I added various solvents to a shower gel product to see what would happen. The main surfactant ingredients in it are Methyl Gluceth 20, Sodium Laureth Sulfate, and Lauryl Glucoside.

First I diluted the sample in water, and added it to a separating funnel along with hexane. After shaking and leaving overnight, the lower water layer was clear and hexane layer was a viscous cloudy white. I extracted the water layer (which was a bit soapy but clear), leaving the hexane layer in the funnel.

I then added methanol to the separating funnel. Adding methanol immediately caused the white soap to clump together at the interface, and the hexane layer now appeared clear. (I don't think I shook the separating funnel in this step)

I then added fresh water to the separating funnel again. This caused the white to diffuse into the lower methanol/water layer so that it could be extracted. The hexane layer was now clear.

I am trying to understand why these things happened. Here is what I think:

  • The surfactant molecules (e.g. SLS) have long hydrophobic tails, and hydrophobic heads. In water they will arrange themselves into micelles etc with the tails pointing in so they aren't exposed to the water. In hexane they will arrange tails pointing out so the heads aren't exposed. The majority of the molecule being hydrophobic, it will be more favourable to arrange tails-out in the hexane, so this is why the hexane layer went cloudy and water was clear (but still soapy).

  • When methanol was added, the surfactants clumped together at the interface, and this might be the surfactants arranging with their heads pointing to the hydrophilic methanol and the tails pointing to the hexane at the interface. But why didn't they do this with the water/hexane layers? Water is more polar than methanol (polarity index 9 vs 5.1) so maybe water is too polar even for the heads to be pointing to in that way. It probably isn't because of density because the density of SLS and others is a bit over 1 g/ml.

  • Adding water to the methanol layer maybe adjusted the polarity just right so that surfactant could dissolve in it, but I'm not sure how that works. Maybe since methanol has a slightly non polar region, that would help solvate the tails while the water solvates the heads?

Let me know if my thoughts sound correct or if there's something I haven't considered.


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