Thursday, June 27, 2013

Chemistry Thursday: Why oil and water don't mix or more on solublity

Mixing two things is all about solubility. If we say that something is water soluble, it means that it will mix or dissolve in water. If we say something is oil soluble, it means it will mix or dissolve in oil. If we try to mix an oil soluble thing in water, it will not mix, and we say it is immiscible. Why don't oil and water mix?

Solute: The thing that is dissolved. It can be a solid, liquid, or gas. We'll only be discussing solids and liquids as gases aren't really used that much in homecrafting cosmetic chemistry. 

Solvent: The liquid into which that thing is dissolved. It could be water, alcohol, oil, and so on. A gas or solid could be the solvent, but we won't be addressing this on this blog. 

Soluble: When the solute dissolves or mixes easily into the solvent. There is a limit as to how much will mix in the solvent, and that's called solubility. Solubility is dependent upon temperature and amount. We can increase solubility by increasing the amount, but when the temperature goes down, the solubility will also decrease (most of the time). 

Insoluble: When the solute won't dissolve or mix easily into a solvent. 

Precipitate: The solid left over when something dissolves. It can also mean a solid that forms in a mixture when we've used too much of it. (See the picture of toner with the layer of stuff at the bottom!)

Miscible: Micibility is defined as "the property of liquids to mix in all proportions, forming a homogeneous solution." If we say aloe vera is miscible in water, we are saying that it will mix in water. If we say something is immiscible, it means it won't mix in the solvent. So oil is immiscible in water. 

Salt: A salt is a compound created from a metal - like sodium or calcium - and a non-metal - like sulfur or chlorine - creating an ionic bond. 

Polar: When a molecule has one end that is more negatively charged and one end that is more positively charged. This is a result of the electronegativity of the atoms involved in the bonding. The larger the difference between the electronegativities of the atoms, the more polar the molecule. If the polarization gets too much, it becomes an ionic bond.

Ionic bonding: Ionic bonds are between a positively charged metal atom and a negatively charged non-metal atom. (These are salts!) It's easier to break an ionic bond than a covalent bond. 

Covalent bonding: Covalent bonds are those between non-metal atoms, like oxygen or nitrogen or carbon. They are about sharing electrons, and they are much harder to break than ionic bonds. They can be polar bonds or non-polar bonds. Molecules are about the covalent bonding.

Water is covalently bonded, meaning that the oxygen and hydrogen are sharing electrons. It is considered the universal solvent because so many things will dissolve in it. It's polar, meaning that it has a slightly negative end (the oxygen atom) and a slightly positive end (the hydrogen atom). When we put something like a salt into water, it dissolves. How? 

When we add a salt to water, it ionizes, meaning it breaks into its positively charged metal ions (Na+) and negatively charged non-metal ions (Cl-), and those ions are hydrated or surrounded by water. The sodium cations and chlorine anions stop being a salt, meaning the ionic bond is broken, and they float around separately in the water. (Water in which a salt has been dissolved is an electrolyte - it conducts electricity because of those charges!)

Click here for a cool video on salts dissolving in water! (I can't seem to embed video. Weird!)

When we add a covalently bonded thing to water, it can still dissolve, but it does it in a different way. Sugar is a decently sized covalently bonded molecule. When we add it to water, the sugar molecule doesn't ionize or split into ions - it remains as a sugar molecule. Clumps of molecules dissolve - consider the sugar cube - but the molecules remain as sugar molecules. They don't ionize. They aren't considered electrolytes because they don't conduct electricity.

If you want to get into more detail, check out this video!

Salts ionize and molecules don't, but they can both dissolve in water. Why doesn't oil, a molecule, want to dissolve in water?

As I mentioned, water is a polar molecule, with a slightly more positive end and a slightly more negative end. Something like sugar has polar features as well - we can see this with all those OH bonds (O being slightly more negative, H being slightly more positive), so it is attracted to the negative and positive sides of the water molecule.

There are forces that hold our atoms and molecules together - ionic and covalent bonding, for instance - and there are forces of attraction between those molecules, ions, and atoms. If the attraction between the solute and the solvent - say, salt and water - is stronger than the attraction between the solute - like clumps of salt - then the mixing will happen. If the attraction between the solute and solvent - say, oil and water - isn't stronger than the attraction the solute has for itself and the solvent has for itself, then the mixing won't happen.

In the case of oil and water, oil isn't polar. It's non-polar, meaning it has no electrical charge and it isn't an electrolyte. It is more attracted to itself than it is to the water. So it won't mix. Because oil is lighter than water - remember the idea of specific gravity? - it floats on top of it.

So why do we care about solubility and mixing? Because it affects just about everything we do in cosmetic chemistry! From using emulsifiers to make oil and water mix to dissolving extracts to mixing a lotion while warm, it's all about the solubility of ingredients and how to bring them together in a stable way! Join me tomorrow for more fun with chemistry when we look at how this applies to what we make! 


Row said...

It's an excellent addition to a face mask. I add 1/4 tsp to mine. It has the same nature as clay in that it will draw all the impurities out of your skin. Some people do break out as the toxins are expelled.

I was also looking for new uses(I had to buy 2 oz) and came across this very interesting tidbit. Haven't worked up the nerve to try it.

Row said...

It's an excellent addition to a face mask. I add 1/4 tsp to mine. It has the same nature as clay in that it will draw all the impurities out of your skin. Some people do break out as the toxins are expelled.

I was also looking for new uses(I had to buy 2 oz) and came across this very interesting tidbit. Haven't worked up the nerve to try it.

Anonymous said...

what about the hydrophobic/hydrophillic portions of the oil molecules? The oil is considered to be non-polar because of the hydrophobic heads of the molecule.

Susan Barclay-Nichols said...

Hi Anonymous. (Please re-write your comment with your name or an identifying thing as I don't allow anonymous comments and will delete it by the end of Sunday, September 8th.)

I'm afraid I don't know what you mean about hydrophilic portions of oil molecules. Are you thinking about a surfactant/emulsifier that has a hydrophilic and hydrophobic part, something that is considered polar?

A triglyceride, like lauric acid or oleic acid, for instance, has a glycerol backbone, but it doesn't have a hydrophilic part and a hydrophobic part, just a hydrophilic everything. Oils and butters - a bunch of triglycerides together - aren't considered polar. There are oils that are considered more polar than other oils, but compared to things like water, alcohol, glycerin, and salt, they aren't polar. (For instance, coconut oil would be thought of as more polar than olive oil because of the smaller ratio of glycerin to fatty acid, but coconut oil isn't polar.)