Stearic Acid Neutralization in Emulsions

Hi,

In some formulas where stearic acid is being used as an emulsifier, I see it being neutralized to pH 6.5 and other times I see it neutralized to pH 7.5 or 8. I had learned that stearic acid should be neutralized to ~pH 7.5 -8, because otherwise if it is lower, not all the stearic acid will be neutralized? Is that correct? Basically, I am asking what is the optimal pH stearic acid should be neutralized to when it is used as an emulsifier. 

Thank you for your help. 

Comments

  • BobzchemistBobzchemist Member, PCF student
    This is one of those "it depends" questions. If what you are looking for is the complete conversion of stearic acid to a stearate, then your pH is going to need to be alkaline - just how alkaline will depend a bit on your neutralizing agent, but it's usually about 9.0.

    You are right that if the pH is lower, not all of the stearic acid will be neutralized/saponified. There are some benefits to having free stearic acid in a formula, so that's probably what's going on with the lower pH formulas - but at pH 6.5, I would strongly suspect that there is very little stearate being formed.
    Robert Zonis, Sr. Formulation Chemist, Beaumont Products "All opinions and comments expressed are my own, have no relation to Beaumont Products, are fully copyrighted, and may not be used without written permission."
  • heraklitheraklit Member, PCF student
    So what's the HLB of various stearates (sodium, potassium, tea etc.)?
    15?
  • @Bobzchemist What are the benefits of having free stearic acid? Thickening properties? 
  • Bill_TogeBill_Toge Member, Professional Chemist
    @heraklit the HLB system only applies to non-ionic surfactants; stearates are anionic, so they don't have HLB values at all
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • MicroformulationMicroformulation Member, Professional Chemist
    edited September 2015
    It would have a saponification value. I believe Stearic acid is around 209 or so.


    markfuller@microformulation.com Microformulation.com Microformulation Cosmetic Consulting provides Custom Formulations for both large Commercial accounts as well as smaller entrepreneurs. We can provide Naturally compliant Formulations under the NSF, NPA, Whole Foods and USDA Organic Certifications. BS.Pharm Albany College of Pharmacy, Union University.
  • heraklitheraklit Member, PCF student
    Oh yes! Thanks Bill.
  • The benefits of having excess, unsaponified Stearic Acid would be the "creaminess" that it would give a cream.  Brushless shaving creams often have unsaponfied stearic acid to give the cream a smooth, creamy consistency ideal for shaving.  These shaving creams are very much like old style facial creams in the 1920s, 30s and 40s.

    David
  • Chemist77Chemist77 Member, PCF student
    If there is unneutralized stearic acid in the formula, will it impart the pearlescence???
  • Yes, it should.  This was often the result in what used to be known as "vanishing creams".  Here is a link:

    http://www.cosmeticsandskin.com/aba/vanishing-cream.php

    David
  • Chemist77Chemist77 Member, PCF student
    Thanks @David08848 I had seen this in the past but never had a good look. Wow I missed the important part.
  • Thank you i think you all just solved my puzzle I had finally put it together that stearic acid was causing the drop in my creams pH...and planning to buffer with koh... But I ideally need a pH of around max 6 would that be an issue as not all will convert to  potassium stearate
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