Ionic surfactants (anionic or cationic) for emulsion stability

In an emulsion with non-ionic emulsifier, what percentage of anionic or cationic emulsifier (minimum) is needed for the purpose of increasing the stability?


  • PharmaPharma Member, Pharmacist
    edited November 2021
    As a rule of thumbs, 5-10% of total emulsifiers/co-emulsifiers should suffice. An exception to this rule may be lamellar gel networks and α-gels where >20% weak anionics (such as glyceryl stearate citrate) may be included. However, in that case, the anionic emulsifier is often combined with fatty alcohols (about 1/6 to 1/10) and non-ionic emulsifiers with large hydrophilic head groups may be included to increase stability (in that case at +/- 10% of total amount of emulsifiers/co-emulsifiers).
  • @Pharma thanks for explanation.

    In one emulsion i use 1% Polyglyceryl 6 distearate+ 3% setearyl alcohol. Does setearyl alcohol count as co emulsifier? I mean should i add 0.1% or 0.4% ionic emulsifier to this to be like 10% total emulsifier?

    I want to add cationic emulsifier to get that powdery feeling with stabilization. As they are expensive, the least expensive one i found is Stearamidopropyl Dimethylamine. Is this a good option to add in skin lotion for a boost of stability and that powdery feel of cationic surfactants? 
  • Also by large hydrophilic head groups, do you mean lower HLB? 
  • PharmaPharma Member, Pharmacist
    About 0.4% (a typical partner for your combo would be sodium stearyl lactylate). Sodium stearyl lactylate as well as stearamidopropyl dimethylamine may, due to their pH sentitive ionicity, be added at even higher % depending on pH. The former gets less anionic at low pH, the latter less cationic at higher pH. I never work with stearamidopropyl dimethylamine but I would really give it a try. I like cationics and think they feel really nice (I played with Emulsense and Varisoft EQ 65 combined with polyglyceryl esters and they play reall real nice together)! One drawback is that you can't use anionic polymers for thickening...
    Regarding you second Q: No, I mean high HLB. Traditionally Tween 20 but for ingredients from renewable resources, polyglyceryl-10 laurate would make a suitable substitute (start in the range of 5% with regard to total emulsifiers and go from there).
  • AbdullahAbdullah Member
    edited November 2021
    @Pharma i am using sodium stearoyl lactylate 0.8%+ glyceryl stearate 3.2% in another emulsion. That is a good product and economic that i really want it to be. The only drow back is that at pH below 5 SSL become grainy. 

    I want to have one product at pH 4 and as i am using petrolatum and +10% glycerin, i want to have some cationic surfactant or whatever ingredient that can reduce the tackiness.
    Also i am using Ethyl lauroyl arginate in that product which is cationic.

    Do you have any suggestions for non-ionic polymer other than guar gum and cellulose? 
  • @Pharma Just to check that i have understood correctly, of i use ionic+fatty alcohol+ non-ionic surfactants at 5% total, should i use 5-10% of this 5% which is 0.25-0.5% ionic, 0.25-0.5% non-ionic and 4-4.5% fatty alcohol? 
  • PharmaPharma Member, Pharmacist
    Abdullah said:
    @Pharma Just to check that i have understood correctly, of i use ionic+fatty alcohol+ non-ionic surfactants at 5% total, should i use 5-10% of this 5% which is 0.25-0.5% ionic, 0.25-0.5% non-ionic and 4-4.5% fatty alcohol? 
    Correct. With weak ionic ones such as lactylates, better go with 0.5% whilst strong ionic ones such as SLS or SLES work well with the lower amount. It always depends a bit on what you have in your system, so this is just a rule of thumbs.
    You could also use cellulose and starch derivatives or chitosan. Sadly, most aren't as performing as the anionic polymers, they require higher inclusion rates and tend to feel slimy/tacky. Different cationic and non-ionic synthetic polymers (polyesters, polyamides, quaternized polymethacrylates, PLGA...) are used for example in pharma... I don't know if any of these are available for cosmetics and/or perform good enough as cosmetic gelling agents.
  • @Pharma in cationic surfactants, which one is weak and which one is strong?
    1. Stearamidopropyl Dimethylamine
    2. Behenamidopropyl dimethyltryptamine
    3. Behentrimoniom cloride
    4. BTMS
    5. Distearyldimonium Chloride

    And how do we know what is weak and what is strong ionic? 
  • PharmaPharma Member, Pharmacist
    1 and 2 are weak because these are ternary amines. You have to add an acid to protonate them and turn them into positively charged molecules. The pH of most cosmetic products is not low enough to protonate 100% but only enough so they can act as ionic emulsifiers.
    The three others are strong because these are quaternary amines. They have a permanent and pH independent positive charge.
  • @Pharma thanks 😊

    The pH would be about 4. Is 1 and 2 weak even at pH 4?
  • PharmaPharma Member, Pharmacist
    I don't know the pKb of the two... should be in the range of 5-6. Assuming it were 5, only 90% would be protonated at a pH of 4 whilst 99% would be protonated a pKb of 6. Number 3-5 are always at 100%.
    90% is good enough to feel and behave like a cationic emulsifier but it makes things a bit susceptible to pH shifts and 'molecular' interactions.
  • @Pharma what is pkb?
    And does 1 pH lower than pkb mean 90% protonation in everything? 
  • PharmaPharma Member, Pharmacist
    Yes, 1 unit below pKb means 90% protonation, 2 units 99%, 3 units 99.9% and so on.
  • Pharma said:
    Yes, 1 unit below pKb means 90% protonation, 2 units 99%, 3 units 99.9% and so on.
  • @ketchito i tried to make a lotion according to this advise. 

    As my English is not so good maybe i didn't understand this correctly. 

    Please let me know what do you understand from this and what was my mistake. 
  • @Abdullah I hope I also understood correctly. Even though sodium stearoyl lactylate and sles are both anionic surfactants, sodium stearoyl lactylate has a longer carbon chain, which makes it more useful as a emulsifier, while the shorter chain lenght makes sles a good detergent. To put it in a different way, it'd very unlikely to replace sles in a shampoo by sodium stearoyl lactylate (you'll have less foam and detergency). Each have very specific attributes, so I wouldn't replace one by the other.
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