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Can I Thin Out a W/O Emulsion by Adding an O/W Emulsifier?
Hi,
I’m developing a ZnO (20%) moisturizing sunscreen, with ~33% oils and ~19% water (final formula concentrations pending). My emulsifiers are Glyceryl Stearate (3%) and Sorbitan Stearate (1%), with some thickeners/stabilizers at low concentrations (e.g. Xanthan, 0.1%).
The emulsion is stable and spreads easily enough, however the product is supposed to be dispensable through a pump. As it currently stands, the lotion is more like a butter that is too viscous to be poured.
I’ve seen that some similar (W/O) products on the market have low concentrations of emulsifiers typically used for O/W emulsions. Is this out of ignorance, or does it help decrease viscosity without destabilizing the emulsion? I have been thinking of experimenting with Glyceryl Stearate Citrate, but I do not know if it would be a waste of funds (nor how much to start out using).
One other thing to consider: We are sticking with natural/biodegradable ingredients, so some of the traditional solutions aren’t available to us.
If not the emulsion, do you have any other suggestions to decrease viscosity?
Thanks,
AD -
7 Replies
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I am a total novice, but I think if you provide a few more details, you are likely to get more informative answers:
-1- Any particular reason why you are using such a high concentration of oils? I would imagine that you could accomplish your goal of being truly moisturizing with far less in the way of oil.
-2- Please confirm if you are using any butters, or only oils liquid at room temperature. Some oils are more viscous than others, so if you insist on keeping the concentration of oil as is, perhaps you can switch to different oils to help accomplish your goal?
-3- I am curious as to how you settled on 20% ZO. It is true that this is used by a lot of formulators, but what makes you so sure that you need to use such a high concentration in order to achieve a good UVA and UVB protection level? -
@adonason Mixing both O/W and W/O emulsifiers would make the emulsion more stable, which usually translates in more viscosity, which is the contrary to what you want to achieve.
As @suswang8 commented, the high viscosity might be due to the use of butters (or waxes), or high amounts of powders (ZnO).
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You need to post the entire formula and a process. Right now it’s impossible to say anything or give you constructive advice. Several points to mention:
1) In general W/Os do not “like” high oil phase (yes sounds counterintuitive but true)
2) Generally speaking, the smaller droplets the thinner is w/o emulsion, so apply high shear and keep oil phase above 25% (which you already did) so that droplets are round
3) ZnO w/o sunscreens are extremely hard to stabilise, are you using a lot of waxes? Maybe that’s the reason why it’s thick (although if you reduce it might separate)
4) As a caveat, I am not an expert in sunscreens by any means, but based on my knowledge gathered from webinars and some suppliers info your product might not have the SPF you want.Croda’s coated zinc oxide CZ-300 gives an SPF 1 per 1% of zinc oxide (just as an example). Assuming you are using treated and properly dispersed ZnO, and follow all other rules, your SPF is still low (unless you have other sun blockers).
I know it’s not absolutely precise but run your formula through BASF simulator (assume z-cote as your zinc). I have a suspicion will hit not more than 15 with 20% of Zn alone. -
ketchito said:@adonason Mixing both O/W and W/O emulsifiers would make the emulsion more stable, which usually translates in more viscosity, which is the contrary to what you want to achieve.that only applies to O/W emulsions; it’s not possible for W/O emulsions to form with O/W emulsifiers present
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Bill_Toge said:ketchito said:@adonason Mixing both O/W and W/O emulsifiers would make the emulsion more stable, which usually translates in more viscosity, which is the contrary to what you want to achieve.
that only applies to O/W emulsions; it’s not possible for W/O emulsions to form with O/W emulsifiers present
@Bill_Toge You’re correct, I should’ve been be more precise about my comment. Thanks for bringing that up.
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Bill_Toge said:ketchito said:@adonason Mixing both O/W and W/O emulsifiers would make the emulsion more stable, which usually translates in more viscosity, which is the contrary to what you want to achieve.
that only applies to O/W emulsions; it’s not possible for W/O emulsions to form with O/W emulsifiers present
Thanks for the comments, you two.
That’s interesting, do you know why that is? Do the hydrophillic components of the emulsifiers bind to each other instead of any water present?
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@adonason the long and short of it is that thermodynamics favour O/W emulsions, because water has a much stronger tendency to aggregate (due to hydrogen bonding between water molecules) than oil does; W/O emulsions can only be formed when this is minimised
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