Pressure build-up in 15% Ascorbic acid emulsion during stability testing

Dears,

We've been experiencing significant pressure build up/bubble formation after 2 weeks of 40 degree stability testing with an emulsion. There is minimal discolouration of the Ascorbic acid at this stage, so we are doubting whether this is related to the oxidation of the ascorbic acid. We cannot seem to solve the problem and would appreciate any thoughts.

The formula includes the following and is cold process.

Oil Phase:
- 2.5% Sepinov EMT 10 (Hydroxyethyl Acrylate/Sodium Acryloyldimethyl Taurate Copolymer)
- 1.2% Sucrose Stearate 
- 0.5% Sepiplus 400 (Polyacrylate-13 & Polyisobutene & Polysorbate 20)
- 5% Ecosilk (Isoamyl Laurate and Isoamyl Cocoate) --> to reduce stickiness of ascorbic acid
- 12% Sweet almond oil
- 1% Tocopherol 

Water Phase:
- 3.5% Dermosoft 1388 Eco
- 2% Sodium PCA
- 2% Glycerin
- 1% Panthenol
- 0.3% Sodium Phytate
- 0.1% Xanthan gum
- Water up to 100%

Post:
- 0.1% Ethylhexylglycerin
- 15% Ascorbic acid
- 5% Lactic acid
- 5% Sodium hydroxide solution (20% Sodium hydroxide solution)

Should point out that this is to have a 6 month expiry date from production as obviously the formula contains water and ascorbic acid. 

Would highly appreciate any thoughts.

Best regards

Comments

  • BelassiBelassi Member
    - 15% Ascorbic acid
    - 5% Lactic acid
    - 5% Sodium hydroxide solution 

    acid + base -> salt + water
    Cosmetic Brand Creation. Concept to name to IMPI search to logo and brand registration. In-house graphic design inc. Pantone specs. Cosmetic label and box design & graphics.
  • PerryPerry Administrator, Professional Chemist
    What's the beginning and ending pH?
  • BelassiBelassi Member
    I suppose I could do the stoichiometry, but that's your job as formulator. Perry's question gets directly there with an empirical method.
    Cosmetic Brand Creation. Concept to name to IMPI search to logo and brand registration. In-house graphic design inc. Pantone specs. Cosmetic label and box design & graphics.
  • PharmaPharma Member, Pharmacist
    ... We've been experiencing significant pressure build up/bubble formation after 2 weeks of 40 degree stability testing with an emulsion. There is minimal discolouration of the Ascorbic acid at this stage, so we are doubting whether this is related to the oxidation of the ascorbic acid...
    You can't necessarily correlate pressure/gas with discolouration. A gas takes a 1'000 times larger volume than a solid. In case of ascorbic acid, this would be hydrogen gas -> H2 is highly flammable and hence easy to test ;) . Though usually, ascorbic acid does not degrade like that but reacts with oxygen to form water or reduces other stuff = no bubbles and no pressure build-up but instead rather an underpressure.
    The main coloured degradation product of ascorbic acid is dehydroascorbic acid but it's not the only possible degradation product. It has just a beige colour and is therefore not always visibly recognized or only at higher concentrations. 0.1% degradation (of your 15%) will not be seen by the naked eye but could, in theory, produce a fair amount of pressure. Neglecting molecular weights, this would increase your product's volume by 15%. Similarly, a probably barely visible degradation of 1% of the added ascorbic acid would turn a 100 ml bottle into a 250 ml balloon.

    What does sometimes happen is degassing (liberation of air dissolved in water). Boiling your water before use helps against that. But that's purely speculative and I don't see anything which would stand out as a bubble-builder... true, knowing your product's pH might prove helpful.
  • PerryPerry Administrator, Professional Chemist
    edited March 3
    The type of packaging might matter too. What type of container are you using?

    I once worked on a VO5 Hairdressing problem  where some lots of tubes  (aluminum) were generating gas and bloating. The exact same formula put in a different tube didn't have the problem. I never did figure out what was causing it but it had something to do with the coating of the tube.  We changed tubes lots and the problem went away.
  • PharmaPharma Member, Pharmacist
    @Perry Aluminium (zinc and some alloys too) + acid = hydrogen gas. The effect may be stronger if antioxidants are present. The acid can be as simple and weak as diluted vinegar.
    Some tube coatings don't completely seal the whole inner surface or are susceptible to certain 'solvents', rendering them permeable enough for chemical reactions.
  • Perry said:
    What's the beginning and ending pH?
    Thank you for all the responses, we will take a good look!

    To respond to Perry's question, initial pH is 3.8, final pH is 3.5 after 4-week stability testing. Though this might be due to the fact that the Ascorbic acid has not properly dissolved yet immediately after production.

    Should point out that the pH of the water phase is very high (pH 9) due to the Sodium phytate.

    The material is glass. We use 15ml transparent glass samples to perform testing at this stage (brown borosilicate glass will be the final packaging).

    Please also find a picture of the sample with the white paper towel used as a colour reference. Note that most discolouration has occurred at the surface, there is less discolouration throughout the emulsion. There is approximately 15% expansion in volume after 4 weeks of stability which is very significant.


  • PharmaPharma Member, Pharmacist
    IMHO if your product was white in the beginning, that beige colour there means it's likely dead and doomed.
    BTW might be microbial contamination/fermentation too. This might also explain the drop in pH.
    Oh, and you shouldn't bottle and stabi-test something that's not yet dissolved to the point it should.
  • Pharma said:
    IMHO if your product was white in the beginning, that beige colour there means it's likely dead and doomed.
    BTW might be microbial contamination/fermentation too. This might also explain the drop in pH.
    Oh, and you shouldn't bottle and stabi-test something that's not yet dissolved to the point it should.
    Good point, will look into that. Btw the picture of the sample was 4-week stability, so the effects are more obvious vs the mentioned 2-weeks and demonstrate the issue better.
  • Hi All, I hope you are all doing well during these crazy times.

    We are still struggling with this formulation issue and cannot seem to solve it. Would love to get your thoughts on this. Here are the findings up until now based on further testing:

    - It does not seem to be related to the packaging in which the samples are stab tested
    - When the Ascorbic Acid is taken out, the problem is resolved, so it is related to the Ascorbic acid
    - The problem already arises without any discolouration of the AA happening, so we highly doubt it is just due to oxidisation.
    - We've also made sure the AA has been completely dissolved, so it does not seem to be related to that
    - We've tested with a Sodium hydroxide solution as well as Sodium citrate solution to adjust pH which also does not seem to impact it
    - We've tested with Phenoxyethanol/Ethylhexylglycerin as an alternative preservative, but actually it seems to make it slightly worse
    - We've tested with two forms of vitamin C, the crystallised form as well as the ultra fine powder form from DSM, both produce the same issue
    - Initial results show - this is not yet confirmed - when we increase the level of sucrose stearate, there seems to be more bubble formation, but it's not the whole problem as without it, it's still occurring

    Any thoughts would be much appreciated. 

    Have a good day!
  • PharmaPharma Member, Pharmacist
    Did you knock-out other ingredients such as tocopherol, lactic acid and most of the water phase?
    Assuming the issue is of chemical and not microbial nature, leaving preservatives and the like out might be okay for figuring out what's going to happen.
    Did you also use plastic packaging?
  • Thanks for your response. We did not leave out Tocopherol or lactic acid yet, do you think it could be interacting with the AA somehow?

    Will also test with knocking out the water phase ingredients. 

    Have not tested with plastic packaging yet. Only two different kinds of glass: clear glass jars and borosilicate amber glass vials.
  • PharmaPharma Member, Pharmacist
    Tocopherol is likely to interact with AA, lactic acid might... something has to, doesn't it?
    Hmm... glass should theoretically be as inert as plastic. Metal jars/tubes would have been my first guess (we've probably covered that already but I've forgotten it).
  • Haven't used any metal jars or tubes, so I guess that's not causing it. Tests are currently running.

    One more thing; I had a suggestion at some point that there might be traces of enzymes in the formula that could cause gas formation. Have you had any experience with that?
  • Bill_TogeBill_Toge Member, Professional Chemist
    was it aerated to start with?
    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
  • PharmaPharma Member, Pharmacist
    Enzymes are present as contaminants in/on most anything. But most do not produce gas, are not stable over time, are inactivated by heat (those which go into the product at 70°C will likely be destroyed), require specific conditions such as electrolytes and pH... some exceptions are enzymes found in laundry detergents which withstand harsh conditions and function under varied conditions but those do not produce gas. Common enzymes I recall which may result in formation of gas are (my brain is still halfway asleep) carbonic anhydratase and cytochromes. The former requires carbonate as educt though this will be gone if you bring your water to a rolling boil for a few minutes because by doing so, it degases (air, including oxygen and CO2 will 'evaporate') whereas cytochromes are sensitive to chelates such as phytate.
    Bottom line is: It's more likely that microbes are responsible.

    What I've noticed during my PhD is that oxidation (of ascorbyl palmitate but also certain additives used for enzymatic assays) is somewhat unpredictable. Simply changing a detergent/emulsifier or the buffer whilst maintaining the same pH and overall composition greatly impacts oxidation reactions. Also, presence of air (basically oxygen and CO2) in the medium is detrimental.
  • Thanks for you reply! You've been incredibly helpful.

    Very interesting. I should point out, we don't heat the formula whatsoever, it's cold-process. So perhaps that still opens up the enzyme option? Interestingly, there is only gas formation in the presence of Ascorbic Acid, so it must be that if there is enzymatic activity, it's mainly reacting with the AA.

    The reason I raise the enzyme option is also because of the phenomenon that the gas formation increases as we increase the sucrose stearate. And perhaps the sucrose acts as a source for the enzymes?

    Regarding your second point, are there any other emulsifiers you would recommend? Or in general, is there any additive that could enhance the stability of the AA (in an O/W emulsion) that we've not included?
  • Bill_Toge said:
    was it aerated to start with?
    Hi Bill! There is slight aeration at the start, but few very small bubbles. Then it seems new bubbles are formed building up significant pressure within a short period of stability testing. This is only the case with the AA contained, not excluding the AA (with exclusion of AA, there are also bubbles at the start, and no gas formation during stab).
  • PharmaPharma Member, Pharmacist
    Cold process, okay. Still, chances that you could blame enzymes is still very low whereas chances that it's microbial are higher.
    I'd try a test batch with boiled water. Simply boil it for a few minutes, fill it in a container up to the brim and close it as good as possible (to avoid contact with air), let cool and use that water directly without introducing too much air in the process. Hence, the final product shouldn't contain much oxygen. If you still have the very same effect (zero effect is unlikely, oxygen is everywhere)... then I'm like ??? Maybe try adding metabisulfite to quench oxygen?
    Sucrose stearate is a surfactant, it can speed up chemical reactions (lower surface tension equals better contact between molecules and it helps tocopherol to get in touch with AA*) and it also stabilises bubbles (instead of 'evaporating', gas gets trapped as visible bubble).
    *Tocopherol is known to act as oxidation booster under certain conditions and it regenerates AA if in contact with it (because one is in water, the other in oil, only an emulsifier brings them together). This means that one AA molecule might redox-cycle several times instead of being a one way ticket or, the other way round, it activates tocopherol better/more often so that tocopherol can do damage to whatever compound it does damage to. Given the respective % of the two, the latter option seems more likely but I can't see how tocopherol could produce gas.
    Unfortunately, the effect of emulsifiers on oxidation stability is not predictable, there is no good or bad, only trial and error will tell.
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