Here's what I know and what I think about pH and hair products. This is "redox" chemistry (oxidation and reduction). One of my teachers said to our class, "Next week, we'll be doing redox," followed by an evil laugh. I love it because it applies to so many living systems we work with every day. I think you will too.
©Science-y Hair Blog 2013
Get ready for a chemistry smackdown on hair cuticle folklore. Forget all that stuff you've heard about cuticles being open or closed as a result of different pH. Cuticles being "open" when it is the result of a solution you put on your hair is a side effect of hair swelling in the wrong pH, in strong detergent, or in just plain old water if it's wet for too long. Likewise for cuticles being "closed."
The shortest story possible: If you do not want to suddenly damage the structure of your hair in an irreversible way - keep it out of solutions with a of pH 3 or under, and out of solutions with a pH 9 or above. Especially do not leave your hair in these solutions for a long time. For that matter - keep those sorts of solutions away from your skin and eyes also!
These extremes will tend to cause the hair to swell, take on acid or base, and structural damage is the result.
©Science-y Hair Blog 2013
Examples of things you might want to avoid putting in your hair to avoid those extremes: pH 2 is undiluted lemon juice, straight vinegar can be pH 3, so can soda drinks. Why you would put soda in your hair, I don't know. But you put it on your teeth when you drink it, which I find disturbing. To get to pH 9, you need (diluted) sodium hydroxide, lithium hydroxide, potassium hydroxide, or guanidine hydroxide used in hair relaxers or ordinary soap (the kind made from fats and lye).
If you remember one thing from this blog post:
Rinse-out products are safest for hair between pH 4 and 6.
Leave-on products are safest for hair between pH 5 and 6 (or 4.5 and 6 for bleached or lightened hair).
Treatments which use heat or are meant to be more than superficial are safest between pH 5 and 6.
To prevent sudden and irreversible damage to hair, stay between pH 4 and 9.
©Science-y Hair Blog 2013
The slightly longer story
Keep in mind that products marketed to the entire family are assumed to have contact with eyes, nose, and mouth during use and when rinsed off. So a neutral or near-neutral pH around 7 may be ideal for safety reasons.
You also need to know that certain preservatives are more active in a specific pH range. That means that the product pH needs to be kept tightly within a specific pH range to assure the preservative will work well and continue to work for the full anticipated shelf life.
These non-hair considerations are a big deal because irritating people's eyes and noses and mouths is dangerous and potentially litigious, and choosing a preservative which will work with the variety of ingredients in a product and stay stable for 1-2 years is one of the more difficult aspects of product formulation.
©Science-y Hair Blog 2013
Electric hair
Your hair is mostly protein and oils and water (if you ground it up and measured what is in there - that's what you'd find). Those things carry charges - positive or negative. Hair tends to have a greater number of negative charges when you're running around during your day. We're dealing with the surface of the hair because that's where these solutions mostly act. Once hair has swollen in an inappropriate pH solution, you begin acting on the interior structure. Bad news!
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| Brown rectangle is a hair, at right the net charge is negative because there are more negative charges than positive ones. |
Charge is sometimes pH-dependent. If you move an item from an acid environment to an alkaline or basic environment, it will switch to an opposite charge as it gains or loses electrons to the environment. It's electrons that represent charge in the first place.
Hair (proteins and smaller particles) have an "isoionic point." That is the pH value at which it is in balance between positive and negative charge - there is no "net charge," the same number of positive charges and negative charges exist at the surface. Your hair is taking up neither acid nor base from the solutions it is in contact with. It's stable under these conditoines. Everything is groovy.
If you put your hair in a solution lower than 5.6-6.2, such as vinegar or lemon juice or a citric acid rinse, it will take on acid (that's protons or + charges). If you put it in a solution higher than 5.6-6.2, it will take on "base" or OH- charges. That's right - when you use baking soda-water on your hair and it feels extra-soft, it's soaking up base and taking on that slippery, slimy feel shared by many bases. Shifting the electrostatic charge on your hair takes a toll because you can't do that without altering the chemistry of the proteins and oils that make up your hair in some way.
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| Hair at its isoionic point between pH 5.6 and 6.2. Positive and negative charges are in balance. |
What is healthy hair's isoionic point hair? Around pH 5.6 to 6.2.
But there's more! Has your hair been oxidized by bleach (highlighting, permanent dye, swimming pools) or by the sun? In that case the isoionic point decreases because oxidation changes the chemical composition of the surface of your hair. You might be able to go a little lower in pH - and benefit from it. This is one reason why products for color-treated hair often have a lower-than-neutral pH and are marketed based on that feature.
So what!? We rinse a lot of these products off in pH ~7 water, right? Yep. Though even brief, repeated extreme pH insults would still be a bad idea.
©Science-y Hair Blog 2013
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| Below pH 5, acids enter the hair if left on hair for a long time. Hair loses its charge balance. Above pH 7 to 8, "base" enters hair. The - and + charges above show how these charges are added by the solution - it's a visual aid, not an absolute reflection of the whole oxidation/reduction picture. |
So why are some products formulated with a lower-than-5 pH?
Because bleached hair can go lower, and many people's hair can potentially tolerate a lower pH if they are quick about it. That's where the isoelectric point (different from "isoionic") comes into the equation. Your hair's isoelectric point is way down between pH 3.4 and 4.5. Isoelectric means the hair temporarily has zero charge. If you read about these things online or in the popular media, you will encounter the idea that a quite-low pH product, having knocked the charges away, leaves your hair with tightly-closed cuticles and therefore very resistant. That is not an adequate explanation for me! I prefer to think of it as the point at which your hair is "non-reactive." Non-reactive refers to what happens between hair and things you put on your hair. "Tightly closed cuticles" implies that once you close them, they cannot be opened like a locked door. Or something. It doesn't hold up to scrutiny, such as the question, "Okay, but then what will keep them closed?"
We must ask ourselves - if the hair is charge-free, how will it bond to cationic-charged conditioners for better slip? Indeed, it seems that cationic conditioners are more strongly adsorbed to hair (adhere more firmly) above pH 3.7. You need some negative charges on there, people! A lower-than-4.5 pH would be completely counter-productive in a cationic conditioning product.
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| Representation of conditioner bonding to hair based on the attraction of opposite charges (left). Pink + charges at right represent conditioner not bonding to hair at pH values below 4.5 when hair has a "zero" charge. |
©Science-y Hair Blog 2013
The take-home message:
For short-term, superficial treatments like shampooing or conditioning or rinses the isoelectric point sets the very bottom limit: Below pH 5 your hair is out of balance, but not necessarily in danger. You're not going beneath the surface of hair with these treatments and they are short-lived. Stay above pH 4 and below pH 6 and you're in good shape. For a pH 4 or 4.5 hair product (like a gel) that will take many hours to dry in your hair - that might be a problem. It's still superficial or a surface-only treatment, but the length of time it stays wet and therefore active might cause problems. Probably better to be around pH 5.5 for that, unless you have lots of highlights or bleaching with hair dyeing.
It's those longer-lasting and more invasive treatments like permanent waves, hair bleaching, high-heated treatments in which you absolutely must keep hair in that 5.6-6.2 range, especially because it's not just the surface you're dealing with - perms and bleaching are deeper than the cuticles. Hence the "acid perm."
Why does my hair not respond well to acidic rinses?
So if low-ish pH isn't necessarily evil to your hair, then why can vinegar or citric acid rinses leave hair feeling dry and rough? Let's say your hair is porous (even if just the ends are) - those solutions with a less-friendly pH get into your hair more easily and have access to a lot of surface area. You just got a bigger dose.
Acids have corrosive action - they donate those "+" charges that lead to rusty metal. Acids can dissolve things like calcium. Acids can destroy fats and proteins. Think of what happens if you put lemon juice in milk (it curdles). Or if you put a nail in a glass of Coke. Weak acids like vinegar or citric acid can strip off some surface oils and proteins. Acids may interact with your water, with the mineral deposits on your hair, the ingredients in your products. The possibilities are endless. There's nothing wrong with an occasional vinegar or citric acid rinse as long as they're diluted properly but chemically, it can be a wild card.
Some people have very resilient hair that can tolerate acidic rinses whereas other people's hair will swell and take on acid immediately. Some people's hair can tolerate acids but not bases. Everybody's hair is different for so many reasons.
Some people have very resilient hair that can tolerate acidic rinses whereas other people's hair will swell and take on acid immediately. Some people's hair can tolerate acids but not bases. Everybody's hair is different for so many reasons.
©Science-y Hair Blog 2013
Test pH at home:
Get yourself some pH strips in the aquatic pets section of a pet store or the swimming pool section of a department store, or from your local drugstore. Make sure they measure below pH 7 as well as above. Test your homemade rinses and adjust accordingly. You can test other products also. Conditioner often gives a less-than-accurate reading because it's not all liquid (there are fats in there). But you can get a rough idea.
What you do with this information is up to you. In my world - if I wanted to chemically relax my hair, I'd do it and hope it didn't eat the skin on my scalp. If I wanted to wash it with bar soap, I would do that (but no thank you to the soap scum). And I'd be armed with the knowledge that my hair had been chemically altered and therefore I would need to give it extra gentle care. If you want to use a citric acid or vinegar rinse - go for it. If it's too strong the first time, whip out your pH strips and find a pH your hair can tolerate.
Chemical and Physical Behavior of Human Hair
Robbins, 1994. 3rd Ed. Springer-Verlag, New York
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