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Sodium Benzoate and Quarternary Compounds
Hi folks! I have been a long-time reader
of the Chemists Corner blog and lurker on these forums. I’m finally
jumping into the discussion for the first time today with a question that I
have often speculated about.
I have read in a few places that sodium benzoate
is incompatible with quaternary compounds.“Benzoic acid is used as a preservative at
concentration range of 0.1–0.5%. It demonstrates antimicrobial activity only in
unionized form. Furthermore, it is only effective in acidic formulation with
optimal activity at or below pH 4.5. It has moderate activity against
gram-positive bacteria, molds, and yeasts, but is less effective against
gram-negative bacteria. Benzoic acid is incompatible with quaternary compounds
and nonionic surfactants.”source: Alok K. Kulshreshtha, Onkar N. Singh, and G. Michael Wall. Pharmaceutical Suspensions: From Formulation Development to Manufacturing (New York: Springer, 2010), 121.
What’s the story here? What causes the incompatibility? Is it the cationic nature of ingredients like
Honeyquat and other quaternary conditioners? I guess I was surprised to see
that nonionic surfactants are also called out as incompatible, but not cationic
surfactants. Most importantly: What is the nature of the conflict? Will it inactivate the preservative
properties or have a more series formula-ruining effect? And is it only benzoic acid/sodium benzoate that suffers
from this incompatibility? Are the other organic acids safe?
I have seen a lot of formulas out and about on the internet
that contain Honeyquat and are preserved with sodium benzoate or another
preservative that utilizes sodium benzoate in its formula. Are the creators of
these formulas just unaware, or do they know something I don’t know?Interested in your insights!
Bri
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3 Replies
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Interested in this, too, but can’t comment.
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Bri, the author appears to be making a broad, somewhat pedantic statement about the incompatibility of benzoic acid with nonionics. They also state that caveat for other preservatives as well. They are not versed well in the science and the art of preservation. Lesson here: don’t believe everything you read, or at least apply critical thinking to it. They are likely referring to the deactivation of certain preservatives by TWEEN 60 and TWEEN 80. That applies to parabens, and somewhat to phenolics, but shouldn’t be deemed the same pathway for other preservatives, and certainly not other nonionic surfactant chemistries. Use PEG ethers and you won’t go wrong. As for quats, if you are talking straight alkyl/benzyl quat surfactants, such as CETAC, StearAC, etc., you might want to use something else than benzoic acid, and not much of it, since those compounds offer more antibacterial and antifungal activity than most realize. “Honeyquat”: they still make that crap??
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Chemicalmatt:
Thank you so much for your insight here. I had asked a friend working as a chemist professionally (albeit, in a very different field outside cosmetic formulation) and he was at a loss to explain this author’s admonitions. That coupled with your analysis leads me to believe that I should stop worrying about using my nonionic surfactant in conjunction with sodium benzoate.As I was furiously trying to research this on my own, I started to have some doubts about the hydroxypropyltrimonium honey. A lot of competitive formulas are still using it and saying “oooh, look! We have honey!” But I’m starting to wonder if the expense is worth the marketing story. You last statement raised even more doubts in my mind. Care to elaborate on your disdain for this ingredient?Again, thank you for solving this mystery for me!Bri
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