However, with a high-speed mixer the thermodynamic (based on HLD) O/W emulsion can be obtained.Ī complication is provided by viscosity. If mixing is done by hand and the oil is thrown in quickly, the result is likely to be that the water accepts it is a minority and you get a horrid W/O failed mayonnaise. Mayonnaise is an O/W emulsion with 70-80% oil very close to the inversion limit. the blue triangle spreading from right to left into the orange. But if you started with a normal 60% O/W emulsion and started to add oil, you might find you had to go to 85% oil before it flipped, i.e. the yellow triangle spreading from left to right into the blue. So you might find you still have a W/O emulsion up to 60% water, i.e. It depends on % surfactant, HLD, viscosities and stirring speed. When you do such experiments you find that the point of flipping isn't fixed. For a while the system will keep producing a W/O emulsion till the whole thing flips into what it knows it should be, an O/W emulsion. Now keep stirring with the slow addition of water. But examination of what we've produced would most likely show a W/O emulsion. this should naturally be an O/W emulsion. To explain the funny triangles, let us imagine having stirred together 15% water in an oil, using a surfactant giving HLD=-1 within this system, i.e. It has to be included, but the real point of this page is further down - so please carry on reading! Should shift to an O/W emulsion when more W is added, but things might get stuck the wrong way round. More importantly, these complications can mean that those who play at this boundary can end up with a mess. By having some o inside the W in the o/W/o the oil continuous phase isn't quite as overwhelming. These extra complications can be considered as nature's way to redress the balance of the two species. Inverted O/W is similarly often w/O/W with water drops inside the main oil emulsion drops inside a continuous water phase. So inverted W/O is often o/W/O where the small o means a drop of oil inside a larger water drop inside a continuous oil phase. I have deliberately avoided cluttering the already complicated images with the fact that the inverted phases are often multiple phases. However, even if HLD0 will flip into an O/W emulsion when the % oil drops below 25% in the domain of more than 75% water. The horizontal line in the middle is at HLD=0 and shows where systems will naturally change from Type I (O/W) when HLD is lower to Type II (W/O) when HLD is higher. At greater than the sphere close packing limit of 74% there simply isn't a way to pack the oil spheres with water surrounding them, so the system has to flip.įollowing Salager (who yet again has produced so much clarity through his regular use of such diagrams) we can produce a step graph which shows the issues involved. The key to this is that although the surfactant system is "happy" with an O/W emulsion, the overriding fact is that water is below a critical % in the butter and thermodynamics make it preferable to have the small amount of water dispersed in the oil phase.Ī simple (simplistic) explanation for this comes from Ostwald. I have had to invoke the ideas here in a number of very tough formulation challenges involving (or avoiding) inversion and have found them to be very powerful.Ĭream seems to be a rather conventional O/W emulsion. Those who don't know the HLD aspect of inversion have (and I have read many academic papers showing this) a very hard time because nothing makes much sense. Once you start to understand inversion by including HLD, a difficult problem becomes much less difficult (though still challenging). These and other frustrations involve emulsion inversion. Sometimes you deliberately want to change from an O/W to W/O but it doesn't happen when you want it. Sometimes you want an O/W emulsion but get a W/O. HLD - Hydrophilic Lipophilic Difference.
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