Tuesday, March 9, 2010

Fun with chemistry: Solubility

The cold and headache are taking their toll on me, so please enjoy this post on solubility before I start a series on skin types and formulating creations!

Solubility is a pretty important part of making bath & body products, but we don't talk about it much. In essence, solubility is the ability of a solid, liquid, or gas (the solute) to dissolve in a liquid solvent to create a homogeneous solution (homogenous meaning a mixture that shows no variation in properties, like Kool-Aid dissolved in water - you don't see big particles in there).

The solubility of the solute depends on the type of solvent (water, oil, alcohol, and so on), the temperature, and the pressure. (There is a concept called the STP or SATP, which stands for standard temperature and pressure. I was taught it was 25˚C and 1 atm or or the pressure at sea level, but it can vary depending upon the textbook and instructor.)

If something is soluble in water, the solubility generally goes up when you increase the temperature and generally goes down when you reduce the temperature. Something like salt dissolves readily in cold water, but dissolves better and faster in warm or hot water. This is one of the reasons we dissolve extracts or allantoin or other solid things in warm water or add it to the heated water phase - it will dissolve or mix easier and create a nice homogeneous mixture.

If two things are immiscible, it means they will not mix or dissolve well (like oil and water without an emulsifier). Some things are very soluble - salts, for instance - and some things are kinda soluble, meaning they aren't completely soluble or need some help to be soluble, like an increase in heat or some serious stirring. If we add too much of a very soluble thing or take something that isn't completely soluble and add more of it or fail to raise the temperature, we get a precipitate, which is a solid of the solute that can fall to the bottom of our creation. We see this kind of thing when adding too much salt to water (the stuff on the bottom is the precipitate).

In this toner, I dissolved honeysuckle and green tea extract at 0.5% each. If I put too much of this extract in or used the wrong kind of solvent (oil, for instance), the extracts would fall to the bottom and form a gooey mess, which would be the precipitate. As you can see, there is no gooey mess! And yes "kinda soluble" is a science-y term!

If you've made a toner and found a precipitate at the bottom that requires you to shake it before using, or if you've made a lotion in which your allantoin or MSM crystallized, you've either added too much or added it at the wrong temperature or used a solvent that won't dissolve the compounds. (Try mixing shea butter in alcohol - won't work! Now dissolve it in oil! Success!)

A rule of thumb is "like dissolves like". Water dissolves water soluble things; oil dissolves oil soluble things. Water is polar; oil is non-polar. (If you want to learn more about this topic, I suggest this wiki page for a great place to start!)

So why am I sharing all of this chemistry fun with you on such a lovely March day? Because there are good reasons for suggesting 0.5% of an extract other than the effect it might have on your skin or bank account. When we're formulating, we need to take into account the possibility that our lovely extracts or additions might precipitate out and cause an icky mess on the bottom of our bottles, which is not very pretty and a waste! Many manufacturer's data sheets or bulletins on ingredients include the solubility - generally expressed by moles of the ingredient - so you can see if how much you should add.

There's a cute java application showing the solubility of salt here. It's fun to keep increasing the moles until you see the precipitate! (Or you can try this at home with a glass of water and some salt, or in your bath with some Epsom salts!)


Li said...

When using allantoin in a lotion or cream, can I add it to the water phase before heating it up?

Susan Barclay-Nichols said...

No, I wouldn't. I would heat up the water phase, then sprinkle it over the top and mix it until it is well dissolved. This ensures it will dissolve quickly and uniformly. Adding it when the water phase is cold might dissolve it a bit, but you're still left with bits than haven't dissolved.

Li said...

Thanks, Susan! I thought it would dissolve as the water heats up.

Lalla said...

I have two questions.

If the solubility is 0,5% at 25°C, does it mean if we dissolve a higher concetration at say 35°C the allantoin will cristallize as it cools down?

If I try to dissolve MSM and allantoin and caffeine in water, will their respective solubility decrease because of the other ingredients?

Anonymous said...

I have a question, if you heat up water at approximately 360 degrease, how soluble would it be?

Susan Barclay-Nichols said...

Hi Anonymous. (Please use your name in your comment or I will have to delete it. Those are the rules around here!) If you heat the water to 360˚ C or F? And what would be soluble? (You can't heat water that hot. It's called super heated and it's really dangerous.)

Anna said...

Hi Susan,

When shopping for ingredients I get confused about the solubility of things. Let´s say a distributor lists that a certain ingredient is soluble in propylene glycol and ethanol. My immediate thought is that it also ought to be soluble in other substances similar to propylene glycol and ethanol, such as butylene glycol. But honestly, that is just me guessing because the name is so similar.

Could you help me figuring this out? Are there a few catergorys that these substances fit in to? I see you mentioning oil and water, which doesn´t confuse me at all. But what is propylene glycol? Is it an alcohol?

This gets really frustrating when I would like to buy an ingredient like Dimethyl Isosorbide, which is listed as a solvent, but I have absolutely no idea what I can dissolve in to it since I don´t know what "category" it belongs to. Could I use it to dissolve the example-ingredient from above instead of using propylene glycol or ethanol? I have absolutely no idea!

Hoping you can help me wrap my head around this!

Thanks for sharing this amazing well of knowledge of yours!