Formulating water-in-oil emulsions is inherently more difficult than oil-inwater emulsions. While there are several reasons for this, probably the most important one is that these emulsions (usually) do not have a clearly defi ned electrical double layer surrounding the emulsion droplets; thus there is a greater likelihood of coalescence. While water-in-oil emulsions have become more popular in the United States in recent years due to their rising popularity in the sunscreen arena, they have enjoyed widespread use in Europe for many years. I have never heard a good explanation for this geographical discrepancy.
I can’t tell you how many times that chemists have told me that the emulsion they made must be water-in-oil since they’“added the water phase into the oil phase!“ This, of course, is nonsense. Another comment I hear is that I have more oil than water in my emulsion, so oil must be the external/continuous phase. More nonsense.
One thing that I have noticed in recent years as we have developed more efficient W/O emulsifiers is that the ratio of oil to water is basically the same whether we have a W/O or O/W emulsion. As a general rule, the amount of water is approximately 60-80%. We should remember that if the internal phase ever exceeds 74%, we no longer have only spherical droplets. The type (O/W or W/O) of emulsion you wind up with is almost entirely dependent on the emulsifier.
Bancroft’s Rule tells us that wherever the emulsifier is most soluble will become the external/continuous phase. One tried-and-true way to determine which type of emulsion you have is to add water to it with propeller mixing. If the water is readily incorporated and the viscosity drops, you can be quite certain that the emulsion is O/W. If, on the other hand, it required quite a bit of mixing to incorporate the water and the viscosity actually increases, then the emulsion is W/O. I find this test to be easy and generally reliable.
There are various things to consider when formulating W/O emulsions.