Our previous kitchen chemistry post discussed soap. Soap consists of sodium salts (or potassium salts) of fatty acids. For example, sodium stearate soap consists of sodium (Na+) and stearate C17H35COO−) ions:
These ions do their soapy job because the charged oxygen end of the molecule is attracted to water (since the hydrogen side of a water molecule has a slight positive charge). The long hydrocarbon tail, on the other hand, is attracted to oil and grease. Mixing soap and water with oil or grease therefore produces little grease droplets surrounded by many, many stearate (or other fatty acid) ions, with their tails embedded in the grease droplet, and their oxygen heads poking out into the water. I’ve only had the patience to draw eight for this droplet:
Because these little droplets are surrounded by negative charges (on the oxygen atoms), the droplets repel each other. This means the droplets stay separate from each other, and cannot combine into larger oily blobs. Because the negatively charged oxygen atoms are attracted to water molecules, the droplets also remain dispersed within the water (so that they can later be rinsed away). In fact, what we have here is an emulsion (or sol) of oil or grease in water, stabilised by the soap. Recall the image with which we began this post series:
Other kinds of emulsion will likewise need some kind of molecule that keeps the droplets separate – usually also a molecule with a distinct head and tail. In mayonnaise, for example, the emulsion of oil in water is stabilised by phospholipid molecules from egg yolks. These molecules also have a “head” attracted to water and a “tail” attracted to oil.
This post brings our kitchen chemistry series to a close, at least for now.