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Urea
I have seen example o/w emulsion formulations in which urea is premixed with water. This is an endothermic reaction and appears to be added in a cold process phase. Is this ok? Does anything else need to be considered?
I have also seen such a premix include ethanol. Is the ethanol needed?
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20 Replies
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IF you are formulating with urea, DO NOT premix it, there is no need, it readily dissolves and is a weak electrolyte at best . Always add it directly to the batch during the cooling phase after heating/combining phases in an o/w emulsion for the same reason you state: it dissolves in water endothermically and will cool your batch as it dissolves. No ethanol is needed but a buffer is. Use lactic acid/sodium lactate at pH 5.5 - 6.0 or add rice starch, the more natural means of stabilizing urea.
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@chemicalmatt Why are people so often promoting lactic acid / sodium lactate as a buffer for urea? It’s no longer buffering at that pH range, let alone preventing the product from a rise in pH which is the main issue with urea.
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Pharma said:@chemicalmatt Why are people so often promoting lactic acid / sodium lactate as a buffer for urea? It’s no longer buffering at that pH range, let alone preventing the product from a rise in pH which is the main issue with urea.What are you suggesting as an alternative @Pharma ? Triethyl citrate? Or, none needed? (After lowering pH with lactic acid… 4.8 to 5.2 range generally.)
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chemicalmatt said:IF you are formulating with urea, DO NOT premix it, there is no need, it readily dissolves and is a weak electrolyte at best . Always add it directly to the batch during the cooling phase after heating/combining phases in an o/w emulsion for the same reason you state: it dissolves in water endothermically and will cool your batch as it dissolves. No ethanol is needed but a buffer is. Use lactic acid/sodium lactate at pH 5.5 - 6.0 or add rice starch, the more natural means of stabilizing urea.
I was tracking …. until you got to the rice starch. Could you elaborate on that? Mode of action….purpose…etc. I am interested now…as I have a big pile of it.
(Since all my formulas use a kiss of starch, I will probably switch them all over to rice starch.) I also have a number of formulas using HydroVance. But I have always paired them up with triethyl citrate, and lowered pH with lactic acid…and is some formulas…also include sodium lactate. -
Pharma said:@chemicalmatt Why are people so often promoting lactic acid / sodium lactate as a buffer for urea? It’s no longer buffering at that pH range, let alone preventing the product from a rise in pH which is the main issue with urea.
I agree with you. LA/SL will not help
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Hi all, I am making this mistake apparently. May I please ask why lactate buffer won’t work?
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ngarayeva001 said:Hi all, I am making this mistake apparently. May I please ask why lactate buffer won’t work?Because science
.In simple terms: less than 1% of the lactic acid / lactate at the pH urea is most stable at has to do the job. Officially, that’s not buffering (besides the point that, according to ‘good buffering practice’, a ~10 fold excess of buffer with regard to the agent to buffer whould be used). Sure, only a small amount of urea is degrading at a time and the equilibrium between free acid and it’s salts would theoretically provide a certain flux of new available acid for quite a while.It would work great if decomposition of urea led to a drop in pH… alas, it does the opposite.A good buffer would have a pKa of +/- 1 (+1 would be better) target pH. The problem is that there aren’t too many options in chemistry of which only very very few are okay in cosmetics.That’s why triethyl citrate and triacetin are used as alternatives. -
@Pharma
What about Gluconolactone? I have heard that it could prevent the Urea from the decomposing. -
What keeps urea from decomposing is a proper pH, no matter how you achieve that.If someone wants to add sodium lactate as humectant and lactic acid to adjust pH; perfectly fine. It’s simply not acting as a buffer. If you @ggpetrov want to use gluconolactone for pH adjustment, that’s fine as well. Just keep in mind to fully hydrolyse it before you adjust pH with it or you’ll end up with a too low pH.Cosmetic products might change pH due to several reasons other than urea decomposition: Adding something such as TEC to prevent a rise and a good preservative to present a drop in pH (which is mostly caused by microbial growth) is pretty much all we can do beyond proper pH adjustment and avoiding heat to make sure that urea remains as stable as possible. Once it starts degrading, a buffer would at best slow down decomposition but it wouldn’t prevent bloating (1% degradation of a 20% urea cream would roughly translate to 100 vol-% carbon dioxide formation or a container under enough pressure nobody wants to be near it) and it wouldn’t hamper possible formation of isocyanic acid and cyanate either.Not buffering away ammonia (the other final degradation product besides carbon dioxide and the reason people want to buffer urea solutions) might probably be considered a benefit because it bears the chance of reeking bad enough to keep those customers not already deterred by bloating from using a possibly unhealthy product 😉 .
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That’s super helpful! @Pharma, is there a general rule of thumb of how much urea vs TEC is good to use? Or is it about a pH that needs to be targeted?
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Also, what’s buffering urea in this combo: Aqua (and) Glycerin (and) Trehalose (and) Urea (and) Serine (and) Pentylene Glycol (and) Glyceryl Polyacrylate (and) Algin (and) Caprylyl Glycol (and) Sodium Hyaluronate (and) Pullulan (and) Disodium Phosphate (and) Potassium Phosphate. It’s a BAFS’ formula so I don’t know the %%% (PatcH2O®)
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No, there isn’t any rule of tumbs. Depending on stability of urea in your system, more or less (or none) TEC has to be used.The BASF formulation uses a phosphate buffer. I don’t like phosphate buffers in cosmetics though it’s effective over a broad pH range including urea’s stable zone.
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On my casual glance at numerous mfg formulas….a common inclusion rate is .2 to .3%
As TEC is a nice ingredient, and even quasi emollient, I think you can’t go wrong with it. I have a cream where I use it at an astonishing 5%. (5% rate is not intended as a buffer.)
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Respectfully @Pharma, and I do respect you, your advice is unsound. Lactic acid/sodium lactate at pH 5.0 - 5.5 is by stoichiometric definition a buffer system and HAS been proven to maintain a urea-bearing system at a pH 5.0 - 5.5, the sweet spot for stability minimizing decomposition into ammonia and biuret. I’ve discovered in my research on rice starch that is was shown - as was potato starch - to stabilize urea solutions at that pH too. The research paper mentioned that as a side note not as a breakthrough idea. Upon reading this the little light bulb went off inside my head: another means of stabilizing urea, how cool is that!. Since then I’ve formulated 20% urea creams using the classic lactate buffer approach backed up by 3.0% rice starch and have seen nothing but success. @Graillotion is exhibiting the same romance with triethyl citrate that I have. I think I’ve enabled her and perhaps an intervention may be needed.
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chemicalmatt said:I’ve discovered in my research on rice starch that is was shown - as was potato starch - to stabilize urea solutions at that pH too. The research paper mentioned that as a side note not as a breakthrough idea. Upon reading this the little light bulb went off inside my head: another means of stabilizing urea, how cool is that!. Since then I’ve formulated 20% urea creams using the classic lactate buffer approach backed up by 3.0% rice starch and have seen nothing but success. @Graillotion is exhibiting the same romance with triethyl citrate that I have. I think I’ve enabled her and perhaps an intervention may be needed.
I am not sure where I will end up…in my search for the ‘grail’ of starch type products, and I do like to have only one ingredient per category if possible (except emollients…then things really go off the rails)…but would you say that the distarch phosphates would do the same thing as the native starches, in regard to urea stability?
Oh…and for the record…. Graillotion…is a ‘He’.
🙂 Don’t let my flower breeding put me in a box! My latest creation.
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Rice and potato….those seem to be opposite ends of the particle size spectrum in starches…..so I can conclude…it is not size that matters?
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chemicalmatt said:Respectfully @Pharma, and I do respect you, your advice is unsound. Lactic acid/sodium lactate at pH 5.0 - 5.5 is by stoichiometric definition a buffer system and HAS been proven to maintain a urea-bearing system at a pH 5.0 - 5.5, the sweet spot for stability minimizing decomposition into ammonia and biuret. I’ve discovered in my research on rice starch that is was shown - as was potato starch - to stabilize urea solutions at that pH too. The research paper mentioned that as a side note not as a breakthrough idea. Upon reading this the little light bulb went off inside my head: another means of stabilizing urea, how cool is that!. Since then I’ve formulated 20% urea creams using the classic lactate buffer approach backed up by 3.0% rice starch and have seen nothing but success. @Graillotion is exhibiting the same romance with triethyl citrate that I have. I think I’ve enabled her and perhaps an intervention may be needed.
[Not a pissing contest]I never mentioned a pH 5, I wouldn’t dare saying that lactic acid/lactate isn’t a well established buffering system (what is a stochiometric definition? The two compounds don’t need ‘to be stochiometric’ to behave as a buffer), and I also didn’t mention that it’s not working.According to my sources (which aren’t cosmetic journals but pharmaceutical in nature), urea is most stable at pH ~6. Lactate buffer would indeed work as buffer up to a pH of 5 (and downwards likely even at 5.5 though that’s of no concern here)… however, as I mentioned, it’s not about buffering (for that, the amount added is often not even enough to buffer considerably beyond the self-buffering capacity of a cosmetic emulsion, let alone handle 10%+ urea) but about setting a pH range wherein urea is most stable. A simple no-brainer.I know that lactic acid/lactate is used to adjust pH in urea products and I’ve found patents as well as cosmetic and even scientific publications wherein they used a ‘lactate buffer’ with best results (from what I recall, the alternatives were poorly chosen and failure wasn’t too astonishing). In most of these it was quite obvious that the autors had no clue what they were actually doing. Again, doesn’t mean something won’t work just because one doesn’t know what’s actually happening.BTW biuret is not generated.Regarding rice starch: I’ve looked into that as well and found what others (for example published in DAZ) have found out too. The first published documents showing starch as a means to stabilise urea used starch as adsorbent in anhydrous formulations… not the same as adding starch to an emulsion. Later, others used starch too but didn’t realise that urea won’t stick on starch once water is added (see where this is going?). Nonetheless, I’d appreciate if you could post the DOI of the publication you mentioned. Always keen on learning new things.Urea degradation is not a simple 1+1=2 and small factors can have considerable effects. End of the day, nobody asks why it’s working but only if it’s stable or not. 🙂
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What @Pharma is saying is :
1. ‘Buffer’ does not have to be strong to ‘suck’ up every Ammonia (from Urea) molecule.
2. Just enough buffer to achieve pH +/-6.2. If you want to stabilise a given amount of Urea to the maximum, you will need A LOT of buffer to the degree that you can bathe or bath with it.
3. I am still not clear about why certain acids and their salts do not make good buffers in context of Urea. After reading so much of Pharma’s arguments, I think he has been talking about pKa. He did not say this straightforwardly, it is only my educated guess. If it is about pKa then Citric Acid can make the ‘best’ buffer to stabilise Urea at about pH 6.2 because one of Citric Acid’s pKa is 6.4.
4. Must be ‘tight’. Pharma mentioned he dislikes Phosphate buffer because its pKa is too wide. Not a specialist to stabilising Urea, in other words.
5. Other than pH and pKa, water content or water activity needs to be low. The lower the better. Nothing to do with stopping bacteria/fungus/mould growth. Urea decomposes quicker with more moisture/water. Urea in Petroleum Jelly practically has no issue with weird Ammonia smell. I keep Urea in vacuum bag, never has Ammonia smell.
My conclusion : Citric Acid may not be the best because Carbonic Acid and Tellurous acid have the nearest pKa to 6.2 AND the tightest pKa compared to Citric Acid. Citric Acid is the best only because it is ubiquitous, and it is inexpensive which even poor kids can buy it.
Reference : My conversation with Pharma three years ago. I am not influenced much by marketing, so my ‘moisturiser’ has always been Glycerol, Urea, Xanthan Gum, DMDM Hydantoin, and Sodium Citrate as buffer made in-situ. Pharma is a pharmacist and he concocts his own poisons with food stuff for himself and his wife. I coincidentally use most chemicals from the bakery supply shop.
https://chemistscorner.com/cosmeticsciencetalk/discussion/3122/improving-ureas-stability-in-cream
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Oops! I made a “mistake” in my previous message.
I keep coming back to this Urea stuff because I love Urea. And because I am madly poor. LOL!
It is NOT meant to be a buffer! Not even a buffer. Just pH. As long as the pH is 6.2 will do the trick. Basically, make water or solution slightly acidic.
Buffer, as @Pharma explained is “a lot” of “it”. Too little of it is not qualified as buffer.
Urea is stable at a given pH. No amount of buffer will stop Urea from explosion. (This is assuming no other chemical is messing with pH whether delayed or instantaneous.)
Keyword here is STOP.I live in a tropical country with temperature as high as 30 Centigrade. Sometimes 40 Centigrade. My amateur Urea moisturiser NEVER bloats nor produces that sweet smell of Ammonia.
I did not understand why people in temperate countries worry so much about Urea getting bad when I am not getting (or making) it bad with this kind of weather. It got me worried, so I started to make duplicates of the exact same formulation of my moisturiser and put it in vacuum dispensing bottle (Can visually see the evidence of gas expansion). Some with buffer and some without buffer.
Only one or two bottles expanded, and it took damn many months for it to happen damn little. I was even waiting for it to pop! I did not bother which ones had buffer and which ones had none because it is not ‘statistically significant’. I said to myself “Why bother”, and so I never bother about buffer. Moreover, I do not make a whole 25kg drum or sell it in malls/supermarkets which will wait for a long time to see strange thing happening. I make about 100g of product each time.
With that said, I think it is due to the water is hard in temperate countries which expedites the decomposition of Urea. This could be the reason why people in those countries worry so much about Urea stability. Water over here is soft; pH is about 6. I never need to add any acid to lower pH of any products as what 99.999999% formulations require. And so, it proves that pH that matters in this context, not buffer.
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How would rice starch impact the skin feel of a 10-20 urea face cream? I happen to like the ‘greasy’ effect of urea but reading about its benefits I want to see if it would improve the texture.
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