Some Liquid Soap Chemistry

It’s only when you dive deeply into the science of liquid soap making that you realise how interesting and chemistry-rich every little step of that process is.  I rarely get time to just meander through a personal project as I’m always caught between feeling guilty about the things I still have on my science work ‘to do’ list or feeling out of time due to the ebb and flow of normal life.  Thankfully the Coronaverse has taken a large number of options and choices away from me (while giving me heaps more to deal with – turns out that the one time people do need and listen to chemists is during a pandemic) so spending two days on a liquid soap suddenly became a thing.

I’m sure that most people who will read this will have a basic understanding of soap making.  For those who don’t and by way of a quick summary here’s the science.

Saponification: Turning oil into soap with the help of a chemical called ‘lye’ (short for alkali).

 

Chemically both vegetable and animal derived oils are triglycerides.  Triglycerides are a combination of one part glycerin and 3 parts fatty acids.  The lye (Sodium Hydroxide mostly for soap bars and Potassium Hydroxide for liquid soaps) can be made into a very high pH solution and added to the oils upon which they start to react chemically to take on a new set of chemistry.

During saponification the triglycerides (which are also known as triesters due to their structure) are broken open by the addition of the lye which cuts through the glycerin/ fatty acid bonds to liberate (free) the fatty acids and glycerin from each other.   The fatty acids quickly combine with the positively charged metal oil from the lye to form what we call a ‘salt’ only these salts are not like the salt you put onto your chips, these salts are bubbly and surface-active and are better known as soaps.  These soaps are anionic (-ve charged).

Meanwhile the glycerin is now left sitting pretty, intact and free.  In this form it is highly water soluble, moisturising and heavy (heavier than water).   It’s worth noting that glycerin is a type of alcohol and is sometimes referred to as Glycerol or spelled Glycerine.  All three are one in the same and are brought to you by saponification.

So when making liquid soap we use Potassium Hydroxide as that produces a softer soap.  Lots of people know that and would repeat that phrase without really thinking any more about it but I can’t help but ask more questions:

• Potassium soap is softer than what though?
• By how much is it softer and how do we measure that?
• Why is this soap softer anyway?

The answers are quite interesting (to me, at least).

I found a paper that gave the Krafft points of different salts of fatty acids.  This was the best paper I could find to answer my question with regards to potassium soap being softer than what?

If I look at the data given for potassium vs sodium soaps, the two most common and most familiar to soap-makers I find that the Krafft Points of Potassium Soaps are much lower than the sodium:

Here’s the data showing the resting (non-changed) Krafft Points of some common soaps:

Potassium Laurate. 10C

Sodium Laurate  25C

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Potassium Myristate. 25C

Sodium Myristate 45C

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Potassium Palmitate 45C

Sodium Palmitate 60C

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Potassium Stearate 55C

Sodium Stearate 70C

 

Great, but what’s that got to do with liquid soap making, you may ask.

Krafft Point (and it was someones name so don’t spell it Kraft). 

Krafft Point is a feature of ionic surfactants. It is the point (temperature) at which the surfactant becomes useful i.e: where its solubility is such that the surfactant can start forming micelles and display surfactant-like behaviour. Another term for this point is its Critical Micelle Concentration (CMC).

Now that may not sound too interesting at this point but it is interesting if you’ve ever made liquid soap and wondered what on Gods name it’s thinking when it just won’t mix into water after you try to dilute it after making it.

What’s happening there has a lot to do with the soaps Krafft Point and while it seems very simple  based on what I’ve just said above (that the Krafft point is a simple temperature related thing) it gets more interesting when you consider how a soaps Krafft Point can be altered!

Altering a Krafft Point.

Regular salts such as Sodium Chloride, Potassium Chloride and others affect the Krafft Point, depressing it (making it lower) up to a point, above which the surfactant may ‘salt-out’ (a term for when the environment is too salty for the soap and it responds by losing its shit).  Now lowering the Krafft Point is a good thing as it means the soap will be fully soluble at a lower temperature rather than having to be heated.  We want our cosmetic products to be fully functional and in their best shape between the temperatures of around 5 – 45C and while that’s not always strictly possible, it’s something to aim for.  Understanding that for liquid soap,  you can stack the odds in your favour by understanding the Krafft point is powerful stuff.

Glycerin and other alcohols (including sorbitol, propane-diol, hexylene glycol, ethanol etc) improve soap solubility in water, thus reducing its Krafft Point.  I instinctively thought of adding more glycerin to my liquid soap in an attempt to increase its solubility because I did notice that my soap batter seemed to be far too attracted to its self to bother with trying to break the waters hydrogen bonds and make a little hole for its self to sit in.  I felt that the glycerin could ‘soften’ the hold the soap had on its self somewhat and in doing so, create an opportunity for a new relationship to form (between the water and the soap).  It turns out that glycerin interferes with the bonding between the soap molecules thus opening up a gap for water to get in. When concentrated the tails of the soaps form bonds between them (Van der Waals) which require some energy to break. The glycerin improves the changes of bonding happening between the water and soap rather than just within the water and within the soap. However, it turned out that just adding more glycerin was only half of the equation, the whole mixture needed to be re-heated for the real solubility magic to happen.  Glycerin + Heat + more mixing = beautifully solubilised liquid soap.

Upon reading further, the requirement for an extra dose of heat wasn’t surprising as I was right, the energy needed to break the soap-soap and water-water bonds was high enough to make the situation difficult, adding that extra boost of some heat energy lubricated the process enough to make forming soap-water bonds much easier and more attractive.

NOTE ON EXTRA GLYCERIN (or other alcohols for that matter). 

Now there has been some talk of adding glycerin to the lye prior to saponification, apparently it’s quite a well known method to speed things up.  Basically people dissolve the Potassium Hydroxide in Glycerin on a stove top (the lye is not very soluble in the glycerin) and then add that to the oils.  I can see that this works by concentrating the potassium hydroxide and speeding up its attack and if people can handle that, fine.  However, it does look quite a bit more dangerous than adding the hydroxide to water and then adding the glycerin to boost solubility later on.

Some people seem worried that the extra glycerin may be reacting with the lye to produce other chemicals, sometimes toxic things. It is highly unlikely that any further reactions than basic saponification go on during this process, even if a microwave is used to get the liquid soap dilution part finished quickly.  The polymerisation of glycerin into Polyglyceryl esters is possible with just heat, lye and glycerin but it takes 8-14 hours and is typically done at temperatures of around 300C and / or pressures around 60 times greater than we find on earth.  Half the problem with the information on the internet is that there’s a glimmer of truth in it but that someone reading a science paper has mis-understood it or taken a theoretical risk and turned it into a certain threat.  Basically you can add glycerin at any stage you like and be fine, I guess it’s up to you but I feel adding it after is safer and easier to control.

Other ways of influencing Krafft Point. 

Temperature affects the Krafft Point as mentioned above, for ionic soaps it is normal for the soap to become more soluble in high temperatures and less so in the cold – this is why some people’s Castile soap goes cloudy in the winter. You can see from the above examples with different fatty soaps, the temperature at which these things become soluble can vary and in some cases be far too high without intervention to make a practical product.  The soap I made contained around 16%  Potassium Palmitate/ Stearate both of which are not that soluble in water under normal conditions.  So I HAD to play with the Krafft Point to stand a chance of getting my Coconut soap clear and lovely under normal conditions!  It’s that simple.

Concentration of the soap affects the Krafft Point – a soap has to be present at its Critical Micelle Concentration to reach this point, below that it may exist as monomers only, rather than micelles.  Now this isn’t usually a problem in soap making as you are creating a concentrate but it is worthwhile knowing and there’s definitely problems at the other end of the scale too when your liquid soap dilute is made too strong – you’ll end up with floaty bits of a surface hard layer if you exceed the holding capacity of your water.  Remember that the soap is forming micelles and there’s only so much space for them to sit and spread out in, after which point they start to become attracted to the ‘other’ surfaces such as the interface between the air and the top of the product.  If that happens, dilute your soap.

pH affects the Krafft Point too.   Solubility is always affected by everything (it’s an applied, rather than absolute measure) and pH is an important factor of that.  Different oils saponify into different fatty soaps, usually with carbon chains ranging from C12 – C18 but sometimes outside of that on either end.  These individual fatty soaps each have their own solubility parameters and pH ‘sweet spots’.  What changing the pH does is change the degree to which the head (or functional group) of the soap is activated (protonated or deprotonated).  If the pH gets too extreme the soap can lose their heads altogether but within the soaps sensible mid-range,  this property of soap can help you reach an optimal degree of solubility.

But there are multiple fatty soaps in a vegetable soap, which pH do I aim for?

With a blend such as that which we find in liquid soap, there’s usually a ‘majority rules’ pH point where the soaps all sit happily in the mixture.  As this does depend on your particular soap recipe it can vary between makers but the solubility sweet spot is typically between pH 9-11.  One thing I found with my particular recipe is that it settled at a pH of 9 when first made but reached its peak balance point between solubility and mild feeling when adjusted up to pH 10.  Keeping this in mind, it’s pertinent for liquid soapers to keep that pH meter handy at all times as everything you do to your soap during dilution could affect its pH.  You can try and calculate this by looking at your particular oil formula and calculating what your dominant (or most abundant) fatty soap chemistry is then tracing back its happy place pH.  Otherwise just observe the clarity of your soap as you gently raise and lower the pH. Same, Same.

So with that I’ll sign off…

While soap has been widely studied and saponification has been a reaction that humans have undertaken throughout history, it’s still surprisingly hard to piece together the science that helps us answer the ‘why’s’.  I guess that’s part of what keeps it relevant and interesting as both a hobby and a job.  It always excites me to dive into something that seems ‘known’ only to find that we really know next to nothing at all.  Keeps life interesting.

I hope you are well and if, like me, you’ve not been (I got a dose of flu, it really sucked and my lungs are still a bit dodgy, probably not helped by that long summer of bush fire smoke but hey ho, if I’m here for a good time and not a long time so be it) just do your best to enjoy it.

Amanda x

PS: Along my reading travels I found this paper on bush fire retardant foam. Apparently that’s now a potassium soap base due to its high environmental tolerance.  Lots of pink foamy stuff was dropped onto the places around us this summer so it’s good to know that I can whip up a few tonnes of this and keep us safe.