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Amphoteric and Anionic surfactants - how to use together
So I’ve read online that CAPB is amphoteric but at a lower ph of less than 5.5-6 it becomes cationic. If this is the case are all formulations containing CAPB and an anionic surfactant only stable when the ph is kept high enough? What exactly are the rules when combining amphoteric and anionic surfacants?
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12 Replies
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These zwitterionic surfactants display a negative charge at high-pH values due to charge domination of the carboxylate group.55 They are positively charged at low-pH values due to the charge domination of the amine group. The isoelectric point is the pH at which the negative ionic charge of the carboxylate groups is exactly counterbalanced by the positive ionic charge of the alkyl ammonium groups. Streaming potential measurements of micellar solutions of betaines show that the isoelectric point is greater than pH 9 for C12-14 alkylbetaines. When alkylbetaines are mixed with sodium lauryl ether sulfate, the ionic attraction between the positively charged betaine headgroups and the negatively charged sulfate headgroups cause a dramatic increase in the packing factor of the mixed micelles. The isoelectric point of micellar cocamidopropyl betaine is 6.25. Therefore, at pH 5.5, cocamidopropyl betaine is slightly positively charged. This charge is just enough to confer an increase in packing factor on sodium lauryl ether sulfate micelles to ensure that the mixed micelles are wormlike, and, therefore, they exhibit a “thickened” rheology. On the other hand, cocamidoethylbetaines have an isoelectric point (IEP) of less than pH 3.5, which means that this cosurfactant is essentially negatively charged at all pH values above 3.5. It has been proposed that this low IEP is due to the formation of six-membered ring conformation that brings the carboxylate group into the direct vicinity of the amine group, thereby effectively neutralizing a proportion of its charge. Correspondingly, cocamidoethylbetaines are less efficient “thickeners” of anionic surfactant compositions.
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Does this mean at pH above 6.25 CAPB will have negative charge?
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That will never happen since CAPB has a quaternary amine with a permanent positive charge. That’s why it’s sometimes argued if CAPB is a real amphoteric or not.
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So what will change in CAPB is pH is above 6.25?
Nothing?
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I don’t remember what are the pKa values for CAPB, but if at that pH the molecule is neutral, going up in pH wouldn’t change its ionic state since it’s hard to remove an hydrogen from a methyl group, and it’s also harder to do that from an amide group (intramolecular stabilization).
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@GeorgeBenson for SLES with CAPB together are stable in large border of pH then for you desired pH there is no issue.
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You need to realize that the cationic charge of CABP in <6pH is NOTHING in compare with anionic surfactant. It is perfectly stable and does not interfere with the anionic charge. That is also why you can use cationic polyquats in anionic systems.
If you would use something like cationic emulsifier with anionic emulsifier (surfactant), then insoluble substances would form.
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Agree with @Paprik here @GeorgeBenson : charge density, pKa and ratio is not the main consideration. The mass ratio is however, and the “classic” ratio of anionic: amphoteric is 2:1 based on solids. If you are formulating a conditioning shampoo with cationic polymers, then increase ampho slightly and never use a highly charged anionic (i.e. sulfates, phosphates). Another helpful hint from an Old School formulator (that would be moi): use an amine oxide as builder and your dilution-deposition is even better.
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@chemicalmatt anionic: amphoteric at 2:1 do what? I didn’t get this point.
Also how much amine oxide or ratio with CAPB and anionic should we use for best effect?
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@Abdullah That mass ratio is the ideal (“classic”) ratio for building cleansing products like shampoos and body washes. Floor cleaners, laundry detergents, hard surface cleaners are a different story. The “classic” viscosity builder is an alkanolamide (cocoamide DEA in the old days) at the “classic” ratio 4:2:1 anionic: amphoteric: alkanolamide. I’ve found substituting coacoamidopropylamine oxide or lauramidopropylamine oxide for the amide results in good viscosity & foam build but gains better deposition of the polyquaternium/dimethicone (or guar HPTC if you’re one of those people) upon rinsing.
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May i know what is the reason for using surfactants in their ratios
i.e anionic : amphoteric : non ionic
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@chemicalmatt thanks.
I was also looking for an alternative to glucosides as non-ionic surfactant for shampoo other than DEA and MEA.
Looks like i should try amine oxide.This surfactants ratio is also interesting. I have always used high amount of anionics compared to others. At max 1 part non-ionic+ amphoteric combined to 3 parts anionic. I should try this too.
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