Formulating With Stearamidopropyl Dimethylamine

Last night I was experimenting with stearamidopropyl dimethylamine in a conditioning shampoo recipe. I melted in in my surfactant phase and was prepared to add citric acid to the water phase to neutralize (5.88:1 ratio SD to CA) before combining, but I decided I would take a pH reading beforehand. It was already at ~4.6 due to the usual 0.2% EDTA I usually add to water. Since EDTA is already an organic acid, and the literature says the water phase should be about 5.5 before combining with surfactants, I decided to hold off on adding more CA. After adding surfactants, my pH shot up over 8 (working with glucosides), and then I added another 0.6g of CA to bring down the pH to 5.5 again.

Anyhow, the results were good. Nothing separated and the solution was clear (odd, I was expecting the SD to cause pearling). My question is this: Was I right to hold off adding additional citric acid to the water phase until combining with surfactants? Or should I have added the citric acid anyway?



  • Chemist77Chemist77 Member, PCF student
    Citric acid is generally called pH adjuster among other things so I suppose logically the adjustment is best done at the end of the process.
    I am sure other learned members can chip in too with a much better explanation.
  • CosChemFanCosChemFan Member
    edited August 2015
    Formulating with SD is a bit interesting because the literature contradicts itself (at least mine does). I had to ask the supplier and they said you can neutralize SD either in the beginning or at the end. You still get the positive charge. I've used it neutralizing it both ways and the results were the same to me. You would think they would just list one method of neutralizing this stuff, but it's one of those things I guess. 

    For a shampoo it's easier to just throw the acid in the water phase. If you put SD in the surfactant phase add the acid to the water as well and combine. For conditioners adjust the pH at the end. I personally add the SD to the water phase and not the surfactant phase.

    If your pH in the water phase with SD is already acidic then just combine the phases because it's already neutralized.  

    And no, SD does not make a pearlized product. I was surprised by that too when I first used it.
  • Chemist77Chemist77 Member, PCF student
    edited August 2015
    Well I forgot that important aspect quite mistakenly that YES indeed it is needed to neutralize to pH 3.5-4.0 in case of SD and then at the end if process you can adjust the pH. Getting desired pH in the middle of process would be a bit cumbersome.
  • I guess my question is this: Does it matter what kind of acid you are using to neutralize the SD? Is the SD to acid ratio important, or is it really a matter of adjusting your water phase to a specific pH before combining with SD?

    I got the idea of melting the SD in the surfactant phase here:

    My understanding is that SD is supposed to be an alternative to dimethicone, but I did not notice any conditioning effect at 1%. I don't know if that's because there was an error in my process, or if I simply didn't use enough SD.
  • Chemist77Chemist77 Member, PCF student
    edited August 2015
    Generally its citric acid for various reasons including cost, I disperse in my water phase and adjust the pH to 3.5
    I am not sure what else is there in your formula but @1% alone I suppose u won't have any perceptible effect.
  • @thebrain can you say what are your surfactants in this Shampoo as SPDMA is cationic conditioner? 
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