This post was promoted by a rise in formulators looking for a more wholesome alternative to Sodium Hydroxide in their pH adjusting.
There are some amino acids that can shift pH upwards on account of their basic pH but as you will see below, using them for this purpose does not always work out well.
Arginine is an amino acid and amino acids are famous for reacting together to build peptides and proteins. In a cosmetic sense, amino acids can be added into a formula individually to increase skin hydration or condition and strengthen the hair. In addition, they are frequently used as key actives in Mesotherapy cocktails where they are injected under the skin as anti-ageing and restorative agents.
Despite their family name, some amino acids act as bases (raising the pH) in a cosmetic formula. This fact has become increasingly interesting to formulators looking to raise the pH of their formulations in a ‘natural’ and biocompatible way, a way that avoids ‘fillers’ or ‘harsh ingredients’ or, in a way feels/ sounds more natural or organic. While the idea of utilising ‘basic’ amino acids for this purpose isn’t new and is, in fact indicated by some major cosmetic ingredient manufacturers, my concerns remain and have their origin not in the big picture but in the detail, the chemistry and that’s what I want to dive into here.
The problem of equivalence.
In terms of teaching, the promotion of Arginine as a drop-in alternative for Sodium Hydroxide for pH modification should stop. Chemically speaking, arginine has the capacity for introducing to the formula a level of chaos and vulnerability that sodium hydroxide simply doesn’t possess as we shall soon see.
The curious chemistry of Amino Acids.
Behaviour in solution (water)
Amino acids such as Arginine and Leucine behave mostly as bases in a cosmetic formula, increasing the pH of water when they are added. Arginine sends the pH to 12.5 while Lysine takes it to 10.5. This, and the fact both amino acids are quite stable in their structure across the typical cosmetic pH range of 4-8 are what make them suitable for considering as pH adjusters in the first place.
pKa value = A concentration-independent way of expressing the strength of an acid (or base). The lower the number, the stronger the acid, the higher the number, the more likely the ingredient will perform as a base. Ingredients can have one or more pKa values depending on their chemical structure and complexity. Ideally cosmetic pH adjusters would have either one pKa value only OR have more than one pKa but that they exist in a close pH band.
Amino Acids Vs The Rest.
Sodium Hydroxide is the gold standard for pH adjustment moving up (from acid to alkali).
When you increase a formula pH with Sodium hydroxide you are using a chemical that is very small and nimble (molecular weight 39.997g/mol). It neutralises to a salt and water, requiring very little addition to your formula (in most cases) to adjust a formula pH in the majority of cases. It is readily water soluble and has a predictable titration curve for the most part, although that will of course depend on what in your formula is making it acidic. As we will talk about pKa values in a moment, Sodium Hydroxide has a pKa of -1.8 and a pKb of 15.7 (correction, thanks Perry) meaning that in a cosmetic setting it will ALWAYS act as a base. The fact that it is a small molecule means it’s less likely (compared to Sodium Bicarbonate with a molecular weight of 84g/mol) to weigh down or salt-out (destroy) your formula. That the chemical reaction it undergoes in a neutralisation setting is simple (it literally makes water), highly predictable and efficient make worthy of a gold star!
When you use Amino Acids to adjust the pH of your formula you are using a molecule that is heavier, takes up more physical space and is more temperamental than Sodium Hydroxide. Arginine has a molecular weight of 174.2g/mol, Lysine 146.19g/mol and both have three pKa values plus an isoelectric point.
Isoelectric Point: A pH at which the Amino Acid (or other ingredient) carries no charge and so won’t act as a pH adjuster. In some cases we can put this to use as a pH buffering agent.
The three pKa values occur because the amino acids have three functional groups – the parts of the chemical that can participate in acid / base reactions. For comparison, sodium hydroxide and Triethanolamine have only one pKa value each (-1.8 and 7.74 respectively) while Sodium Bicarbonate has two (6.4 and 10.3).
On the amino acid, each group has its own discrete pH value where it becomes active (the pKa). The whole amino acid also has a pH point or range where adding more won’t budge the pH at all for a good amount of time (the Isoelectric point). This makes using them as pH adjusters quite complicated.
Having more than one pKa is not the end of the world. On the lowering-pH side we have Citric Acid, a commonly used adjuster with three pKa values (3.1, 4.7,6.4). Comparing that to other acids we might use to adjust pH down in a cosmetic setting there’s Lactic with one pKa (3.86) and Acetic (vinegar) also with one (4.76). Remember the lower the number the stronger the acid so in this case, for pH adjustment citric is strongest.
With regards to molecular size, Citric Acid is a good example of a common pH adjuster that’s also relatively large, sitting at 192.124g/mol. This is significantly heavier than the amino acids we discussed earlier, throwing doubt on the relevance of molecular weight as a measure of whether a chemical is fit-for-purpose as a pH adjuster. For reasons that will become more obvious as we go on, it’s fair to say that while molecular weight is worthy of consideration, it’s not the most important feature of the chemical.
Unlike Amino acids, Citric does not have an isoelectric point. It’s three pKa values are very close together and this is because its three functional groups share the same chemistry (-OOH or carboxyl groups). By comparison, Arginine and Lysine Amino Acids contain two types of Acid/ Base curious functional groups – the Ammonium group: NH4 > NH3+ and NH2+ plus the carboxyl group -OOH. Not having an isoelectric point makes Citric a much easier molecule to work with for pH adjustment. The absence of nitrogen-containing functional groups is also a big winner as we will explore next!
The Chemical Reactivity of Amino Acids
Ammonium Ions and Nitrogen.
Amino acids contain ammonium functional groups. These are chemical bonds in which nitrogen is surrounded by hydrogen and bonded to carbon. In our amino acids, these nitrogen functional groups exist in what we call a primary position. Other options include having the nitrogen in a secondary or tertiary position. The naming relates to how many carbons the nitrogen is bonded to. Another nitrogen-containing neutraliser that’s common to cosmetic science is Triethanolamine only this is a tertiary rather than primary amine.
Nitrogen chemistry is involved in one of the persistent ‘nasties’ issues in cosmetic science and that’s the presence or formation of Nitrosamines:
While amino acids, being primary amines are not the most likely nitrogen-containing molecules to form nitrosamines in a formula, their presence does create an opportunity for nitrogen-centric chemical reactions and not all of those are desirable. Again, I must point out that nitrogen is not ‘nasty’ or to be avoided – it’s abundant in our skin as I’ve already said. What I’m talking about here is the potential for a nitrogen-rich functional group to cause formula troubles and I’m particularly talking about this in the context of it only being in the formula to adjust the formula pH.
Here is a thorough assessment by the CIR review into nitrosamine formation.
Vitamin C goes brown faster with Amino Acids.
One common and simple reaction that can occur in your amino acid-adjusted formula is the Maillard Reaction. This is where vitamin C reacts with the amino acid turning it brown and basically breaking it down.
Vitamin C is difficult to work with at the best of times as it is highly reactive and vulnerable to oxidation. Adding amino acids to your formula, especially those that aren’t strictly necessary introduces an added problem or layer of complexity in this regard. There are some interesting papers on vitamin C / amino acid interaction in the food industry:
Non-enzymatic browning in citrus juice
Non-enzymatic browning reaction in L-ascorbic acid/basic amino acid systems
Sugars break down with Amino Acids.
Many cosmetic formulations contain sugars and both glucose and fructose react with amino acids under normal storage conditions to form bi-products that go on to discolour the juice. While this is more an issue of aesthetic than safety it is still something most brands would prefer to avoid or at least minimise.
Your amino acid may form a Schiff base
Schiff bases happen when a nitrogen-containing compound forms a coordinate complex with a metal ion. I guess it’s similar to a chelation complex only more colourful and potentially less helpful.
This article by the bumbling biochemist has some more useful information on this.
Metal ions are not uncommon in natural formulations. They can come in on the back of your herbal extracts, your clays or your colour pigments. Should they find themselves attracted to and then bonded onto your pH-adjusting amino acid your formula may change colour, become lumpy or granular or otherwise turn into a shit storm. Then again, this may not happen at all but as it’s possible, it should could be worth keeping in mind if your formula starts looking like a rainbow. You are not hallucinating!
Weird things happen when Arginine meets Citric Acid.
While I must confess to not yet being able to fully articulate what happens when Arginine and Citric acid get together, it is clear that in an aqueous environment, they react in a way that’s non-linear and therefore somewhat harder to manage and/or fully predict in terms of pH response, formula aesthetics and stability. Citric is present in most fruit extracts and some herbal preparations meaning it might be in a formula even when you didn’t intend for it to be:
Unravelling the nature of citric acid:L-arginine:water mixtures: the bifunctional role of water
pH Adjusting with Amino Acids – A Problem of Extrapolation?
Amino Acids are OK or even preferred in some (specific) settings.
A great example of where the pH-adjusting benefits of an amino acid are preferable is Innolex’s Emulsence HC – a palm-free cationic conditioning wax that sends your formula pH to around 3 prior to neutralisation. I’ve used this ingredient in many formulations and can attest to the benefits of neutralising this particular ingredient with the suggested Amino acid vs Sodium Hydroxide or even bicarbonate of soda (which is pretty much always a dud choice). The amino acid doesn’t just work better, it is the only common option that works well, improving all aspects of the formula both in use and in-pack.
Here’s a formula from Inolex showing how Emulsence HC is used to create a conditioning spray and how much Arginine is added to make the adjustment.
Lubrizol has also been known to mention amino acids in their technical documentation, listing them as one among many ‘neutraliser options’ for their carbomer family of polymers. While that appears at first glance like evidence for treating amino acids as just another pH adjuster that’s not quite the case. In this technical documentation, Lubrizol is focused more on what can work for their polymer than what will work safely in your formula.
As we have seen above, Amino Acids can and do react with other commonly-found cosmetic ingredients so while it is correct to state there are amino acids that can adjust a formula pH upwards, extrapolating that to assume they work in all (or even most) cosmetic formulations likely problematic.
The Summary.
As much as we call cosmetic science ‘chemistry’, in many formulations, we are actively trying to avoid chemistry happening! I mostly teach people to think of what we mainly do is closer to physics than chemistry – we help things exist physically together and share space without changing each other. We go to great lengths to avoid chemical reactions happening most of the time, adding preservatives, stabilisers, antioxidants, UV protectors and packaging appropriately. While pH adjustment is one of the chemical reactions we need, to accept an invite, it is counter-productive to introduce an unnecessary layer of chaos at this point.
Do Amino Acids Even Delivery What We Want?
The fact amino acids are being widely considered for pH adjustment is a function of their perceived naturalness, gentleness and multi-functionality. While this is sometimes true, with some amino acids made via biofermentation, we can’t overlook the fact that many are synthetic in origin. Synthetic amino acids may or may not meet the criteria for addition in an organic or natural formula.
Price is another consideration for the cosmetic formulator albeit perhaps only a minor consideration. Amino acids are significantly more expensive than the alternatives (Sodium Hydroxide or Triethanolamine – although TEA has its own issues, some but not all of which are common to amino acids) and that’s without considering the extra R&D costs that could be incurred to thoroughly evaluate formula, packaging and micro stability after using this chemistry to adjust pH.
A third take-home point is the dose. The best pH adjusters are the ones that do their job very efficiently and at low addition rates. pH adjusters add complexity and ionic activity to the water phase and that, in turn, can destabilise emulsions and de-solubilise solutions. For those reasons it is best practice to use the pH adjuster that does the job in the most invisible way and that is why we must consider aspects such as pKa, isoelectric point and molecular weight when weighing up our options.
Fourth is the reactivity – Amino acids are chaos when compared to the other typically used options. It is entirely possible that in adopting this pH-adjusting strategy you are creating a formulation that has a lower shelf life than it otherwise might have. It may be more irritating, less aesthetically pleasing, less efficacious and less physically stable. Then again it may not. It’s all in the detail!
All in all, brands and formulators, teachers and workshop facilitators who are promoting amino acids as a more natural, safer or gentler alternative than Sodium Hydroxide for pH adjustment would be wise to think more deeply and adopt an applied rather than absolute mindset. I could be wrong, but I feel it likely amino acid pH adjustment could, in many cases, be a bum move.
Comments