Home Cosmetic Science Talk Formulating General “Citric acid is an electrolyte and those crash carbomer gels”

  • “Citric acid is an electrolyte and those crash carbomer gels”

    Posted by Graillotion on February 15, 2023 at 6:42 pm

    I was reading old forums today….from one certain reputable contributor….and came across this statement.

    Unfortunately in the format I was using….I can only read his answers…and not the questions…. so there is a possibility I have taken his comment out of context….since I do not have the context.

    Any thoughts on the title line? How does this also translate into polymeric emulsifiers?

    Would using a monobasic acid like lactic mitigate this vs a tribasic acid like citric?

    Aloha

    @chemicalmatt

    Bfd replied 1 year, 2 months ago 2 Members · 1 Reply
  • 1 Reply
  • Bfd

    Member
    February 16, 2023 at 10:47 pm

    A neutralized carbomer unit has negatively charged carboxylate groups on a carbon backbone. In an aqueous gel, the carboxylate groups repel one another (electrostatic repulsion), giving rigidity to the structure. They all want their own space, no overlapping.

    The problem with adding electrolytes is that they screen the negative charge on the carbomer units. The distance of repulsion between units is diminished the higher the ionic strength of the system becomes. With positively charged ions sandwiched between the negative carboxylate groups, carbomers get cozy together and viscosity plumets.

    At some point with addition of acids, the pH drops low enough (<5) that you’re protonating the carboxylate groups back to carboxylic acid. You might think this means you’ve reset to your starting point, but the persistent high ionic strength of the solution will prevent the carbomers from ever contributing meaningfully to viscosity again.

    At pH 6.5, used at the same concentration, citric acid will contribute 3x more protons than lactic. Three protons are significantly worse than one, but one will still cause some electrostatic screening.

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