Thick + Thick = Thin?

October 23, 2009 6:30 PM

What happened to my Shampoo? I mixed two different Shampoo brands about 70/30 and liked the resultant mixture. The next time I mixed them 50/50 and overnight the mixture lost all it's "thickness" (viscosity, I guess) and still works fine, but is as thin as milk if not thinner.

The two brands are Pert Plus and Avalon Organic Botanicals "Nourishing Shampoo" (bought at Trader Joe's). I *believe*, but am not totally sure, that the Pert Plus was the 70 per cent one when it worked fine the first time.

posted by Rafaelloello to science & nature (6 comments total)

Well, no beauty experts have come forward in almost 2 hours, so I'll take a guess: I think you broke an emulsion. If I remember correctly, Pert is a thick creamy product, sort of pearly looking - this is almost certainly an emulsion, not a simple thickened solution (like you would get by adding carrageenan or corn starch, etc). I don't know what the Avalon shampoo is like, but I'm guessing it's not thick and creamy like the Pert.

Emulsions are only stable in a certain range of proportions (other factors like pH play a smaller role). When you mixed the Pert with the other shampoo, something fell outside the range that the emulsion could tolerate, and so it collapsed into a simple unemulsified mixture. Do you see 2 distinct layers of liquid, like oil and water in a salad dressing? That would be pretty definitive for a collapsed emulsion.

For further geekery, emulsions are a type of colloid. Colloidal dispersions can be thin, like milk (which is actually a colloid), while emulsions are usually thick, like mayonnaise or hand lotion. Colloids can be destabilized by adding something that will cause the suspended particles to aggregate, resulting in a flocculant precipitate sitting at the bottom of a thin (non-viscous) solution. If you see a layer of solids (either fine like silt or puffy like curdled milk) at the bottom of your bottle, you've probably destabilized a colloid.

Hopefully this will give you enough of an answer to do a little more digging if you're still curious.
posted by Quietgal at 7:58 PM on October 23 [3 favorites]

yeah, and you can re-emulsify if you're curious. mix back to 70/30 and shake, see if it resolidifies.
posted by wuzandfuzz at 8:13 PM on October 23

Very good Quietgal and wuzandfuzz! Yes, the Pert is creamy and the Avalon is clear. I do have it in the green transparent Avalon bottle and I have looked for a water/oil like separation but I don't see any, it looks to be very homogenous; it's still the color of Pert, but thin as milk. Also, the hole in the cap of the Avalon bottle is quite small, and hasn't clogged with any chunks and I'm getting pretty close to the bottom. I've been using it for a couple weeks this way and I'll attempt to re-emulsify on Monday (I bicycle commute and shower at work, it's my work shampoo that thinned out). I do have both shampoos here at home and I was thinking about trying to duplicate the phenomena with different mixtures. The mixture is just so darned thin, it's like something happened to both shampoos, not just the Pert emulsion. Is it possible that they are both emulsions and, perhaps from changing pH, *both* of them broke down.

I have this funny feeling my wife is going to wake up tomorrow and wonder what all these labeled containers on the bathroom counter are;-)
posted by Rafaelloello at 10:04 PM on October 23

If you're going to take it that far, be sure to document it for the internet. Cockeyed has built a reputation on vaguely silly "science-y" stunts, and this seems like a low-rent version of those.
posted by subbes at 10:39 PM on October 23

subbes: Will do (hoping my self-esteem is strong enough to endure being the perpetrator of a a low-rent version of a vaguely silly "science-y" stunt which all the interwebs will view).
posted by Rafaelloello at 12:11 PM on October 24

I looked into this a little more, and I'm pretty sure you broke the emulsion of the Pert Plus. This is a so-called 2-in-1 shampoo, meaning it is both a shampoo and a conditioner, and the way those products work is pretty interesting. In summary, they contain silicones in an emulsion with the soap, and when the emulsion breaks during rinsing, the silicones are deposited on the hair, leaving it smooth and shiny. (The PertPlus FAQ confirms this.)

There is probably very little silicone in Pert Plus - believe me, a little silicone oil goes a long way - and it would not form a visible layer of liquid like the salad dressing I mentioned. In fact, it would probably cling to the plastic bottle walls in an invisible thin film.

If simply rinsing your hair (i.e. diluting the Pert Plus shampoo) is enough to break the emulsion, it makes sense that adding the other shampoo could also break it. You're adding more soap to the system, which probably destabilizes the emulsion even faster than simply diluting it with water. (The physical chemistry of soaps and detergents is complex and fascinating, but I'm not an expert so I'll leave it at that.)

I looked up the Avalon shampoo and I can't tell from the ingredient list whether it's also an emulsion or not. So I don't know why the viscosity of your final mixture was so low - again, I'm not an expert in colloidal systems and I don't know what else might be going on in your bathroom chemistry experiment.

Lastly, I'd be surprised if you could re-emulsify your shampoo blend, even after bringing it back to the right proportions, by simply shaking - most emulsions are fairly hard to form. (You can make mayonnaise at home but it requires a helluva lot of beating, not just shaking.) It takes a lot of energy to disperse the liquids into sufficiently small droplets to form a stable emulsion, and high-shear mixers are used industrially. You could try putting the stuff in a blender or food processor, but you'd have to make sure you're not mistaking foam (bubbles) for an emulsion. For your 15 minutes of YouTube fame, focus on breaking the emulsion, not trying to re-form it. And be sure to post it to Projects!
posted by Quietgal at 7:59 PM on October 26