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So I have been wondering why you can see the following:

Using 10 ml oil and 1 ml water with and emulgator leads to a way better emulsion than if you use 10 ml water with 1 ml oil (if you use the same amount of emulgator)

btw: as an emulgator we used the yellow part of an egg

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  • $\begingroup$ Could you clarify your question? $\endgroup$
    – A.K.
    Oct 5, 2018 at 15:57
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    $\begingroup$ My question is why oil in water is makes a more stable emulsion than water in oil using the same amount of emulgator. $\endgroup$
    – Cornelia_88
    Oct 7, 2018 at 10:32

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The two combinations (10/1 and 1/10) of oil and water will give different emulsions: the first will be water-in-oil and the second will be oil-in-water. The difference lies in the continuous phase; the minor phase will be tiny droplets, with a size depending on how much energy you put into shearing it. Neither emulsion will be particularly stable; that's why we need emulsifiers: to stabilize the emulsion, and also to make it easier in the first place.

The yolk of an egg is primarily fatty (that's why it tastes so good). It's so fatty that it does not absorb the egg white, which is 90% water and about 10% protein. The yolk does contain some hydrophilic groups - but not enough to emulsify the albumen. But the yolk can emulsify/stabilize a small amount of water, and the yolk will disperse easily in (some) oils. So the yolk, spread out in oil, can stabilize a smaller amount of water. But you can't get the yolk well dispersed in water: it agglomerates quickly. And extra oil just makes the destabilization quicker and more obvious.

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    $\begingroup$ Thanks for the answer! Do you also know the mechanism behind it, why oil in water makes the destabilization quicker? $\endgroup$
    – Cornelia_88
    Oct 7, 2018 at 10:34
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    $\begingroup$ While egg yolk has some hydrophilic groups (not enough to be really stable in water), the oil probably has much less hydrophilicity (depending on the oil, of course). The benefit of the oil is to disperse the egg yolk and allow it to coat tiny globules of water (which have to be made tiny by shear energy from the mixer blade - the process is not spontaneous). After the water is sheared into tiny droplets and coated, the emulsion is viscous enough to be stable for a while. Oil alone doesn't stabilize water droplets well, so they are larger and settle quicker. $\endgroup$
    – James Gaidis
    Oct 7, 2018 at 13:12
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    $\begingroup$ So the emulgator is the important part right? It decides whether you get oil in water or the other way around, something explained by the Bancroft-rule. $\endgroup$
    – Cornelia_88
    Oct 7, 2018 at 16:23
  • $\begingroup$ Pretty much. Emulsifier is a more common term. It is a combination of hydrophilic and hydrophobic parts in the same molecule; it is uniform and homogeneous, but has the ability to coat tiny droplets and make them seem opposite to their real nature. The chemical magic is that emulsifiers (also called surfactants) can be synthesized with different balances of hydrophilic and -phobic groups. This allows formulating with different oils and different amounts and different stabilities. And yes to Bancroft's Rule. $\endgroup$
    – James Gaidis
    Oct 8, 2018 at 13:27

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