The hydrophilic-lipophilic balance system (or HLB) was created by William Griffin the 1940s as a way of figuring out which emulsifier would work best with the oil phase of an emulsified product. All emulsifiers have a hydrophilic head (water loving) that is generally composed of a water soluble functional group and a lipophilic tail (oil loving) generally composed of a fatty acid or fatty alcohol.
The theory behind HLB is that emulsifiers showing greater solubility in water would be better for oil in water emulsifications; emulsifiers showing great solubility in oil would be better for water in oil emulsifications. The lower HLB valued emulsifiers are better in water in oil as they are more lipophilic; the higher valued HLB emulsifiers are more hydrophilic. (Solubility of a molecule generally increases with temperature, hence the reason for heating and holding our lotion ingredients. And most emulsifiers we use are in a pellet or flake form, so heating is the only way to incorporate it into a liquid environment!)
The HLB value of an emulsifier is determined by the hydrophilic portion of the surfactant. The equation is as follows - HLB = % hydrophilic portion by weight of the molecule divided by 5. (We don't need to know this to use the HLB system, but I'm the kind of girl who has to know everything, so I thought I'd include it for like minded people!) So the higher the number, the higher the portion of the molecule is hydrophilic, and the more water soluble it will be.
Take a look at this polysorbate 80 molecule (HLB 15). Based on this number, we should expect this is an emulsifier that would be better suited for oil in water emulsifications because it has such a high HLB number. (And from experience, we know poly 80 is well suited for adding oil to watery things like toners or facial cleansers - not a lot of oil, but enough that we want it not to float on top of the product!) We don't see any fatty acids or alcohols on this chain, so it's going to be a hydrophilic emulsifier with a high HLB value.
This is sorbitan stearate (HLB 4.7). Based on this number we can predict this molecule would be better suited for oil soluble applications, like a water in oil lotion.
You will never use one emulsifier alone for a lotion - you might for bubble baths, body washes, perfumes, and so on, but for a lotion, you need a system. We want to combine a low HLB emulsifier (the one that loves oil more) and a high HLB emulsifier (one that loves water more) at a level that will match the HLB of our oil phase. And we'll want to add enough of these emulsifiers to ensure we get proper, stable emulsification of our product. (I've seen it suggested that you start at 2% and at 4% emulsifiers in your lotions. I'm going to suggest 4% for now to ensure we have enough in the lotion to be successful.)
So how can use we the HLB system to create awesome emulsification systems? Join me tomorrow for a sample recipe!