Can You Lower the pH of Castile Soap?
Good old, honest, traditional castile soap, loved by hippies and the home-made set the world over for its simplicity and elegance but there is one thing that bothers me about this ingredient and that’s it’s high pH.
Why am I bothered by such a trifling matter?
A couple of reasons:
- I find castile soap overly drying for my eczema prone skin and I feel the very high pH (9-11.5) doesn’t help.
- It stings the eyes.
- Many of my customers want to use castile as an ingredient in an otherwise pH balanced natural product. I’ve had requests from those wanting to use it in a combination face wash to those putting it in cleaning products. To do this I have to be able to adjust the pH.
So is pH Adjustment possible?
Yes and no. Mainly no.
So the pH of castile soap in its natural state is between 9-11.5 which is typical for soap which naturally has a high pH whether it is in a bar form or as a liquid. This is because it relies on Lye (Sodium Hydroxide or Potassium Hydroxide) to rip the oils heads off (that’s how I imagine it) and replace them with ‘soap’ (sodium or potassium salts) while releasing the glycerin from the triglycerides. The resulting saponified fatty acids have naturally high pH levels (usually between 8-11 but most often between 9.5-11) and any residual lye will also contribute to giving the soap bar a high pH (although soap manufacturers usually make sure there is no residual lye).
One of the reports I’ve used to establish the pH of the saponified fats is this one: The Hydrolysis of Soap Solutions. III. Values of pH and the Absence of Fatty Acid as Free Liquid or Solid JAMES W. McBAIN, P. LAURI:NT 1 and LUCILLE M. JOHN, 2 Department of Chemistry, Stanford University, California. It is available to purchase from Springer online.
pH changes with concentration and temperature so these numbers are just to illustrate the point rather than be absolute references but you can quickly see where the high pH of soap comes from.
Potassium Stearate pH 10.05
Sodium Stearate pH of a 5% Aqueous solution is 10.7 (MSDS online)
Potassium Laurate pH 10
Sodium Laurate pH 9.78
Potassium Myristate pH 10.28
Sodium Myristate pH 9.81
Potassium Palmitate pH 9.81
Sodium Palmitate pH 10.1
The above ingredients and their pH’s represent what is formed when you saponify an oil. You are creating new chemicals that have some degree of water solubility and that have relatively high pH values. So, even if ALL of the lye is used up in your soap, your soap will still have a high pH (pH 7 is neutral) because you have created chemicals that have a high pH.
Here is what stearic acid/ Sodium Stearate looks like:
So what happens when you try and adjust the pH down?
There is a precipitation reaction that occurs when you add acid to the castile soap base. This doesn’t happen straight away but it does happen relatively quickly showing us that there is a reaction going on.
Here are the results of an experiment we carried out in the New Directions Laboratory:
So we established that the pH of the New Directions Castile Soap could be reduced from 10.27 to 9.03 using a small amount of citric acid and careful stirring.
Soap and citric acid.
Next we wanted to see if the pH of Castile Soap could be changed by using another method/ acid just in case there was a particular issue with Citric Acid.
We chose to look at diluting the Castile with more Glycerin (as some customers had said that helped to make the soap more mild), with more vegetable oil (based on the theory that any excess lye would soap up the oil and thus lower the pH) or with lactic acid – an alternative to citric.
The results are here. This batch of Castile was a bit lower than the last and measured a starting pH of 9.6.
So with regards to pH adjustment it became clear that none of the above worked but what these results did show, especially with the sunflower oil addition was that it is unlikely the resting pH of the Castile Soap was due to excess lye thus supporting what we know about the pH of the saponified fatty acids.
So what is going on with the Castile Soap when we attempt to adjust the pH downwards?
Well that’s the question.
The typical reaction we talk about it:
Acid + Base = Salt + Water.
But we have an acid and a salt so does that mean we make a salt and water?
Or do we make nothing?
We must be doing something or there wouldn’t be a precipitation happening…..
When using Citric Acid we could rip the sodium from the Sodium Myristate/ Stearate etc and form Sodium Citrate salt but would that really happen? If it did happen would there be a precipitate? The answer is probably not and there would be no precipitate as Sodium Citrate is readily water soluble.
So is there some kind of complex happening? There must be something going on..
There are a few things that might be going on, Let’s look at each one in turn.
Firstly there is a reaction called ‘Protonation’.
So if we remember what our soap is made of (and refer to the hand-drawn picture above) we see the bit of the molecule that makes the soap water soluble has a Na+ (or a K+) part and a COO- bit when it is in water.
When an acid comes along it can donate a proton to the COO- group making it COOH. You can read more about this reaction here.
Converting a soap to an acid is something that can happen but it is unlikely to happen in every situation unless the resulting compounds are less energy intensive to make (thermodynamically attractive). Citric (or lactic) acids are weak acids so they are not exactly powerful enough to do this reaction on their own as the salt is still the favoured format given the (still) high pH and the relative weakness of the acid. However, if this reaction could occur it is true that the salt form of the molecule is more soluble than the acid and in the case of these huge carboxylic acids this head-swap leads to the precipitation of the insoluble fatty acid.
So is it likely that the precipitate is formed by a reaction of glycerin and the citric acid?
Glycerin is a bi-product of saponification and is retained in castile soap to help keep it liquid. It is well known that biopolymers can be manufactured by combining citric acid with glycerin so it is reasonable to question if this is happening here, especially given that the resulting polymer would form a precipitate. However, in order to form a precipitate the polymerisation reaction has to take place at temperatures in excess of 100C and over a prolonged period of time rather than instantaneously at room temperature. So I guess it remains a possibility that something like this is happening but it is unlikely to be what is going on here. Here is some more information on this reaction.
Does the citric acid just reverse (or un-do) the lye reaction?
So we have triglycerides, add lye and get soap.
So why not take soap, add an acid and get triglycerides?
Or even monoglycerides
Or what else….. Fatty acids?
Is there a chance that the citric acid can replace the -O-Na+ with a -OH?
That’s what we discussed in the protonation bit above. It’s not that likely.
OK so what on earth is going on?
Well, my theory centres on something more physical than chemical which is all too often the case with cosmetic chemistry. I think that the addition of acid interferes with the micelle formation and that the cloudiness is caused by the water-hating tail groups being temporarily (or permanently) flipped open or disrupted. Acidity in the water disrupts the status quo of the micelles and if too much acid is added the micelle structure changes completely, the stearates are unable to orientate into micelles which leaves their water insoluble part with no option but to run for the hills – or the top of the beaker in most cases. I think this is happening as the pH is not changing enough to provide the right environment for such a vigorous chemical reaction but I have been wrong before and I am sure I could be wrong again.
The bottom line.
Castile soap is lovely and natural. It can have a pH of between 9-11.5 in its natural state and you have limited scope to change that without changing the micelle structure of the product (based on my logic). Adding excess oil into the formula is something that many people choose to do to re-fat the skin as it is washed so as to avoid the skin being left feeling dry. This is acceptable but won’t change the pH.
So that’s that.
PS: Thanks to New Directions and especially to Alyce for helping with the experimental part of this piece.