Formulating with Acids: pKa and pH

This is from the DECIEM website:
"pKa is the most important aspect to consider in formulating with acids. pKa implies acid availability. When pKa is close to pH, there is an ideal balance between salt and acidity, maximizing effectiveness of the acid and reducing irritation. Higher pH numbers in such a case would increase salt which counter-intuitively would make the formula even more irritating than if the formula was more acidic.
So, Glycolic Acid (pKa 3.6) is less irritating at pH 3.6 then at pH 4.6?
Thank you!

Comments

  • No glycolic is exfoliating/peeling outer epidermis/stratum corneum and as a result you are not likely to see a difference in irritation.
  • Thank you DrBob. I'll never stop learning :)
  • Higher pH numbers in such a case would increase salt which counter-intuitively would make the formula even more irritating than if the formula was more acidic.

    that's a new one on me; have they cited any sources for this claim?
    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
  • edited February 3
    Not to my knowledge. I really do not understand, Deciem doesn't need any fancy sentence to sell his products. http://theordinary.com/product/rdn-lactic-acid-10pct-ha-2pct-30ml
  • That is hard to believe. If I neutralize the glycolic acid or even partially neutralize it to pH 6-7, will it be more irritant for the skin than the pH 3.6? 
  • @em88 I doubt it; to me, this claim looks very much like superstitious nonsense
    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
  • This is the explanation I just received from Deciem:

    Sodium lactate has a high molecular weight. Therefore, it would stay on the skin and not penetrate which can be irritating.

    Experts, do you agree? Thank you
  • As far as I've read, neutral molecules cross the lipid-heavy stratum corneum more easily than ionized ones. So, in an acidic environment where pH<pKa and glycolic acid mostly keeps its proton, maybe it absorbs better than a solution of mostly glycolate ions. But I don't know how that would affect irritation.
  • glycolic acid formulation has 3 to 4 ph.
    While arbutin work good at 6 ph.
    how to formulate of this  combination? 
  • edited April 5
    that page is littered with factual errors, particularly the chemistry part; let's take it apart bit by bit

    first of all, throughout the text, the term 'salt' is used in place of the more specific (and more generally recognised) term 'conjugate base'

    "Lactic acid contains NMF"
    NMF contains lactic acid, the converse is not true

    "AHAs have no known systemic toxicity"
    on the contary, using glycolic acid as an example, a dose of 600 mg/kg is teratogenic (causes birth defects), and the NOEL for teratogenicity is 150 mg/kg

    "First and foremost, acid strength relates to the importance of pKa, which is a measure of acid strength and free acid availability"
    on the contrary, it is the pH at which the free acid and its conjugate base are present in equimolar amounts; it is nothing to do with actual quantity of acid present

    "The pKa is the logarithmic expression of the pH at which the acid possesses ‘free acid’ containing equal amounts of ion and salt."
    1) pH is a logarithmic quantity by nature, and the fact it's logarithmic is irrelevant in this context
    2) salts are ions by definition, so the phrase 'equal amounts of ion and salt' is nonsense
    3) see above for a coherent, non-garbled explanation of pKa

    "Acids, bases, and salts contain ions of the element hydrogen."
    what is an "ion of the element hydrogen"?

    "The lower the pH the more active the acid peeling solution."
    not necessarily; if you put one drop of concentrated hydrochloric acid in a beaker of water, it will have a very low pH, but it will be a very weak acid

    "The more salt you have in your acid, the less potent it is"
    correct (notwithstanding the use of 'salt' to mean 'conjugate base')
    "and potentially the more irritating it can be."
    sources please?

    "The pKa value tells you at what pH you have half acid and half salt. This translates that if you buffer an acid, you convert some of the acid to the salt form."
    the first sentence is true, the second sentence makes no sense whatsoever

    "Acid solutions are “buffered” not to make the acid stable but to raise the pH."
    my head hurts

    "The concentration is not as important as the pH and the pKa. I am asked all the time ‘what is the acid percentage concentration?’ Percentage of acid concentration is irrelevant"
    if this is right, then the laws of thermodynamics are wrong

    "and understanding this ‘acid chemistry’ explains why TCA, or trichloracetic acid (pKa of .26) will be far stronger in a five percent solution than lactic acid (pKa 3.86) or glycolic acid (pKa 3.83) at 30 percent. This is primarily because TCA is almost fully ionized and lactic and glycolic acids are not."
    this is more to do with the chemistry of each acid than its strength; hydrochloric acid has a pKa of -7, and is therefore a much stronger acid than TCA, but it is much less corrosive to metals (and less toxic to life) because it has completely different chemistry
    similarly, hydrofluoric acid has a pKa of around 3 (making it a weaker acid than TCA), but is a much more hazardous material, and is one of the few substances which can etch glass, due to its chemistry

    god almighty, after reading that I need to lie down; the most depressing thing is that whoever wrote this probably got paid an ungodly sum for it
    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
  • @maria @Perry no worries, in a perverse kind of way I actually enjoy tearing apart stuff like this
    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
  • @Bill_Toge
    "in a perverse kind of way I actually enjoy tearing apart stuff like this"
    :joy:
    And please continue doing so! Love it! :+1:
  • edited April 16
    Sodium lactate has a high molecular weight. Therefore, it would stay on the skin and not penetrate which can be irritating.

    This has me confused?  Sodium Lactate's skin conditioning/humectant properties are surely well recognised? ( and in fact the brand sells a NMF product with Sodium Lactate in it)

    Plus, the idea that because a compound has high molecular weight it is categorically more likely to cause irritation?

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