Liquid soap: Tips for good recipes
Liquid soap (LS) is made with potassium hydroxide (KOH) rather than the sodium hydroxide (NaOH) normally used for making bar soap. KOH makes a soap that can be diluted with water to a reliably liquid product. Sometimes a bit of NaOH is also included in a liquid soap, but KOH should be the main alkali used to get a stable liquid soap.
Some people try to make a shortcut "liquid soap" by dissolving grated bar (NaOH) soap in water. This method seems like a great idea, except ... it does not work. A mixture of NaOH soap and water usually does not remain stable. As time passes, the mixture may eventually thicken to a non-pourable gel, or thin to a watery solution, or even separate into layers.
Control the pure soap content. The active ingredient in soap is soap, of course, but liquid soap or bar soap is not pure soap. Bar soap is mostly pure soap with some water, glycerin, and other ingredients. Liquid soap is mostly water with pure soap as the second main ingredient.
When people wash with bar soap, they rub the bar a lot or just a little bit in an effort to get just the right amount of soap for the job. When people use a liquid soap product, they often squirt the same number of dollops of product onto their hand or a washcloth regardless of how much soap they really need. Even if half a dollop is enough, many people will still use two or three squirts. Does that sound familiar?
Because of this tendency to over-use liquid cleansers, it is a good idea to intentionally control the amount of pure soap in each dollop. This reduces waste, cleanses well without drying or irritating the skin, and allows the soap to form lather more quickly. The best way to control the soap per dollop is to control the percentage of pure soap in your product within a reasonable range.
For liquid soap, the weight of the pure soap in the product is roughly equal to the fat weight plus the alkali (KOH and/or NaOH) weight. The non-soap ingredients are everything else -- all of the water and water-based liquids used to make or dilute the soap as well as fragrance, glycerin, thickeners, preservative, or other additives.
Pure soap weight = Fat weight + Alkali weight
Total product weight = Pure soap weight + Weight of all other ingredients
% pure soap = Pure soap weight / Total product weight X 100
For general-purpose handwashing and bathing, try keeping the amount of pure soap in a liquid soap product at 10% to 20% and see what you think. In other words, you want about 1-2 grams of pure soap in every 10 grams of finished product. If you think in ounces, that is 1-2 ounces of pure soap by weight in 10 ounces of product.
Control thickness by dilution. The viscosity (thickness or syrupy-ness) of a diluted liquid soap depends on the fatty acids used. Many liquid soap makers choose to dilute their liquid soap with just enough water so it is thick like pancake syrup or honey. The oleic acid content is the key to success with for controlling thickness by dilution alone with no added thickener.
If you want a honey-thick soap by dilution alone, the oleic acid in the recipe must be about 50%, give or take a bit.
If the oleic acid content is much lower than 50%, the soap will usually be water-thin even if it is diluted with very little water. A 100% coconut oil LS is water-thin because there is simply not enough oleic acid in the soap to create a thicker consistency. To thicken a low-oleic soap, add a separate thickener such as HEC or HPMC (see below).
If the oleic acid content is much higher than 50%, the soap may stay in a non-pourable jelly form even after adding a lot of water. A 100% olive oil (castile type) liquid soap can behave like this. Avoid this problem by reformulating the recipe to contain less oleic acid.
The downside to controlling thickness by dilution alone is that the percentage of pure soap in the finished product will usually be quite high. When I dilute my liquid soap to a honey-thick consistency, the percentage of pure soap in the finished product ranges from 30% to 40%.
For general-purpose handwashing and bathing, the percentage of pure soap in a liquid soap product should be lower -- more in the range of 10% to 20%. Stated another way, you want about 1-2 grams of pure soap in every 10 grams of finished product. If you think in ounces, that is 1-2 ounces of pure soap by weight in every 10 ounces of product.
Add thickness with HEC or HPMC. If you want your liquid soap to be a thick syrup or gel, you may need to use a separate thickener such as HEC (hydroxyethyl cellulose) or HPMC (hydroxypropyl methylcellulose) to reach the desired viscosity. These thickeners will work regardless of the oleic acid content in the recipe.
Add thickness with table salt. For soap that is moderate to high in oleic acid, table salt (sodium chloride) can be used to increase the thickness, although salt will also reduce the lather. Be cautious with salt -- if enough is good, more is definitely not better. If you add too much salt, the soap will become thin again and this cannot be fixed.
To thicken with salt, dissolve 1 part table salt in about 3 parts warm water. Set this brine aside. Using water only, dilute the soap to a reasonable thickness (or to a reasonable pure soap percentage.) Add small amounts of the salt brine and stir until the brine is well mixed into the soap. Let the mixture stand for at least 30 minutes (salt takes awhile to do its magic). Check the thickness and add another small portion of brine if needed. Stop adding brine when the mixture is sufficiently thick.
Control clarity. If you want your liquid soap (LS) to be sparkling clear, then you will want to reduce the percentage of palmitic and stearic acids as low as is practical. Large amounts of these fatty acids can cause liquid soap to be cloudy or even opaque. Lard, tallow, palm, and the butters contain large percentages of these fatty acids.
Also avoid fats that have a high % of unsaponifiables -- avocado, shea, jojoba, beeswax, etc. -- that can also make liquid soap cloudy. Some additives, such as some fragrances and milk, can also cloud the soap.
Castor bean oil adds clarity to liquid soap and is often found in LS recipes at 10% to 30% of the total fat weight.
Limit superfat. Bar soap can contain a high amount of superfat (excess fat) without looking obviously different than soap with lower superfat. Too much superfat in liquid soap will separate out of the soap and float on top. The rule of thumb is to set the superfat to 3% or lower and also to correct for the actual KOH purity (see "KOH purity" below) to ensure there is not too much unsaponified fat.
Account for KOH purity. KOH used for soap making is never 100% pure, because this alkali chemical reacts quickly and easily with water and carbon dioxide in the air. Commercially available KOH is often about 90% pure.
If your recipe calculator assumes the KOH is 100% pure when it is really only 90% pure, then a "hidden" superfat of 10% will be built into your recipe. The excess fat will likely separate out of the soap after dilution.
Check with your supplier about the purity of the KOH you have. If you cannot get that information, assume the KOH purity is about 90%. Be sure to enter the KOH purity into your soap recipe calculator to prevent excess fat in your finished soap. Soapee allows the user to type the actual KOH purity. SoapCalc allows you to choose either 100% KOH purity or 90% purity.
Evaluate lather. Liquid soap can lather more slowly than bar soap, so many LS recipes call for more coconut oil or palm kernel oil than you might typically use for bar soap. The extra lauric and myristic acid supplied by coconut oil and PKO will help the lather to form faster. Unfortunately a higher lauric acid content can make liquid soap harsher and more drying to the skin. A better option to keep your liquid soap lathery yet mild is to keep the pure soap content of your finished product in a reasonably low range.
From our human point of view, it seems sensible that the overall texture of a mostly-potassium soap should smoothly and gradually shift from water thin to honey-like syrup to smooth gel to firm solid as the percentage of sodium increases in the soap. From the soap's point of view, that is absolutely not something it wants to do.
Sodium soaps really "want" to be organized, so that's why sodium soaps prefer to take a solid form or make a ropy gel depending on water content and fatty acid content. The solidity of a bar soap or the ropy-ness of a sodium-soap gel is the result of those sodium soap molecules trying to organize themselves into some kind of structure. Sodium soaps will finally end their quest to be organized only when the water content gets very high. At that point, a sodium soap will form a watery-thin solution.
Potassium soaps are less particular about being organized into a framework -- they're naturally more disorganized. That is why a potassium soap can range in texture from soft sticky paste to smooth gel/syrup to water-thin liquid, again depending on water content and fatty acid content.
As you dial up the sodium content in a mostly KOH recipe by adding more and more NaOH, you're very likely to see exactly what Susie has seen -- a fairly abrupt shift from mostly disorganized potassium soap behavior to mostly organized sodium soap behavior. I won't say it's impossible, but I do think it's going to be tough to find a precise mixture of Na and K and fatty acids and water content that produces the human ideal of a thick syrup or gel that stays pourable and doesn't get ropy. We're wanting a soap that is carefully balanced between sodium-soap characteristics and potassium-soap characteristics. That will be a challenge, because a soap generally wants to behave one way or the other, not somewhere in between.
Yes, the KOH weight needs to be different because it's a different chemical than NaOH and each KOH molecule weighs more than NaOH. When you measure out a given weight of either one, what you're really trying to do is measure a given number of molecules. If KOH molecules weigh more, you're going to have to measure more KOH by weight to get the correct number of molecules.
The lye concentration you use to make liquid soap is more about convenience and practicality than it is anything else. Many liquid soapers use a 25% lye concentration (3:1 water:lye ratio). At this concentration, the paste is reasonably soft, so it's easier to stir and a bit easier to dilute.
If you use a higher lye concentration (lower water:lye ratio) that means there is less water, the paste will be harder to stir, and it can be more difficult to dilute. I've done 33% lye concentration a time or two and the soap was fine, so if you want to go there, there's no big reason why not. I have to say I prefer 25% lye concentration.
Some use a 50% lye concentration to make their paste; the one soaper I know of who does that also relies on sodium lactate to get the paste diluted. I've never tried it, so I can't say much about this personally.
If you use a lower lye concentration (below 25%), the paste will contain more water and will be softer. This might seem to be a good thing, except in my experience lye concentration at 20% or less can make it difficult to get the soap batter to a stable emulsion, even with lots of mixing and fiddling.
Some LS making methods call for all of the water, including dilution water, right from the beginning. These methods require the use of a continuous mixer, something most of us don't have, and hours of mixing, which most of us don't want to get into.
For your first batch, you really can't go wrong with a recipe shared on the Soap Making Forum by member Irish Lass. Here's my take on this recipe:
Olive Oil 65% (can substitute part or all of this with any high oleic oil like HO safflower, HO sunflower, avocado, etc.)
Coconut Oil 25%
Castor Bean Oil 10%
Superfat 3% or lower
Lye concentration 25% (3.0 water:lye ratio)
Use KOH as the only alkali
If you don't know the KOH purity, choose the 90% pure option if using Soapee or Soapcalc
Can use all water to make the soap or can use up to 2 parts glycerin to 1 part water.
I have used 1:1 glycerin:water, 1:2 glycerin:water, 2:1 glycerin:water, and all water to make this and other LS recipes. They all work.
I do not recommend using the 100% glycerin method for safety's sake.
I recommend using distilled water, not drinking water.