Also see these articles -- Liquid soap: Tips for making it and Liquid soap: Tips for diluting and thickening
People often want to know if a bar soap recipe can be adapted into a liquid soap recipe. Although this is technically do-able, the adapted recipe might not make an ideal liquid soap. There are factors to think about when designing recipes for liquid soap that do not apply to recipes for bar soap. Some of these factors are --
Calculate the KOH weight correctly
Correct for the actual KOH purity
Choose the right lye concentration
Limit the superfat
Avoid ingredients that reduce clarity, if clarity is important to you
Use ingredients that increase clarity, if clarity is important to you
Adjust the fatty acids for better viscosity and lather
Minimize the chance of microbial growth
At the end of this article, I include an "extra credit" section where I discuss the reasons why I do not recommend using a dual-lye mixture (a blend of KOH and NaOH) to make liquid soap.
Calculate the KOH weight correctly
If you adapt a bar soap recipe to make liquid soap, do not make the mistake of weighing the same amount of KOH as you would if you were using NaOH. Use a soap recipe calculator to convert a recipe from NaOH to KOH to ensure you get the right weight for the KOH.
For a given total weight of specific fats, you must use a specific amount of alkali molecules to correctly make soap from those fats. Since each KOH molecule is 1.403 times heavier than an NaOH molecule, you must weigh more KOH to get the correct number of molecules.
Correct for the actual KOH purity
Many soap recipe calculators assume the KOH used for soap making is 100% pure. In reality, that is never true, because potassium hydroxide reacts quickly and easily with water and carbon dioxide in the air.
The purity of most commercially available KOH is in the range of 85% to 95%. 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%.
If your recipe calculator assumes KOH is 100% pure when your actual KOH is only 90% pure, then a "hidden" superfat of 10% will be built into any recipe you create with that calculator. This much excess fat will probably separate from the soap after dilution.
Be sure to enter this real KOH purity into your soap recipe calculator to prevent excess fat in your finished soap. Some online soap recipe calculators such as LyeCalc allows the user to enter the real KOH purity, whatever number that happens to be. Changing KOH purity in LyeCalc... SoapCalc only allows the user to choose 100% KOH purity or 90% KOH purity. The 90% purity setting often gives decent results. Many other calcs do not allow the user to enter the KOH purity at all.
If you are using a soap calculator that does not let you adjust the KOH purity, you can "trick" the calculator by adjusting the superfat setting to compensate --
Adjusted superfat % = Desired superfat % - (100% - Real KOH purity %)
Example -- You want the desired superfat to be 2%, but your KOH is only 85% pure. What superfat should you enter in the soap recipe calculator to get the correct superfat?
Adjusted superfat % = 2% - (100% - 85%) = 2% - 15% = -13%
Enter the number -13 (yes, that is a minus 13) for the superfat percentage. This will give the same result as if you could enter 85% for the real KOH purity and enter 2% for the desired superfat.
Choose the right lye concentration
Choose the lye concentration that is the easiest and most convenient for you to use.
Many liquid soapers use a 25% lye concentration (3:1 water:lye ratio) with good results. The paste stays fairly soft, so it is easier to stir and quicker to dilute.
Lye concentrations below 25% (even more water) will make the soap paste easier to stir and dilute, but, in my experience, that much water makes it harder for the soap batter to reach a stable emulsion (trace). I do not care to use lye concentrations of 20% or less (4:1 water:lye ratio or larger) for this reason.
Lye concentrations higher than 25% can be used if desired. I've made liquid soap with a 33% lye concentration a few times (2:1 water:lye ratio). It may have reached a stable emulsion a little easier than at 25%, but the finished paste was a little firmer, so it was a bit harder to stir and dilute.
A few liquid soap makers use a 50% lye concentration (1:1 water:lye ratio). The one soaper I know of who does this also relies on adding sodium lactate to get the paste diluted. I've never made liquid soap with 50% lye, so I do not have an opinion about the pros and cons.
There are still other methods in which all or most of the dilution water is added to the soap batter right at the beginning or a bit later at trace. I have not tried these methods nor do I know of other experienced soap makers who have used them, so I do not have an opinion about how well they perform.
Limit the superfat
Bar soap with a high amount of superfat (excess fat) may not look any different than bar soap with a lower superfat, although a high superfat soap may not lather as well and may not be as firm. But it is easy to ignore these differences.
Diluted liquid soap cannot contain high amounts of superfat. If there is too much superfat, the excess will make the soap cloudy and will eventually separate and float on top. This is a difficult issue to solve once it happens, so many people minimize the chance of separation by following these rules of thumb when designing recipes --
Limit superfat to no more than 3%
Correct for the actual KOH purity as described earlier in this article
If you want your liquid soap to contain a higher superfat without separation, polysorbates or other emulsifiers will be required to emulsify or solubilize the excess fats. Or try superfatting with water-soluble fats such as turkey red oil (sulfonated castor oil).
Avoid ingredients that reduce clarity
If you want your liquid soap (LS) to be sparkling clear, keep the percentage of palmitic and stearic acids as low as is practical. Lard, tallow, palm, and the butters (mango, shea, cocoa, etc.) contain large amounts of palmitic and stearic acids. Large amounts of these fatty acids can cause liquid soap to be cloudy to opaque. In my experience, a recipe high in these fatty acids may also have problems with separation, even though the superfat is low.
Fats that have a high % of unsaponifiables -- avocado, shea, jojoba, beeswax, lanolin, etc. -- can also contribute to cloudiness.
Many additives commonly used in bar soap, including some essential oils and fragrance oils, mica and pigment colorants, food purees and juices, dairy milk, etc., can also cloud liquid soap.
Avoid using tap or drinking water for making or diluting liquid soap. This water contains hard water minerals that will react with soap to form soap scum. This soap scum will reduce clarity as well as lather.
Use ingredients that increase clarity
Castor bean oil adds clarity and is sometimes used in larger amounts than is typical for bar soap -- from 10% to 30% of the total fat weight. Higher amounts of castor may reduce lather, however.
Use distilled or reverse osmosis water or filtered rain water for making and diluting liquid soap, if you want the product to be as clear and lathery as possible.
Adjust the fatty acids for better viscosity and lather
Liquid soap can lather more slowly than bar soap, so many liquid soap recipes compensate by using more coconut oil than is typically used in bar soap.
Unfortunately a high percentage of coconut oil can make liquid soap more drying to the skin and makes the finished liquid soap more watery. You will have to experiment to find the right balance between lather, mildness, and viscosity that is best for you.
Remember that diluting the soap to a lower pure-soap content can help the soap lather more easily, so keep that in mind as an alternative to boosting lather by raising the coconut oil content.
Keep the oleic acid around 50%, if possible, to make it easier to control the viscosity. Even if you plan to use a cellulose- or gum-based thickener, you will need to add less thickener if the soap is already as thick as possible on its own.
Minimize the chance of microbial growth
Avoid adding ingredients to your soap that are food sources for microbes, no matter how nourishing or healing you believe these additives may be. In particular, do not use honey, aloe, milk, beer, or any other sugary or starchy liquids for diluting liquid soap. Use water only!
Ingredients that are sources of sugar, other carbohydrates, and protein create a great environment for bacteria, fungi, and other micro-organisms to grow. You will not necessarily be able to see obvious contamination or dangerous levels of microbes, so it is best to avoid the problem by eliminating or minimizing these additives in your liquid soap.
There is some truth that liquid soap is self preserving due to its alkaline (high) pH, but it is also true the self-preserving effect of high pH gets weaker as the soap becomes less concentrated.
That means soap paste (undiluted KOH soap) is good at self-preservation. Diluted liquid soap may not be as good. Using a preservative in diluted liquid soap is good insurance, especially if you sell or give your soap away. Be sure the preservative you choose is effective at a high (alkaline) pH -- many are not.
The use of a preservative is only one tool to keep your product sanitary and safe. Other good management practices include using packaging that limits contamination of the product by the consumer (for example, use pump dispensers or squirt bottles rather than open jars), reducing the chance of microbial growth by minimizing sources of food, and manufacturing your product using good sanitary practices.
In summary
I am sure I have not covered everything there is to know about designing a good liquid soap recipe, and I am quite sure other liquid soap makers may quibble with some of my ideas and suggestions.
Even so, I hope this article has given you some ideas for creating good liquid soap recipes. If you have questions about this article or suggestions for improving it, please feel free to contact me.
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Extra credit: Why not use a dual-lye mixture (KOH and NaOH) to make thick liquid soap?
From our human point of view, it seems sensible to add some sodium hydroxide (NaOH) to a potassium hydroxide (KOH) soap recipe to increase the viscosity of the diluted soap. It seems reasonable that adding more and more sodium hydroxide should gradually shift the texture of the soap from watery thin to honey-like syrup to a thick gel and finally to a firm solid.
It is tempting to use a "dual lye" mixture of NaOH and KOH to make a nice, thick liquid soap rather than deal with the hassle of using salt or other separate thickeners after dilution. Unfortunately, soap does not want to behave predictably.
Sodium soaps really "want" to be organized and tidy. That's why sodium soaps prefer to be a solid bar-soap or form a ropy or stringy gel, depending on water content and fatty acid content. The solidity of a sodium 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 give up 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 no matter what the water content is. That is why a potassium soap can range in texture from soft sticky paste to a smooth thick gel or syrup to a water-thin liquid, 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 are likely to see the soap abruptly 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 KOH and NaOH for a given blend of fats that will consistently produce a stable, thick liquid soap, batch after batch. You have to be able to predict in advance what blend of KOH and NaOH is needed to get the desired results.
It makes more sense to make liquid soap with all KOH and then add a source of sodium -- table salt, other sodium salt, or NaOH (bar) soap -- after the liquid soap has been diluted to get the desired results. Adding sodium after dilution means I can test each batch of soap and precisely add the optimum amount of sodium to give the best thickening. Learn more about ways to thicken liquid soap...
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