Monday, May 20, 2013

(Long) Weekend Wonderings: Altering the pH of our products, freezing fragrance oils, and using butylene glycol as a co-preservative

In the Weekend Wonderings comment post, Liz asks: First off I have to say your blog is so informative- you are my bath and body hero! And I have a question for you, with summer coming up I wanted to make a sunless tanning lotion. I bought some Dihydroxyacetone and Erythrulose from Making Cosmetics. It says not to use alpha-hydroxy acids with the dihydroxyacetone but the "final product should be in the pH range between 3.5 and 5". I'm trying to figure out what to use to lower the ph. White vinegar? I don't really want to smell like that if possible. Thanks so much with sharing your knowledge so freely with us!

The first thing I suggest is to make sure you have a good way of measuring pH in our products. I'm not a fan of using pH strips as they aren't really that accurate, so if you plan to make products of this nature, you might want to invest in a pH meter. I love my meter, but there are many of them to choose from from our lovely suppliers, and I'm sure you'll find one that you can afford and love.

So how to lower the pH of our products? I don't think I'd want to use vinegar because it's not predictable how much acetic acid it contains - every type of vinegar is different - plus there's a smell involved! Citric acid is a very effective way to reduce pH - I've found that 0.2% in my body washes can reduce the pH up to 1 point, making it more acidic. Lactic acid is another way to reduce it, although I have no suggestions for usage.

Click here to see how I use citric acid
Click here to see how I used it in a body wash!

Having said all of this, some people love their strips, so perhaps they'll comment and let us know how they make them work well for them! And any suggestions for reducing pH that don't include AHAs? 

Related posts: 
Chemistry of our skin: pH and the acid mantle (scroll to the middle for pH adjusting)
Chemistry of our skin: pH and our skin care products (updated for 2012)

I'll be writing more about pH in a few weeks as I resume the Chemistry Thursday series! There are so many misconceptions about acids, bases, and pH that I thought it would make for an interesting series of posts!

In the same post, Yvonne writes: I have a question about fragrance oils. I have so many of them which I don't use up before the year is up as I only make products for myself (the supplier told me they are only good for 1 year). Could I freeze them like I do my carrier oils?


Related post: Heating, holding, freezing, and thawing our ingredients

In this post, melian asks: I have a question for weekend wonderings. On the Dish (forum) at one time the statement was made: "Butylene glycol may act as an additional preservative in your lotions." Though I keep it in my notes, I didn't keep track of who said it, so I can't go back to them. Is this true? I've already done a search on the blog and read everything about butylene glycol.

I'm not sure about butylene glycol, but I know that glycerin can be considered self preserving by the FDA, "Some cosmetics, i.e., those containing more than about 10% ethanol, propylene glycol, glycerol, etc., and cosmetics in self-pressurized containers, are self-preserving and are not likely to become contaminated with microorganisms." (Original post: Can glycerin act as a preservative?)

I've also seen 15% to 20% suggested (Dweck) or 50% suggested in this discussion thread in the Chemist's Corner forum. I could post references all day, but suffice it to say that a certain amount of glycerin can behave as a preservative for our products.

How would this work? In a product, the water is bound to other molecules (say, Epsom salts) and isn't free for usage by the microbes. In other cases, the water is bound by humectants like sorbitol or glycerin (anywhere from 10% to 20% will bind water). So water activity is actually a measure of the amount of free (unbound or active) water molecules present in our products. Water activity increases or decreases with with increases or decreases in pressure and temperature. pH also plays a role.

So why am I talking about glycerin instead of butylene glycol? Because they are all poly-alcohols or polyols, and these ingredients bind to water molecules in the product, which is how they could work as preservatives.

Having said this, I wouldn't trust these polyols as the only preservative in a product because figuring out the water activity isn't as simple as adding 10% or 20% of something to a product - click here to see the equations and information. Besides, who wants 10% to 50% humectant in a product? It would feel really sticky on your skin. (I do have a 25% glycerin foot cream I make when my feet are really trashed, but I need to wear socks and I'm covered in fluff in the morning!) I think using them can offer a boost in preservation (and it reduces the freezing point of your product, which is great if you're shipping your product in the winter or storing it on your workshop in colder months).

Have a question? Hie thyself over to the Weekend Wonderings comment post and share your thoughts. I go through all the comments you make every week on this blog, but I check there first! 


melian1 said...

thx swift for the info on butylene glycol and glycerin. i have no intention of using it as a preservative, tho! cosmetic chemistry is complex and interesting.

Liz said...

Hi, Liz again, of the ph question. I usually use citric acid to lower ph, but I thought it was an AHA? Am I wrong?

Susan Barclay-Nichols said...

Hi melian! It is fascinating, which is why I love questions like yours!

Hi Liz! Can you clarify your question? Are you asking if citric acid is an AHA? If so, no. I don't think so.

Yvonne said...

Hi Susan,
Thank you so much for answering my question about freezing fragrance oils. I tried freezing my fragrance oils and most of them will not freeze (only 2 did). Do you have any idea why? Do you think that just by having them in the freezer it will extend the shelf life even though they are not frozen? Also do you know why hemp seed oil and walnut oil do not freeze? All the rest of my carrier oils freeze well. Just in case anyone is wondering, my freezer works fine everything else freezes no problem.

Thanks for your time Susan. You are awesome!!!

Take care,

Liz said...

Hi Susan- sorry to not be clear before. Wikipedia is telling me the most common AHAs are- glycolic acid, lactic acid, citric acid and mandelic acid. Is that not right? Wouldn't surprise me. Maybe go with the vinegar to lower ph just to be on the safe side- unless you can think of another suggestion? Crossing my fingers...

Ray said...

AHA is a term used to describe the position of the hydroxy group on the molecule. Why would this make you not want to use it to lower pH? It doesn't affect its ability to lower the pH of the system and is commonly used for this purpose. Plus anhydrous citric acid is purely that, while vinegar is a mix containing acetic acid as well as other salts, etc.

Liz said...

Hi Ray- the ingredients I bought for making a sunless tanner aren't supposed to be mixed with AHAs, but the final products ph needs to be between 3.5-5 ph. So it needs to be lowered without using them.

Alexis said...


Making Cosmetics has three formulas for sunless tanners on their site and in each formula they state to use citric acid to lower the pH.

Something to always keep in mind when reading recommendations for ingredient usage is that you really need to determine who they are writing it for and how the writer is using the word or words you may have in question. Two main categories for word usage are chemical/science speak and common/marketing speak. Problem is you have to figure out which category is correct by reading the site and doing some investigating. In the particular case of
dihydroxyacetone, when it states not to use with AHA's, they mean the more common/marketing cosmetic speak, like AHA Fruit Acids, glycolic acid and the like at percentages that would qualify the product to be classified as exfoliating. Citric Acid is technically, from a chemistry point of view, an AHA, but using it cosmetically as an AHA (exfoliating) ingredient is difficult because it is very irritating when used as the sole AHA in an exfoliating product.

Assuming the formulas given by Making Cosmetics are good, using a small amount of citric acid (0.2% - 0.5% maybe) to lower the pH of your sunless tanner should be fine. Having said that, please keep in mind that my recommendation is nothing more than an assumption based on the information I found on Making Cosmetics site and I have not used the ingredient in question.

Anonymous said...

Hi there Susan, Thanks for the info. I am making a lotion with 5% zinc oxide and 10% willow bark extract and the PH came out to 7.5! I want to lower it to between 4.5 and 5 and just wanted to make sure if I do this with citric acid the acidity wont make the zinc oxide ineffective will it? Is it ok to have a lotion with zinc at a PH of 4.5? I tested 5% zinc in water has a PH of 8.5 so I dont know if its ok to lower it THAT much...
Thanks again! This post (and your entire blog) are incredibly helpful, thank you so much!

Your fan, Stephany