Colloids

With summer comes employment, and with employment comes less learning, and with less learning comes less blogging. In addition, my summer studies are centering around broader philosophical studies than what is topical for this blog, so expect a decrease in posting for the summer months.

However, during a pub-crawl with my friends a few weeks ago, the subject of colloids came up. I poured the beer out too fast, and it foamed over. I knew foam to be a colloid, I knew alloys to be colloids, but I had no recollection of what distinguished a colloid from a solution. Both are heterogeneous mixtures of substances with molecules dispersed fairly regularly throughout a medium. Generally, solutions are liquids that have solids dissolved in them, though they can also have a combination of liquids. Colloids don't have a specified state: In fact, the type of colloid depends upon the states of the dispersion medium (analogous to the solvent) and the thing being dispersed through that medium (analogous to the solute). So, really, in a prima facie way, it seems that colloids are just a more general terminology for solutions.

So I broke out my gen-chem book, and found out I was mistaken -- the difference between colloids and solutions is the size of the molecules, or groups of molecules. In both, a molecule or ion is solvated, or completely surrounded by the solution. But in a colloid, the groups of molecules are much larger, between 1 * 10^3 pm to 1 * 10^6 pm (picometers). For comparison, the bond length of a Helium to Helium molecule is 300 picometers. Another common example of a colloid is found in soap -- when soap molecules interact with grease, they embed into the grease while keeping a single part of the soap molecule on the outside of the grease. The part embedded in the grease is attracted to oily things, and the part on the outside of the grease is attracted to water -- so running water will then push the grease along. This is a colloid composed of a clump of molecules attracted to each other, but dispersed in another medium, and much larger than a molecule in a solution. Another good day-to-day example of colloids can be seen if you go for a walk at the park. If you've seen light streaming through the branches, this is because the light is reflecting off of dust in the air. In fact, this is a common way to distinguish between colloids and solutions, and is known as the Tyndall effect (this picture demonstrates a colloid of a solid in a liquid). The molecules in a solution are so small that they don't interfere with the visible light spectrum, but the molecules or groups of molecules in a colloid are large enough to do so.

So this brought me to another question regarding chemical philosophy: While we can observe the Tyndall effect to distinguish between colloids and solutions, do the sizes of the molecules matter very much aside from the fact that they interact with visible light? I've done kinetics experiments revolving around a solutions ability to absorb light. So, even though we can't observe the interaction with our eyes, the molecules do still interact with light, don't they? Is the terminology of solutions a bit too simplistic? After all, there is a point in solutions where you have to ask, what is the solvent and what is the solute? What if you have more than two liquids and a solid? Proteins can grow to reach sizes like this, and yet they are only one molecule solvated by water. Does that make our DNA colloidal, and what point does this distinction elucidate? After all, we could also just say that solutions with really big particles in them interact with the visible light spectrum and be done with it. But then we'd be expanding the terms of solution to include things that are clearly not mixed in the same way that salt water is mixed, such as mayonnaises, or beer foam. But they are also slightly different from a solid block of, say, iron. But just because there seems to be this odd in between zone where we're uncertain about how to classify and understand given solutions, it seems rather ad-hoc to just make up a term and rationalize a distinction. So, what's the point of colloids, and what terms should we use in distinguishing between types of solutions, if indeed we ought to revise them at all? I'm going with the ambiguous and open-ended ending.