-
Science behind Emulsions (looking for a deeper understanding)
I have a few questions about the fundamentals of creating emulsions. For starters, is there a reason that the oil phase and water phase are created separately? Would an emulsion loose stability, or not become as opaque were this not the case? For example; Typically the water phase is mixed together and heated to ~75C and the oil phase materials are mixed together in a separate container and heated to ~75C. Once both phases are the same temperature they are then mixed together (just a recap of what we have all done). Would it make a difference towards the outcome of an emulsion were the aqueous phase heated to ~75C and then the oil phase added step wise directly into it (not so much that it dropped the temperature), eliminating the need for a separate container (If so, why)? Why is this not the common approach?
My second question about the science behind the emulsions has to do with the order of addition. Is it important to add oil to the water phase in a O/W emulsion as opposed to adding the water to the oil phase? I should probably test this out, but there are so many variables and stability studies that I just don’t want to deal with. Using rationalization (I know, far inferior to empirical evidence) I would say that it should not make a difference as long as you have emulsifiers present to allow the two phases to interact. The more abundant phase should “naturally” become the outer phase. Does it work this way? If not, or if so, why?
Thank you, and I look forward to your input and feedback. Of course, as you know, feel free to respond to only one or both questions. I am in search of opinions and experience, not paper citations, so please do not feel as though you need to support your input with scientific publications. If you would like to though, it is not discouraged.
-
7 Replies
-
Emulsions can be created with force alone - think about shaking a vinegar-oil salad dressing - you’ll have a good emulsion for a few seconds, then the oil droplets start to merge/coalesce, and then the emulsion will fall apart.Emulsion stability happens when you keep the oil droplets from coalescing. This can be done either physically, or chemically (or both).
Physically, you can prevent coalescence by keeping the droplets from ever touching each other - that’s done by increasing the viscosity/yield value/gel strength of your external phase.Chemically, you can also prevent coalescence by keeping the droplets from combining once they have touched each other - that’s usually done by having the correct surfactants and correct amount of the surfactants in your formula. Good chemical stability is usually the result of having a cohesive bi-layer (or larger) of surfactants surrounding each droplet of your inner phase, so that when the droplets touch, only the surfactants touch, and the cores do not.One of the reasons oil and water phases are prepared separately is to ensure that most of the surfactants easily get to their proper places in/around the droplets, instead of forming separate surfactant-only micelles. -
another reason the oil phase is prepared separately is to allow all the solids to melt before they’re added to the water phase, ensuring that a uniform emulsion is formed
if you need to, you can sequentially add the oil phase ingredients to the water phase (for instance, if you’re producing it on an industrial scale and there’s no premix vessel available on the plant), but you’ll need a sharp eye to ensure the ingredients are all fully melted and a good shear mixer to ensure it gets thoroughly mixed
for an O/W emulsion, it doesn’t matter whether you add the oil to the water or vice versa (e.g. we had an O/W sunscreen formula where the oil phase had such a large volume that the only practical way to make it on the plant was to add the water to the oil phase), but for a W/O emulsion you must add the water to the oil - if you do it the other way round you’ll end up with the water phase becoming continuous, and the oil phase will fail to mix with it
the reason is that the attractive intermolecular forces are stronger within water than they are within oils; water droplets have a much greater tendency to coalesce than oil droplets, which is why water will form the continuous phase unless it’s physically prevented from doing so, and also why W/O emulsions generally need more rigorous stabilisation than O/W
also, the nature of the continuous phase depends on the nature of the emulsifier(s); the relative volumes of the continuous and dispersed phases are irrelevant
-
“for an O/W emulsion, it doesn’t matter whether you add the oil to the water or vice versa (e.g. we had an O/W sunscreen formula where the oil phase had such a large volume that the only practical way to make it on the plant was to add the water to the oil phase)”^Just curious would that still be considered an O/W emulsion? Perhaps I’m not understanding the deeper meaning of o/w vs w/o. Are o/w and w/o somehow ratio dependent, and not dependent on what’s added to what?
-
You can make an oil-in-water emulsion (with the right surfactants) by continuing to add water to oil until the water droplets get big enough that the emulsion ‘flips’ and the water becomes the continuous phase. Some people deliberately use this as a technique to enhance stability.
Look at this picture and imagine what happens as the blue water droplets get bigger:
-
@Bobzchemist thanks for the clarification!
-
@bobzchemist what you’ve described there is a phase inversion, which didn’t happen with this product - what happened was more like this (please excuse the crude MS Paint diagram!)
1. the oil phase is heated until uniform
2. the water phase is added with stirring
3. the mixed oil and water phases form a crude O/W emulsion
4. the batch is sheared, forming a fine O/W emulsion
Log in to reply.