Friday, June 28, 2013

Chemistry Friday: Why do we care about mixing and solubility?

I've shown you this picture many times before, but I present it again as an illustration for this post!

Both bottles contain the same liquid - my toner recipe - but I added a ton of extracts to bottle 2. When I made the product and added the extracts at cool down (below 45˚C), it looked great. But when it cooled to room temperature (around 20˚C or 70˚F-ish), I saw this mass of stuff at the bottom of the mister. That's a precipitate! And as I mentioned in part one of this post - why don't oil and water mix? - it's all about the solubility.

There are a few reasons we care about solubility...
  • We don't want to use more ingredient than will be soluble in our product and end up with precipitate. 
  • We want to know with which solvents - oil or water - our ingredients will work and into which phase we should add them. 
  • We want to know how temperature affects solubility and homogeneity of our products, for instance, with lotions being emulsified well. 
  • We also like to know why the world works the way it does, and why oil and water won't mix without an emulsifier! 
If you read a data bulletin or listing at a supplier for an ingredient, you'll see a list of things with which it will mix and with which it won't. You might see listings for water, alcohol, perhaps other polar solvents, and oil.

In this data bulletin for Amaze XT, it reports that "AMAZE XT polymer is soluble in water and ethanol (< 30%)." What this means is that the ingredient is water and alcohol soluble. 

Or look at this data bulletin for Cosmocil CQ, which notes its solubility as "Soluble in water, ethanol, glycerine and propylene glycol. Insoluble in hydrocarbons, vegetable and animal oils, and aromatic solvents." Again, this ingredient is water soluble. 

You might also see the solubility for an ingredient listed (although I couldn't find a single example for this post!). For instance, many of my powdered extracts are listed (somewhere!) at 0.5% at 25˚C in water. Which means that I can use 0.5 grams of something like rosemary extract in 100 grams of water and it will stay dissolved at 25˚C. If I use more, it will likely sink to the bottom as a precipitate.

The suggested usage rate of an ingredient could be based upon the solubility or it could be based on another thing, such as safe as used rates, tested rates of efficacy, or even how badly something might smell in a product. Always stay in the suggested usage range of your ingredient! 

You know what I noticed in researching this post? Very few suppliers have suggested usage information and solubility on their pages. They don't mention if the ingredient is heat sensitive or not, so we're left trying to figure out how to use the ingredient. That seems ridiculous to me! 

Solubility tends to go up as temperature goes up for liquids and solids. When I mixed in 1 gram or 1% powdered extract into the toner at 45˚C (cool down), it dissolved nicely, but when the mixture reached room temperature, the extract precipitated or solidified out of the mixture. If I heat it up again, it will return to its lovely dissolved state. There isn't a rule for how much more you can incorporate into a product at a higher temperature - those solubilities are figured out experimentally.

As an aside, it tends to go up for gases in lower temperatures. That's why we put pop in the fridge! The solubility of carbon dioxide goes up as the temperature goes down, so cold pop is fizzier than warm pop!

And as a further aside, this is why we wash our hands, clothes, dishes, and stuff in warmer water because it means the solubility of the stuff on the plates, clothes, and hands increases and washing becomes easier!

Allantoin has a solubility of 0.57 g/100 mL (25 °C) or 4.0 g/100 mL (75 °C). This means I can get 4.0 grams of allantoin to dissolve in 100 ml of 75˚C water (a little warmer than our heated water phase), but only 0.57 grams will remain dissolved at room temperature. So 3.43 grams of allantoin will end up at the bottom of the container as a precipitate. And allantoin can be like little shards on your skin! 

If you see something listed as mixing in water and having a solubility in water of 1% at 25˚C, you know you should only use maximum 1 gram in 100 grams of product in the water phase. If you see something listed as having a solubility of 10% in oil, you know that you should use maximum 10 grams in 100 grams of product and it should go into the oil phase. How to know if it should go into the heated or cool down phase? Check out this post on that topic, as well as this post on how to know how much to add (that is about solubility and safe usage rates). That depends upon heat sensitivity, not solubility.

What does this mean for adding a bit of this and a bit of that? Will the solubility of one ingredient affect another? Possibly. Again, these things are determined experimentally, so you might have to play around to figure out if using MSM with allantoin will change the solubility of one or the other. It probably won't, but if you include things with a common ion, it can change the solubility big time! 

Things that look mixed well at higher temperatures might fall apart at lower temperatures because the solubility of solids and liquids tend to be higher when the temperature is warmer. This is really evident in lotions - something that looked well emulsified at 70˚C or 45˚C has separated by room temperature. It's amazing what heat and mixing can do - it can make water and oil stay emulsified without emulsifiers for a short period of time, but that mixture won't be stable once we get to room temperature!

This is why heating and holding is so vital! If you can get everything to the right temperature and mix it well, the chemical emulsification doesn't have to do as much work because its solubility in water is higher. If we get it right, it means it will stay stable when it cools down when the solubility goes to a lower percentage.

Isn't chemistry awesome? Join me tomorrow for more fun with stuff and things and...I have no idea what I'm going to write about tomorrow, so we'll both be surprised, eh?


Artemis said...

Hi Susan,
I emailed you last week about oil cleansers and what kind of concentration of emulsifier to use and haven't received a reply yet, so was wondering whether you had received my email?
Thank you!

Susan Barclay-Nichols said...

Hi Artemis. I received it. I've been enjoying some time off with my family, and haven't really looked at my e-mail much.

I think if you did a search on the blog about emulsifiers and solubilizers, you'd find the answer you seek. I've re-created products like this and I've suggested substitutions for the oleates. It really is something you have to make and try yourself to see what works for your skin type with the ingredients you can find easily. I have no idea how this product feels, but you do, so I think it's a better idea for you to try to recreate it rather than having me do it.

The almighty LabRat believed that it was better to send us off on a fishing trip rather than handing us fish, and I learned so much that way! I cursed his name repeatedly at first as I couldn't understand why he wouldn't share the information with us, but I came to love the journey as much as the end result. I'm trying to follow his tradition. I really encourage you to go on a search on the blog to see what you can find because I think you'll learn more that way. Then get into the workshop and try what you've found. You will make some mistakes, but you'll learn so much.

As I wrote in a previous post...Terry Pratchett wrote, "Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life." Resolve to be on fire!

This isn't to say that I won't help out, but rather than waiting for me to wade through the 2395 unread e-mail messages I have in my inbox, it might be a better use of your time to see what you can find on the blog!

Let me know what you find and what you eventually make!

Artemis said...

Ok, sorry to bother you. I will experiment and let you know when I get it right!

Clare said...

Hi Susan,

I'm an occasional reader though I'm trying to read more regularly as your explanations are so useful. I'm a complete newbie to this, so far I've only made lip balm (semi-successful, more practice required!) but I dream of making The Ultimate Moisturiser for my grumpy pernickety skin.

I'd like to ask about this post. Everywhere says that allantoin can be used up to 2%, but if the solubility at room temperature (and all products are going to end up at room temperature) is only 0.57%, surely it's impossible to use more than that? Why is the usage rate always stated as up to 2%? How do we know that other extracts or actives are soluble up to their maximum usage rate? I don't want to spend lots of money on lovely ingredients only to waste them if they won't dissolve at the recommended usage. Can you help explain?

Thanks very much

Susan Barclay-Nichols said...

Hi Clare! I've answered your comment in today's Weekend Wonderings. The short answer is that allantoin is more soluble in things other than water. Check out the post for the longer answer!