7% Lactic Acid in a 10% Zinc Oxide Daily Wear Sunscreen - Do AHA's only sun-sensitize at low pH?

ZinkZink Member
edited February 2017 in Formulating
I've been working on a pH 7 silane coated zinc oxide (10%) moisturizer/sunscreen for a while, and I wondered if I should get the pH down to a more acid mantle and potassium sorbate friendly pH 5.

To get there it needs 7% l-lactic acid, at pH 5 the free acid value is ~6% effectively making it equivalent to ~0.5% l-lactic acid at pH 3. (1997 In vitro percutaneous absorption of alpha hydroxy acids in human skin).

Is this likely a good addition for a daily wear formula? L-lactic acid can normalize ceramide populations in the stratum corneum (1996 Effect of lactic acid isomers on keratinocyte ceramide synthesis, stratum corneum lipid levels and stratum corneum barrier function) and possibly stimulate dermal collagen synthesis at this dosage.

BUT, could it at a relatively high pH still lead to sun sensitivity and be better used in a occasional use nighttime formula?

References exploration of AHA/BHA penetration and pH by a PhD chemist here: http://www.labmuffin.com/fact-check-friday-why-does-ph-matter-for-ahas-and-bhas/

Comments

  • johnbjohnb Member, Professional Chemist
    Before you go much further in this, have you considered the reaction of zinc oxide with lactic acid?
  • Bill_TogeBill_Toge Member, Professional Chemist
    also, how come you have to add that much lactic acid just to adjust the pH?
    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
  • @johnb yes, I'm running accelerated stability right now, but all seems fine likely due to the silane coating. I see brands like Elta MD use ~1% lactic acid.

    @Bill_Toge free uncoated zinc ions that increase pH have to be counterbalanced AFAIK. No other basic components in the formula. Most if not all mineral only sunscreens are pH ~7, most organic sunscreens are pH 5 from my experience.

    Had a Phd in dermatology help me do some digging, and CIR has an answer to this question based on a few small studies:

    The Expert Panel compared the increase in the number of SBCs associated with AHA pretreatment to SBCs produced as a function of increased UV exposure alone. AHA pretreatment caused less of an increase than did raising the UV exposure to 1.56 MED. The increase in UVR damage associated with AHA pretreatment was of such a magnitude that it is easily conceivable that aspects of cosmetic product formulation could eliminate the effect. For example, inclusion of a sunscreen with an SPF of 2 would eliminate the effect. Likewise, addition of color additives or vehicles that produce even a small increase in UVR reflectance would eliminate the effect.

    CIR 2013 "Safety Assessment of Alpha Hydroxy Acids as Used in Cosmetics" http://www.cir-safety.org/sites/default/files/ahas.pdf

    It should be fine, but there might be a decrease in SPF factor which you'd have to to in vivo tests to find.




  • ZinkZink Member
    edited February 2017
    Reconsidering i'm going to go with a lower amount, will test 2% l-lactic acid for a slight reduction in pH (the original pH 7 is fine and optimal for ZnO stability to avoid the potential for excessive ZnO ionization).

    At 7% you could still feel a slight at pH 5 - not what you want. Now what are good options for gentle preservatives happy at pH 6-7?
  • Bill_TogeBill_Toge Member, Professional Chemist
    it's unlikely to be caused by zinc ions in solution... zinc ions are Lewis acids, even uncoated zinc oxide is insoluble in neutral water, and a significant amount of multiply-charged ions in solution will have a major impact on emulsion stability

    what's more likely is that the acid is dissolving the zinc oxide and being consumed in the process; this would also reduce its efficacy as a sunscreen
    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 AFAIK the reduced efficacy has only been demonstrated in vivo, so it's possible that the physical in vitro SPF rating stays the same but that the addition of the acid does sensitize to some extent making erythema more likely and hence giving lower in vivo SPF results. Or maybe it's a bit of both. 
  • johnbjohnb Member, Professional Chemist
    But isn't the in vivo result the important aspect?

    Regarding the preservative, check what your quoted Elta MD uses: Phenoxyethanol, Iodopropynyl Butylcarbamate
  • @john certainly important for your SPF rating. it really depends what's important to you.

    Phenoxyethanol and Ethylhexylglycerin is also commonly used. 
  • johnbjohnb Member, Professional Chemist
    edited February 2017
    Quote:
    it really depends what's important to you.

    The most important part is if the product does what it is supposed to do. You can do as many in vitro tests as you like but if you don't get the performance in vivo you may as well give up.

    By the way, I am aware that Phenoxyethanol and Ethylhexylglycerin is also commonly used. What I was doing is offering an answer to your question posed  earlier and your reference to the Elta MD products:
    Quote:
    Now what are good options for gentle preservatives happy at pH 6-7?
    My reply being:
    Quote:
    check what your quoted Elta MD uses: Phenoxyethanol, Iodopropynyl Butylcarbamate
    One final point, you claim to have had two PhD graduates helping you in this (one a chemist, one a dermatologist). Fine, but please don't use them as ammunition against any replies you may get here. I have a PhD and I have spent most of my working life amongst PhD graduates. I can tell you that some of the biggest fools I could ever imagine have been amongst those people.
  • @johnb think you're reading a bit too much into my response. I was grateful for your reply, let me elaborate.

    "It depends what's important to you"; different companies and formulators have different goals with formulas. Some are OK with lower SPF ratings, others are not. Also in some markets in vitro performance may be paramount and a better benchmark of actual UV protection knowing that in vivo tests can be manipulated with anti inflammatories. Personally I'd really like to get SPF25-30 out of 10% ZnO if possible, but I'm not concerned getting it higher than that.

    Thanks for pointing out what Elta MD uses. For the other people reading this thread i also pointed out that Phenoxyethanol and Ethylhexylglycerin is commonly used.

    I have one Phd derm graduate working with me, the other link was BY a Phd chemist and a good blog post exploration, of course, having a degree in itself doesn't necessarily mean anything, but the post holds its own imo. Personally I decided to get out of academia after a measly BSc ;)


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