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help me understand CMC / debug my shampoo
Hi, Please help me understand some chemistry. I am scratching my head a bit.
1. I’m trying to make a minimalist shampoo with SCI + CAPB. What is the approximate, very minimum % I should add of each surfactant to reach CMC and ensure no monomers are floating around in water, ready to latch onto my skin?
2. Also, can I use just SCI, without a co-anionic? Why is it suggested that 2 or more anionics should be combined to increase mildness? How does this affect micelle shape/CMC?
3. Could I theoretically make a shampoo with CAPB as only surfactant?
Thank you in advance.
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4 Replies
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I think you have a misconception about what the CMC means. It is simply the lowest concentration of a surfactant system before micelles are created. However, just because some surfactant molecules assemble themselves in the form of micelles that doesn’t mean there aren’t any free surfactant molecules (you called monomers) floating around.
This video gives a visualization of what is happening on a molecular scale.
https://www.youtube.com/watch?v=tn4Cjcc5fxw But the point is micelles are only a temporary state for any single molecule.1. No point exists to achieve what you’re asking.
2. Yes, you can use just SCI. Because using SCI alone is more irritating than using it with a secondary surfactant. I’m not certain how it affects micelle shape but more surfactant probably leads to larger micelles. But it depends on the packing so I don’t really know about these two surfactants.
3. Yes. But it would be expensive and wouldn’t work as well as standard shampoos.
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this is helpful, thank you @Perry. but why is it suggested that 2+ anionics should be combined to increase mildness?
why is SCI + Sodium Cocoyl Glutamate + CAPB theoretically more mild than say just SCI + CAPB?
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Mildness is, on a scientific level, not well understood and based mostly on observation and trial & error experiments. There are some rules of thumb regarding mildness of surfactants (you can google these). Turns out, mixing a mild one with a harsh one renders the latter milder.BTW if you start with a super diluted form, only ‘monomers’ are present up to the solubility limit where free monomers reach their maximum, this is called critical micelle concentration. Above this CMC, you will still have the same maximum concentration of free monomer, pretty much like most solutes (take table salt as an example). With table salt, all added surplus will simply not dissolve and sink to the bottom. However, with surfactants, these molecules will still go into solution but then form micro-aggregates, so called micelles. These are the entities which actually do all the work a surfactant is commonly used for. Once your shampoo is in consumers hands and thereby diluted with water, its concentration obviously falls. In fact, if you use too much water or start with a too diluted product such as you plan on making, the concentration may fall below CMC and that means your shampoo will no longer work as a surfactant/cleaning agent = epic fail. -
thank you for that super clear description! so my take is to use a concentration well above the cmc to account for the possibility people will dilute.
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