more surface active agents are often needed to stabilize these emulsions. Sorbitan stearates and oleates are effective emulsifiers when
used at 0.5 to 5.0% sorbitan isostearates, being branched chain materials, give a very uniform particle size for w/o emulsions.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach