Stearic Acid Neutralization in Emulsions

Formulating

Stearic Acid Neutralization in Emulsions

Posted by beautysci on September 10, 2015 at 4:56 pm

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.

Beeehappy replied 6 years, 6 months ago 8 Members · 11 Replies
11 Replies

Bobzchemist

Member
September 10, 2015 at 6:10 pm

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.
heraklit

Member
September 10, 2015 at 8:29 pm

So what’s the HLB of various stearates (sodium, potassium, tea etc.)?
15?
beautysci

Member
September 10, 2015 at 8:40 pm

@Bobzchemist What are the benefits of having free stearic acid? Thickening properties?
Bill_Toge

Professional Chemist / Formulator
September 10, 2015 at 10:42 pm

@heraklit the HLB system only applies to non-ionic surfactants; stearates are anionic, so they don’t have HLB values at all
Microformulation

Professional Chemist / Formulator
September 10, 2015 at 11:45 pm

It would have a saponification value. I believe Stearic acid is around 209 or so.
heraklit

Member
September 11, 2015 at 5:52 am

Oh yes! Thanks Bill.
David08848

Member
September 11, 2015 at 7:46 pm

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
Chemist77

Member
September 12, 2015 at 2:02 am

If there is unneutralized stearic acid in the formula, will it impart the pearlescence???
David08848

Member
September 12, 2015 at 7:05 pm

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
Chemist77

Member
September 13, 2015 at 3:43 pm

Thanks @David08848 I had seen this in the past but never had a good look. Wow I missed the important part.
Beeehappy

Member
September 1, 2018 at 7:46 am

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