Triolein As An Historical Molecule

 
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For some of us triolein is actually our favourite molecule. It turns out triolein is the molecule that Benjamin Franklin used to calm the waves on Clapham pond in England. Franklin was able to demonstrate the spreading properties of surface-active materials. Although he didn't perform the calculations, this was the first experiment in which molecular dimensions could be determined. A truly remarkable piece of work.

Of the stilling of Waves by means of Oil

Here are some extracts from Vol 64, Part 1 (1774) of Philosophical Transactions, the journal of the Royal Society. Franklin's writing is in the style of the time (not what we would expect to see in a scientific paper these days!), and quite revealing about the scientific process he undertook. We see an interesting mix of scepticism, anecdotes, precise descriptions and analysis of the results. The material published in the journal was transcripts of letters between Franklin and some other correspondents, including Rev Mr Farish and Dr Brownrigg (some local amateur scientists).

Farish wrote with great skepticism:

...According to his representation, the water, which had been in great agitation before, was instatnly calmed, upon pouring in only a very small quantity of oil, and that to so great a distance round the boat as seems a little incredible. I have since had the same accounts from others, but I suspect all of a little exaggeration.

Franklin in his letters mentioned that he had previously observed ships to have less wake after their cooks had recently emptied their cooking oil overboard. He went on to recall sections of his experiment:

At length being at Clapham where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropt a little of it on the water. I saw it spread itself with surprising swiftness upon the surface; but the effect of smoothing the waves was not produced; for I had applied it first on the leeward side of the pond, where the waves were largest, and the wind drove my oil back upon the shore. I then went to the windward side, where they [the waves] began to form; and the oil, though not more than a teaspoonfull, produced an instant calm over a space of several yards square, which spread amazingly, and extended itself gradually till it reached the less side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.

...If a drop of oil is put on a polished marble table, or on a looking glassthat lies horizontally; the drop remains in place, spreading very little. But when put on water it spreads instantly many feet round, becoming so thin as to produce prismatic colors, for a considerable space, and beyond them so much thinner as to be invisible, except in its effect of smoothing the waves at a much greater distance.

Some time later, Franklin tried to use the same technique on sea waves. Although the experiment did not work as he anticipated, he reported his findings as he still believed they were useful scientifically:

...The experiment had not, in the main point, the success we wished, for no material difference was observed in the height or force of the surff [sic] upon the shore;... It may be of use to relate the circumstances even of an experiment that does not succeed, since they may give hints of amendment in the future trials: it is therefore I have been thus particular.

Although Franklin never actually did the calculation, from his experiment it is possible to determine the actual dimensions of the molecules he used. This was the first experiment from which molecular dimensions could be determined. The reason you can do this calculation is triolein has (as we will see in this trail) a water-soluble end and in oil-soluble end. The water-soluble end tends to stick into the water and the oil-soluble end likes to stick out the water, which in this case means into the air. The result is that a monolayer is formed (i.e. a layer of oil that is one molecule thick).

With one teaspoon (approx 5 mL) of trioleum, Franklin covered an area of about half an acre (approx 2000 m2). The thickness of such a layer would be 2.5×10-9 m (or 2.5 nm). To within the uncertainty of measuring areas by eye, this is quite an accurate measurement (the actual molecule is around 2 nm in length). It's quite remarkable that an experiment so simple can yield a result so accurate; it's not necessary to use lots of big fancy equipment to do good science.

This experiment was repeated by Lord Rayleigh 120 years later, who did perform the above calculation, cautiously concluding that there might be molecules with a length of 2 nm. Part of the reason for the caution was that he could not be sure that it truly was a monolayer - was the layer continuous? was it a mix of water and oil or just pure oil?

Historical Importance

All this leads us to look at triolein with a certain amount of affection. Triolein ended up playing an important role in our understanding of surface active molecules and molecular dimensions.