What holds it together

The emulsifier is usually a small percentage of the formula and it is the component holding everything else in place. It is also the one most often substituted casually.

The emulsifier is doing structural work

It sits at the interface between oil and water, lowering the surface tension so droplets disperse and stay dispersed. The film it forms around each droplet is what prevents them from finding each other and coalescing.

So a failed emulsion is usually a failed interfacial film, not a shortage of mixing.

HLB, and what it is for

The hydrophilic-lipophilic balance is a number describing how water-loving or oil-loving an emulsifier is. Oil-in-water emulsions need a higher HLB; water-in-oil needs a lower one.

Oils each have a required HLB, so a blend of oils has a weighted requirement, and matching the emulsifier system to it is the arithmetic that makes an emulsion work first time rather than on the fifth attempt.

It is a guide rather than a law, and it is far better than guessing.

What breaks an emulsifier

Electrolytes, and this is the one that surprises people. Magnesium salts, zinc oxide and high preservative loads dehydrate the ethoxylate chains on many emulsifiers, lowering the effective HLB and pushing the system toward inversion or separation.

Which means adding a mineral sunscreen, or increasing the preservative after a failed challenge test, can break an emulsion that was previously stable. The two most common interventions in a small brand's first year both do this.

Heat, shear and pH extremes also matter, and time simply lets thermodynamics win.

Choosing one

Match the HLB to the oil phase. Prefer an emulsifier system, usually a pair, over a single one, because a primary emulsifier plus a co-emulsifier builds a more resilient film.

Then treat it as fixed. Substituting an emulsifier is a reformulation with a new stability study and a new challenge test, not an ingredient swap, however similar the supplier says the two are.

References