Evidence-based haircare?

July 19, 2011 10:26 AM

Chemists: I'm using baking soda and vinegar to wash my hair. What exactly are they doing?

I've been doing the shampoo-free thing for a couple of months, and like it, but dislike the fact that the no-poo movement is a cavalcade of woo ("OMG toxins absorbed thru ur scalp!" "SLS causes brain damage!") and contradictory advice ("Apply vinegar only to the ends!" "Use honey instead!" "You must make a stiff baking soda paste!" "You must make a weak baking soda solution!" "Leave it on several minutes." "Rinse immediately.") As I troubleshoot my formula, I thought it would be useful to know exactly what the stuff I'm putting on my head is doing!

I scrub my scalp and roots and the middle part of my (long, very thick, straight hair) with 1 tablespoon of baking soda dissolved in 1 cup of water. I know I've got enough baking soda in when my hair stops feeling plain-wet-hair-sticky and feels slippery.

When I've thoroughly rinsed out the baking soda, my hair feels very sticky. (Some no-pooers stop at this point, but while I have not tried to comb my hair in this state, it feels like a very bad idea.)

I then rinse my hair from root to tip with 1 tablespoon of apple cider vinegar diluted in 1 cup of water, and rinse thoroughly in plain water, after which my hair feels slick and comb-able again.

What are these two substances actually do? Is their function in any way analogous to shampoo and conditioner? Is there any benefit to leaving either one, or both, on my hair for a few minutes in the shower, or can I save time by rinsing immediately?

(Snowflakey Details: My hair gets clean, and it stays cleaner longer than it used to, so I'm happy overall, though I'd like to work toward less greasiness. My ends are fine, but my hair is just a bit oilier than I would like near my scalp and in the middle regions. I do have a boars-bristle brush that I try to use to distribute the oil. The advice I'm reading says to avoid greasiness, you should use less vinegar, but then it seems my hair stays "stickier" and I get more tangling and breakage when I comb it out. I'm not so much interested personal experience or what works for your hair; I can find plenty of anecdotes and helpful suggestions on the web. I'd really like to know about the chemistry!)

posted by BrashTech to clothing, beauty, & fashion (10 answers total)

Baking soda is a weak base, but my guess is that half the use of it is just as a particulate scrub.

Vinegar is a weak acid, which will neutralize any remaining baking soda.

Supposedly bases tend to make all the little scaly bits of your hair tend to stick out more, which will make them catch each other, stick together and snarl more. The vinegar would then be undoing that. Darned if I know whether that's true...time for some hair and a microscope, I guess.
posted by Dr.Enormous at 10:45 AM on July 19, 2011

Something with a little higher alkalinity would be diluted borax in purified water. You could also use diluted citric acid in place of vinegar which may have a more pleasant smell. Normally a good base will work to remove oils, but you'll want to balance out the pH afterwards with a mild acid. You might want to still experiment a little with ratios to get the right balance...it sounds like so far the base isn't strong enough to remove the oily feeling (you'll still want some oils to remain...just get rid of the feeling of being overly oil)
posted by samsara at 11:01 AM on July 19, 2011

Read the Wikipedia shampoo entry. Shampoos apparently tend to use citric acid instead of vinegar, but the overall effect is just that of a mild acid, which apparently makes hair smoother and shinier and thus easier to deal with.

Baking soda, on the other hand, strikes me as being potentially bad for your hair, as alkaline substances apparently dissolve keratin.

Really though, the chemistry seems to suggest that the whole no-poo thing is... kind of silly. There's a reason you're finding contradictory and inconsistent advice. The whole thing isn't actually premised on chemistry.
posted by valkyryn at 11:04 AM on July 19, 2011 [2 favorites]

I can't give a full synopsis, but yes, baking soda is a weak base. One of the marks of bases (although this is not a litmus test or anything) is that they often feels slippery to the touch (as well as taste bitter, although I don't recommend you taste test everything).

Bases make the overlapping structure of the cuticle (the outermost layer of the hair) not smooth, basically. The overlapping structure (think roof shingles or fish scales) sort of flare up, which makes your hair feel tangly and snarly as the raised scales catch each other. Vinegar, or an acidic medium, tends to make the scales smooth down so it feels manageable. This has to do with the hydrogen bonds of hair being strengthened in an acidic medium, I believe, so that a more acidic medium makes the hydrogen bonds pull on each other "tighter" and pulls in the scales so it sits smoothly.

I've used vinegar rinses on my hair, and while it does do for a nice rinse, I also don't feel it works as well as traditional conditioners.

Traditional conditioners usually contain, along with acidifiers, a blend of humectants (holds in moisture), gloss-adding, thermal protectors, lubricants, etc etc. Since most of this stuff is designed to stay on the hair surface (thermal protectors don't work very well if it's washed down the drain), your hair tends to get a bit of buildup over time. A clarifying shampoo or even your baking soda/vinegar combo will probably get rid of it, although again, I do find commercial products to work better.

As for how long to leave on your head: I don't see the harm in leaving it a little longer if you feel it gives you better results, but since you're not exactly testing the pH of your solutions, I'd just leave it for as long as you feel like with the caveat that it's comfortable and you're not irritating the heck out of your scalp. Bases weaken the hydrogen bond of keratin (the protein that makes up hair), so I wouldn't recommend doing a basic wash every time.

Shampoo is mostly a surfactants, some thickeners, and usually a little bit of smoothing/conditioning agent.
posted by Hakaisha at 11:06 AM on July 19, 2011 [6 favorites]

valkyryn, did Hakaisha provide enough chemistry for you?

OK, I got your point, and there is a lot of hand-waving going on, but that doesn't mean it doesn't work. It means the practitioners don't know diddly about chemistry... which is par for the course for DIY beauty products, and professional beauty product marketing.

I use the poo-free method, and I've always assumed the basic bicarb joins loosely with the oil molecules, forming weakly-joined soap molecules (or something very akin). Since this process is somewhat inefficient (as opposed to rinsing with true soaps or surfectants), the oils aren't totally stripped away; merely reduced. As noted above, the scrubbing action of the bicarb also contributes.

The vinegar then obviously returns the pH back towards a more neutral level, and mild acids tend to soften and shine protein fibers such as hair and silk (probably for reasons mentioned above).
posted by IAmBroom at 11:19 AM on July 19, 2011 [1 favorite]

Over time, the alkalinity of the baking soda is likely to destabilize the proteins in your hair. While vinegar may smooth the cuticle after it is lifted by the soda, it won't replace the protein.
posted by oneirodynia at 1:37 PM on July 19, 2011

Something with a little higher alkalinity would be diluted borax in purified water.

If you really are trying avoid 'toxins', then please avoid borax. It has recently been reclassified in Europe as Toxic to reproduction ("R60-61: May impair fertility. May cause harm to the unborn child"). Just because something is natural and has been used for a long time, does not mean it's safe. I'd certainly be 100x more worried about borax than SLS/SDS (which is still to say, not very much).
posted by firesine at 2:04 PM on July 19, 2011

The way acids and bases affect the scales is interesting. I've definitely seen the scales on hair in magnified hair on television commercials and in print advertisements. *nods intently*

oneirodynia: what kind of timescales are we talking about? I measured my hair (for SCIENCE!) and it's about 12 inches long, and I read somewhere that hair grows on average 0.5 inch / month. Is subjecting the proteins in my hair to a mildly alkaline solution every other day going to have a significant impact in 2 years? Sounds like semi-reasonable justification for rinsing immediately, though!

Thanks for the note on borax's toxicity, firesine. I'm not actually worried about "toxins," but it's good to know. :) The no-poo thing just seemed to me like a frugal way to lighten my ecological footprint somewhat, and to avoid products with fragrances (because I don't like them, not because I'm afraid of them.)

Thanks for laying the chem knowledge on me, everyone!
posted by BrashTech at 2:52 PM on July 19, 2011

I did some research on baking soda + vinegar when I was putting together a home spray for my roses. When you mix vinegar and baking soda, they react to produce carbonic acid (which decays into carbon dioxide and bubbles away) and sodium acetate, which among other uses is an antifungal.
posted by bq at 7:35 PM on July 19, 2011

Aren't you also potentially damaging the cuticle? I have hair like yours and get the results you describe with Giovanni Tea Tree conditioner after my usual shampoo.
posted by R2WeTwo at 5:20 AM on July 20, 2011