By Richard Bleil
Molecules are fascinating things. Their behavior is based on electrons and the attraction of elements for electrons.
Every element is surrounded by electrons. This means that all touch is an illusion. See, electrons are all negatively charged, which means that all matter is basically coated with electrons. Because like charges repel, when two objects “touch”, what is actually happening is that the electrons on the surface of one object is repelling the electrons on the surface of the other.
The attraction of an element for electrons has been quanitified by Linus Pauling, and the Pauling Electronegativity Scale. A brilliant scientist, I was in graduate school when we lost him. At the time, most publications are papers published in collaboration with others. In fact, there was a paper published around this time that had a longer list of authors than the paper itself, with well over two hundred authors. In the year that Linus Pauling passed away, well into his 90’s, he still published a single author paper.
There are two major kinds of chemical bonds, both relating to electrons, and based on electronegativity. If the difference in electronegativity between two elements is too great, that means that one element is strong enough to literally remove one or more electrons from another. These are “ionic” compounds, like sodium chloride, better known as table salt. The chloride has such a strong pull for electrons relative to sodium that it literally takes an electron away. This results in a negative charge on the chlorine, and positive on the sodium. The chemical bond is, in actuality, nothing more than the attraction of the oppositely charged elements.
If one element is not strong enough to remove an electron (or more) from another, then the elements are forced to share electrons. Electron sharing results in what we call a “covalent bond”. But, there is even a subtlety to this. Just because one electron is not strong enough to completely remove an electron, it may be strong enough for the electrons to be shared unevenly.
Take water. The hydrogen-oxygen bond is covalent, but oxygen has a strong electrognegativity. So, the electrons are more attracted to the oxygen than the hydrogen. As such, they will spend “more time” around the oxygen (actually, there will be a higher “electron density”). This means that the oxygen will not be fully negative, but will have a negative characteristic, a partial negative charge, leaving the hydrogen with a partial positive charge. We call this a “polar covalent” bond, and the partial negative part of the molecule will be attracted to the partial positive of another.
Finally, let’s look at oil, heavy in carbon-hydrogen bonds. Carbon and hydrogen have very similar elecronegativities. As such, the electrons are shared more or less evenly, and there are no positive or negative portions of the bond. We call this “non-polar covalent” bonds.
There’s an old rule that says “like dissolves like”.. If you enjoy a good oil vinaigrette (oil and vinegar) on your salad, you may well know that you have to shake up the vinaigrette to mix it. Eventually, the oil and vinegar will separate out. This is because oil is non polar, and water polar. Water is a great solvent for other polar molecules, like alcohol. They will mix, and remain dissolved, in solution. But oil and water will separate. On the other hand, oil is a good solvent for other non-polar solutes, like moth balls. Mothballs are a chemical called naphthalene, which is non-polar. The naphthalene would not dissolve in water, but it would dissolve in oil
So what is soap? Stains and dirts on our clothes or bodies are generally oil-based. Salts, like when we sweat, would dissolve in water. If our dirt was salt and polar compound based, we could just rinse off in water and we would be done. But stains and dirt are usually oil based. They would come off if we bathed in oil. But, show of hands, how many of us would like to do this?
So there are specialized molecules, originally made of treated fats (salts of fattty acids). These molecules are fairly long, and have two sides, a polar side, and a long non-polar “tail”. In water, the non-polar “tail” dissolves in the oil based dirts, but the polar heads will not. Instead, they stick out of the stain, like matchstick heads. The water will pull at these polar head groups, trying to get them to dissolve in the water. When they do, the non-polar tail will bring with it a bit of the oil, as well as some of the other soap molecules as well. This stabilizes the oil in water. It’s not truly dissolved, but rather, the oil droplet is “suspended” in the water, giving time to rinse it away. We call this a “suspension”, or an “emulsion”. When you see “lecithin added as an emulsifying agent”, basically this is a form of a soap molecule, stabilizing the nonpolar oils in the polar water base of foods.