Why pH matters for AHAs and acids in skincareLab Muffin Beauty Science

Hi I'm Michelle and I have a blog called Lab Muffin. Today I'm going to be talking about why pH matters for alpha hydroxy acids and vitamin C. There are many acidic active ingredients in skincare: alpha hydroxy acids like glycolic, lactic and malic acids; beta hydroxy acid or salicylic acid; vitamin C. Or L-ascorbic acid; hyaluronic acid, retinoic acid and many more.

You might have heard that some acid products need to be a certain pH to work. Why is that? Here's the science behind the skin care tip. Acids are molecules that can give up hydrogen ions or H+. For example, sulfuric acid or battery acid is H2SO4, hydrochloric acid in your stomach is HCl, and acetic acid in vinegar is CH3COOH.

All of them can break up to give up hydrogen ions like so. You can see that the acids in skincare also have hydrogen that can break off. The top layer of the skin, the stratum corneum, is made up of cells embedded in oily lipids much like bricks in mortar. Oil is non-polar, which means it isn't charged, while water is polar, which means it's partially charged.

According to the rule of thumb for solubility "like dissolves like" - that is, things that are uncharged like to mingle with things that are also uncharged, but they'll repel things that are charged. This means that in general, charged particles will not get into skin easily. When acids haven't given up their hydrogen ions they're uncharged so they're able to penetrate the stratum corneum. The uncharged version is called the free acid.

But if they've broken up or dissociated into a hydrogen ion and a negatively charged anion, they're too polar to get through the non-polar stratum corneum very easily. The extent to which the acid is broken up depends on a few things. Let's talk about the two that are most important in skincare. First, the strength of the bond holding the hydrogen ion to the rest of the acid.

This is measured by something called the pKa of the acid. For example hydrochloric acid has a very low pKa, so in water-based solutions it's almost 100% broken up. Acetic acid on the other hand has a much higher pKa so in water only about 1 in 100 acidic hydrogens have broken off. We call this strong and weak.

Skincare acids are all weak acids so they're varying amounts of broken up. Secondly there's also the pH. Of the overall product. PH measures the total concentration of hydrogen ions and low pH means more hydrogen.

The hydrogen ions can come from the skincare acid or from other acids added to the mixture. Let's consider a product where half the acid molecules are in the free acid form so half can get into the skin. If the pH is lower, there's more hydrogen ions around, which means more will attach onto the acid molecules giving more of the uncharged free acid, which can penetrate. This means that a lower concentration of acid at a lower pH will give the equivalent amount of free acid as a higher concentration at a higher pH.

For example, a product with 10% glycolic acid at pH 3 would have 8.7% Of the product in uncharged form, whereas at pH 4.5 You'd need 50% glycolic acid to get a similar amount. In summary, low pH will always increase the amount of uncharged acid that will penetrate the skin, and the pH required to get a high concentration of uncharged acid depends on which acid we're talking about. Why not use a super low pH then? You'd be correct in thinking that if we used a really low pH that would give us the maximum amount of uncharged acid that could penetrate. Unfortunately, skin likes to be at around pH 5 and very low pH can cause chemical burns.

For leave-on products, it's recommended that you use alpha hydroxy acids at a pH of less than four and a concentration between 4 and 10 percent, and ascorbic acid at a pH of around 3.5 And a concentration between 5 and 20 percent. What about the other acids? It gets more complex here because the rest of the acid molecule makes a big difference. Hyaluronic acid is a large polymer made up of thousands of these units, so it's way too big to get into the skin anyway. And it doesn't need to - it's a humectant moisturizer which means it sits on top of skin and holds on to water to keep your skin moist.

For retinoic acid, the rest of the molecule is non-polar enough that the charge doesn't make a big difference. For salicylic acid or beta hydroxy acid it's often recommended that you use a product with a pH of less than 3.5 And a concentration of 1-2% but there's some evidence that it's also non-polar enough that pH. Doesn't matter. That's a quick introduction to pH and acids in skincare.

I definitely haven't covered everything because delivering things through skin is a huge huge topic. I hope you found this video useful, and if you'd like to see more videos like this from me in the future, please comment and let me know!.