I read with some interest the article by Campbell and Rupe in the July 2000 issue of J. Chem. Educ. (1), which describes experiments on the solvent-induced swelling of polymer slabs. As a chemist with a background in fuels and combustion, I found that the contents of this article had a considerable degree of contact with a long-standing interest of my own: the solvent-induced swelling of low-rank coals. I have published a good deal in this area over the last ten years (e.g. ref 2, the most recent in the series) and have often involved Honours students with the work. Low-rank coal means brown coal (lignite). In the USA, deposits of such are to be found in states including North Dakota and Texas.

A safe, effective, and inexpensive demonstration of the swelling propensity of these is possible simply by placing about 1 g of the oven-dried coal, with particle sizes in the approximate range 1-3 mm, into a test tube and adding 5 cm³ of acetone. After an hour or less (preferably with gentle shaking) the coal sample will have swollen, perhaps by as much as 30%. This swelling can easily be measured by determining the height of coal with a ruler, before and after solvent addition. Alternatively, no doubt an overhead projector could be used to show the effect, as in the experiments described by Campbell and Rupe.

I find that in their interpretation of results these authors have invoked some of the ideas also used in interpreting coal swelling results, including the effect of the degree of cross-linkage. When demonstrated to tertiary students, the results of coal-swelling tests could be related to certain basic thermodynamic quantities including the solubility parameter of the solvent. Results for different low-rank coals are relevant to their respective suitabilities for processes such as solvent extraction of chemicals.

In attempting such experiments, please note the following:

1. Use a low-rank coal. Higher-rank ones such as bituminous coals swell only very slightly if at all, probably because they are much less porous than low-rank coals.

2. While acetone has been suggested as a solvent methanol is also very effective, as is pyridine.

I will be happy to give informal advice.

Literature Cited

1. Campbell, D. J.; Rupe, S. B. J. Chem. Educ. 2000, 77, 876-878.

2. Jones, J. C.; Hewitt, R. G.; Innes, R. A. Fuel 1997, 76, 575-577.

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