Different alkalis give soaps of different characteristics. While potassium hydroxide tends to yield soft, highly soluble soaps, sodium hydroxide produces the opposite, and has long been used to make ordinary bar soap. Since I use bar soap for bathing and nothing else, I decided to make a bar that was slightly rich in oil, so as to give it a moisturizing effect, and to potentially allow it to be used as shampoo as well.
I began by adding 30mL of water to a beaker, to which I added 10g of sodium hydroxide, all at once. The lower reactivity compared to potassium hydroxide and the higher ratio of water eliminated any risk of boiling, and the alkali dissolved without any issues, although it did produce an irritating vapor. Once this solution had cooled down, I then added 80g of olive oil, which formed a separate layer just as before. From this point, however, I decided to stir the mixture with a handheld blender (seen below), rather than a stirring rod. Early experiments revealed that sodium hydroxide has a much lower tendency to emulsify, even in more concentrated solutions, and no amount of manual stirring (upwards of four hours) produced a usable soap. Using a blender, however, I was able to produce a thick emulsion in less than thirty minutes. Once the mixture gained a batter-like consistency, I then poured it into a small plastic tub as a mold, which I had previously lubricated with olive oil to facilitate later removal of the soap bar. I then put the lid on the tub to retain moisture, and left it overnight for the reaction to complete.
The next day, I removed the soap from its mold and placed it in a warm location to dry and harden. After four days it seemed hard enough to be used, and can be seen at the top right of the page. Some sodium carbonate had formed on the surface due to atmospheric carbon dioxide, but this easily washed off with water. The soap was effective on both skin and hair, and the excess oil was moisturizing without being excessively greasy. Furthermore, the soap held its shape well when wet, which was surprising considering the fairly short cure time.
Overall this soap was very easy to make, and like the previous soap the ratio of ingredients is simple enough to remember (3:1:8 by weight). The use of a handheld blender significantly improved the process as well, and this could likely be transferred back to the previous recipe to shorten the preparation time. In general I am very pleased with the results here, and will likely continue to make this type of soap for personal use.
Despite the fact that this soap was usable after four days of drying, I found that it became soft after two days of use. Letting the soap dry for a further two weeks eliminated this problem, and it is now stable in water. My assumption is that the soap needs to be dry throughout, and that a wet center allows for rapid water permeation of the entire bar. From now on, I will dry the soap I make for one month before use.
I have also found that if less water is used, the blending process can be shortened to less than a minute; a successful batch of soap was made with this modification, having a weight ratio of 2:1:8 for water, sodium hydroxide, and olive oil respectively, with a small amount of turpentine (0.1% by weight, or 3 drops per 100g of oil) added to the oil as a fragrance. Interestingly, this soap developed no deposits of sodium carbonate; this is likely due the faster overall reaction time, which leaves less free sodium hydroxide to react with atmospheric carbon dioxide.