Okay, here's my answer to Newbie's question in Post 36. In a nutshell, she asked, "...is the organization of the molecules dependent on the process of evaporation?..."
My answer is long and more complicated than what I usually write. But please don't anybody whine at me that it's harder than usual to digest. I can't think of any better, shorter, or less complicated way to explain this. So here's today's dose of soapy chemistry --
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First some explanations of what a bar of soap really looks like if we could peek inside with a microscope --
A bar of soap is a complex mixture of solid crystals entirely surrounded by a film of liquid. The crystals are made of soap molecules fastened together in various 3-dimensional arrangements. Crystals can be big or small, and long or chunky. They can form many different shapes -- plates, irregular blocks, or even long spears.
In a hand-made superfatted soap, the film of liquid surrounding the crystals is a complex alkaline mixture of water, glycerin, and other water soluble chemicals as well as an assortment of dissolved soap molecules. There is a similar liquid inside the crystals as well, but for the purpose of this discussion, the liquid in the crystals is not as important as the liquid surrounding the crystals.
I want to stress this point -- Even though we tend to think of a bar of soap as a dry solid, it's actually a unique mixture of solid particles and liquids. The liquid in a bar of soap is just as important to the overall performance of the soap as are the crystals of solid soap....
Now I want to explain more about the individual soap molecules within this bar of soap --
Remember that a soap molecule is a fatty acid ion combined with a sodium ion. Fatty acids in handcrafted soap usually come from fats. Any given fat is a blend of several kinds of fatty acids.
When a batch of soap is made from fat, the soap molecules within the soap will vary in size and shape, depending on the different fatty acids in the soap. The most common fatty acids for soapmaking, from the smallest in size to the largest, are myristic, lauric, palmitic, stearic, and oleic.
Fatty acids with a simple straight shape are myristic, lauric, palmitic, and stearic. Soap molecules based on these fatty acids pack tidily into a soap crystal. Each kind of soap molecule has different chemical properties. For the purpose of this discussion, I want to share that myristic and lauric soaps are the smallest and are highly soluble in water. Palmitic soap is of medium length and is moderately soluble in water. Stearic soap, the longest molecule, does not dissolve well at all in water.
Last but not least is oleic soap. It is different than the others. An oleic soap molecule is the same overall length as stearic soap, but it contains a double carbon bond that twists the oleic soap molecule into a "U" shape. This bulky shape prevents oleic soap molecules from fitting nicely into the structure of a soap crystal, so the liquid phase in a bar of soap contains many more oleic soap molecules than what you might think from looking at the soap recipe.
When you wash with a bar of soap, the wash water penetrates the surface of the bar and mixes with the liquid phase. The very first soap molecules that touch your skin are the molecules dissolved in the liquid phase.
As you continue to rub the bar with your hands or washcloth, the abrasion removes soap molecules from the soap crystals as well as from the liquid phase. The soap molecules in the liquid phase are often distinctly different than the soap molecules in the crystals. This difference between the liquid and solid phases affects how the soap performs at the sink or in the bath....
So how does all this relate to curing soap?
When a bar of soap is newly made, the soap crystals contain a jumbled mixture of myristic, lauric, palmitic, and stearic soaps with a fair number of oleic soap molecules squeezed into the crystals for good measure.
The liquid phase surrounding these young crystals is large, and it also contains a jumbled mixture of soap molecules. Oleic soap molecules will predominate in the liquid phase due to their bulky shape, but there will be a fair number of all the other soap molecules in the liquid, including nearly insoluble stearic and moderately soluble palmitic soaps.
When you begin to wash with a young bar of soap, it is often the case that the soap will not lather well at first. This is true even though the young soap is relatively soft and contains a fairly large amount of water. The reason for the poor lather is the larger amount of less soluble and less bubbly stearic and palmitic soap molecules in the liquid phase in proportion to the more soluble myristic, lauric, and oleic soaps.
The amount of lather usually increases as you keep rubbing the bar, because you are scrubbing more soap molecules off the soap crystals. These molecules from the crystals mix with the soap molecules from the liquid phase. As more soap molecules accumulate on your skin or washcloth, they will build a larger amount of lather.
Fast forward to the end of the usual cure time of 4-6 weeks. By this time, the water content in the bar of soap has dropped -- it might now be 50% to 70% of the water that was originally in the soap bars. The glycerin and other water soluble chemicals in the liquid phase are left behind and become much more concentrated in the liquid phase.
All that seems plenty good from a human point of view -- evaporate excess water in the soap, leaving behind the soap molecules and other non-water chemicals. The cure is done, right?...
It turns out the process of curing is more subtle than that.
The high concentration of glycerin and other water-soluble chemicals in a bar of handcrafted soap has an unexpected effect on the liquid phase of the soap. Glycerin, along with as table salt and certain other chemicals, has the ability to "salt out" soap when the glycerin concentration becomes high enough.
During salting-out, soap molecules will not remain dissolved in the liquid phase. Instead, the soap molecules will coalesce into solid soap crystals. (I have described how the process of salting-out can be used to clean up soap scraps --
http://classicbells.com/soap/saltOutTut.html)
Not every kind of soap molecule will salt-out to the same degree. Stearic and palmitic soaps salt-out quickly and well. Myristic and lauric and oleic soaps do not salt-out nearly as easily.
What this means for a bar of soap is this --
As the glycerin and other dissolved chemicals in the liquid phase become sufficiently concentrated, the stearic and palmitic soaps in the liquid phase will form solid soap crystals -- in other words, they want to salt-out. These soap molecules also gradually trade places with lauric, myristic, and oleic soaps in existing crystals.
As time goes on, the crystals in a bar of soap will contain more and more of the less-soluble stearic and palmitic soaps and the liquid phase will contain more of the soluble lauric, myristic, and oleic soaps. This shift in the kinds of soap molecules in the liquid phase usually creates a faster building, more abundant lather for a smaller amount of work. The larger amount of less-soluble soap in the soap crystals has the benefit of making the soap more resistant to wearing away from use.
The concentration of stearic and palmitic soaps in the solid crystals begins when enough water evaporates to trigger the salting-out process. But the migration of insoluble soaps into the crystals and the transfer of soluble soaps into the liquid phase is not a fast process. It arguably can take months for this migration to stabilize in a handcrafted bar of soap....
And that, in not a nutshell, is why curing soap is a complex process that goes beyond water evaporation. The glycerin and other dissolved chemicals must be concentrated enough so soap molecules start to salt-out. Only water evaporation will accomplish that task, so it is a necessary beginning to the curing process. But once sufficient evaporation happens, the slower process of restructuring the soap then begins.
Are there ways to force this shift in the crystal structure and the nature of the liquid phase? Yes, if you have the equipment. Commercial soap makers quickly dry their freshly made soap to the desired water content, and then mix the dried soap chips or noodles under pressure using a machine called a "plodder." The soap is then pressed or extruded into finished bars. With careful control of the drying and plodding steps, the soap will form the desired crystal structure.
For handcrafted soap makers, the most practical solution is to allow time to solve the problem.
