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Estimation of Water Activity in W/O emulsions
Is there a method to estimate water activity?
I have seen the charts with water activity (Aw) for various cosmetic and food products. But would it be possible to design a W/O emulsion with 10% water and a Aw of 0.75?
If I knew the Aw of normal saline and seawater can I use this to estimate the decrease in Aw salt would provide at a given concentration? I think this can be done for an aqueous saline solution, but what about saline solution as part of a W/O emulsion?
Would a saturated sodium chloride solution Aw=0.75 in a W/O emulsion yield an emulsion with Aw of 0.75?
If you also add other chemicals that lower Aw (glycerin or salt) can you estimate the cumulative effect?
Thanks for your help!
I think I posted this is the wrong place but cannot figure out how to move it! Sorry!
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18 Replies
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Aw can’t be estimated but must be shown via testing. This has been discussed by David Steinberg at length in the past at several SCC Meetings.
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I understand it must be empirically measured to get the exact Aw, however, it seems reasonable that you can there must be some source where you can make an educated guess?
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perhaps I should not have said calculate?
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David Steinberg was quite emphatic that direct measurement is the only valid methodology. https://www.google.com/url?safe=active&sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjWxpXj5tzrAhXuoXIEHVf2C68QFjACegQIAhAB&url=http%3A%2F%2Fwww.midwestscc.org%2Fblog2%2Fwp-content%2Fuploads%2Fpresentations%2FJan2012CurrentTrendsinCosmeticPreservation.pdf&usg=AOvVaw3zOw_Ft9JkqtWTKfcU5xiq
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Hi @Microformulation,
Thanks for the link. I will have a look.Maybe I am misunderstanding. If you have a paper that measures the Aw empirically of a specific recipe, and that formulation has no glycerol, for example. It seems reasonable to assume the addition of glycerol will decrease the Aw value?
My reason for asking is to assess risk before the actual Aw is measured. If a formulation has a lower suspected Aw then you may be able to do a short challenge test.
Very often tables are used and values assumed for specific types of pharmaceuticals and cosmetics. Below is from Published in AAPS PharmSciTech 2017
Microbial Stability of Pharmaceutical and Cosmetic ProductsH. Dao, P. Lakhani, +6 authors S. Narasimha Murthy
It would be nice to have a more accurate estimate of Aw based on formulation ingredients (in the absence of empirical results) than using an average value.
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You can estimate but that gets difficult in mixtures, you can calculate but that gets difficult with less common compounds (which includes most water soluble cosmetic ingredients), and you can use programs to calculate for you (annual licence fees are likely in the 10-20 K).There is an easy way to do this and all you need is a hygrometer, a tightly sealed container to fit in an open jar of cream and the hygrometer, and a days time
. Relative humidity in % divided by 100 = water activity. Simple! -
Thanks @Microformulation
I have seen the Cosmetics and Toiletries article. Looking for something more detailed.PS I see Albany College of Pharmacy in your signature. Are you from upstate NY?
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Yes, I am. I am from a town about 35 miles south of Albany. Catskill New York and the NYS Thruway was 1/2 a mile behind my house. For more detailed information you may want to look closer at Phil Geis’s book (New Edition released) Cosmetic Microbiology: A Practical Approach, David Steinberg’s Book “Preservative for Cosmetics” or look for information in Food Science. In my experience, Aw has more use in that sector.
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I spent my favourite summer in Windham NY with my grandparents. Our house in Iowa was hit by a tornado and I went to NY while it was being rebuilt. Absolutely beautiful area. What town are you from?
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Sorry, you are from Catskill. I misread.
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In HS I worked landscaping and we did a lot of work in Windham. It is just up the mountain a bit from us. Interestingly enough, I have a colleague in Iowa whose home was just destroyed by a tornado. They call it a dericho. She is another Cosmetic Chemist. By the way, I really enjoy your posts on your teaching aids. We need more Science education at every level.
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Such a small world. Sorry for your friend. I hope she is ok. I don’t know if our tornado was associated with a dericho, but it was an F4 and got us out of school for a while!
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Dr_Sara said:…
Sadly, I don’t have 20K laying about- unless you mean Euros?
Can you point me to a good source to read about calculating the Aw?
…No, not €… and me neither ;( .Problem is, you’re going to find, as @Microformulation said, some values (mostly defined as salinity) in the food sector and others in horticulture but you’re neither going to find all you need nor anything reliable regarding mixtures (which, indeed, can differ from theoretical blends). Furthermore, you can’t be sure that the published salinity values are really the ones used to calculate water activity. You’d be faster making that product and putting it into a large jar to get the actual measurement. I wouldn’t be astonished if ‘making it’ also involved ordering the ingredients first. Seriously, I’ve tried and gave up! I usually don’t give up unless it’s looking real bad. -
water activity is related to the amount of water present, but it’s a function of molecules rather than % w/w; as many cosmetic materials are complicated mixtures with ill-defined molecular weights, the simplest way to gauge it by far is to measure it directly
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Pharma said:Seriously, I’ve tried and gave up! I usually don’t give up unless it’s looking real bad.
@Pharma if you gave up then I do not think it is worth pursuing these calculations for my own edification!
Bill_Toge said:the simplest way to gauge it by far is to measure it directlyThanks, @Bill_Toge. I think I am getting the message!
Maybe I should buy a hygrometer?
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