In the Classroom
Editor’s Note: The three dimensional models (SAMs) in this demonstration do a great job of illustrating the hydrophobic effect, where nonpolar substances are exsolved from water to form two distinct layers. The authors use a “squeezing out” model, and focus on hydrogen bond formation in water to explain the exclusion of nonpolar molecules. The SAMs are valuable in explaining alternative models for the hydrophobic effect which are more consistent with other data. For example, if exsolution were caused by water hydrogen bonding to itself, as in the “squeezing out” model, the process would be exothermic. Surprisingly, it is endothermic (1, 2). If formation of two layers is endothermic, it must be accompanied by a positive entropy change, so that the two (nearly) pure layers that result must be seen as “more disordered” than the solution. Most current theories explain the exsolution in terms of an increase in entropy (3, 4). This situation is a good example of a case where increases in what we intuitively call “order” is difficult to correlate with decrease in entropy (5). Disorder or order is everywhere, and the question is how to recognize the disorder that entropy measures. It might be better not to try (6). Separation into pure phases (incorrectly) appears to involve a decrease in entropy, because pure phases appear to be more “orderly” than a mixture. While the paper above focuses on hydrogen-bond formation in bulk water as the driving force for the separation, the accepted entropic explanation has been used in at least one general chemistry text (7), and the SAMs in the present demonstration might be used as a segue to discussions of its weaknesses as well as its strengths. The SAMs cannot show the increased “order” of the water molecules around dissolved hydrophobic species, possibly caused by stronger than average hydrogen bonding, that is proposed by other authors to explain both the negative enthalpy and entropy of dissolution (8). Nor can the SAMs mimic the self-assembly of the hydrophobic solute molecules proposed by source (9). These papers suggest that entropy as “order” has been useful to researchers, who, when confronted with the paradoxical negative entropy of dissolution in the “hydrophobic effect” described above, looked for the creation of order as hydrophobic substances dissolve (exothermically). Models are never perfect (10), and we believe that even their inaccuracies, if they are made explicit, can be valuable teaching tools. Thinking about how a model differs from its referent can lead to insights about how the real system behaves. The magnetic SAMs require another important caveat: the size of the hydrogen atoms is not well represented. This deficiency can be partially mitigated by painting circles of a contrasting color on the surface of the spheres around the magnets that represent the hydrogen atoms. Hydrogen atoms can also be represented by using longer painted magnets that protrude 0.5 cm from the sphere. We experimented with two 1/4-in. diameter × 1/2-in. length neodymium magnets,1 which press-fit into the holes of 1 1/4-in. wooden models available from many suppliers,2 and two of the 5-mm diameter × 5-mm length magnets described in the demonstration, glued with silicon seal or hot-melt glue into the remaining holes. Models may also be constructed with four 1/8-in. diameter × 3/8-in. length rod magnets3 pressed into 1/8-in. holes drilled in the wooden spheres. In the latter case, the
holes were drilled concentrically with the existing 1/4-in. holes, entirely through the sphere. This gave the proper orientation, and also allowed the magnets to be pushed out with a steel pick from the opposite side if changes were desired. For this reason, it may be advisable to drill 1/8-in. holes through the spheres before magnets are inserted in all designs. Large spheres with four south pole magnets representing oxygen and smaller spheres with four north pole magnets for hydrogen can roughly model proton exchange and other effects, but this is an expensive approach—each sphere will cost $2–$4 and 40 may be required—whose benefits would have to be weighed carefully. Smaller (3/4-in.) woodenball models4 are available, predrilled with 1/8-in. holes, that can accommodate the 1/8-in. diameter magnets.3 If visibility is not a problem, these can reduce the expense. Ferrite magnets are much weaker than neodymium ones and create a much less dramatic effect. Finally, excellent models of water and water clusters can be found on the Web (11), along with extensive descriptions of water structures (12). Notes 1. Forcefield. http://www.wondermagnet.com (accessed June 2002), 877/944-6247 or 970/484-7257, order #16, $1. 2. Science Kit. http://www.sciencekit.com (accessed June 2002), 800/828-7777, #47730-00, Advanced Molcular Model Set, $45.00; #62607-10, 1 pack of 12 carbon (black) balls, $7.95; SargentWelch/Cenco. http:// www.sargentwelch.com (accessed June 2002), 800/727-4368, #WLS-61815, set, $29.95, or WLS-61825F, 12 carbon atoms, $9.90; Carolina Biological Supply. http://carolina.com (accessed June 2002), 800/334-5551, #RG-84-0210, set, $30.15. 3. Forcefield. http://www.wondermagnet.com (accessed June 2002), 877/944-6247 or 970/484-7257, order #26, $0.25. 4. Science Kit. http://www.sciencekit.com (accessed June 2002), 800/828-7777, #62606-00, set of 100 atoms, $29.95 or #62606, 12 carbon atoms, $4.50; Carolina Biological Supply. http://carolina.com (accessed June 2002), 800/334-5551, #RG-84-0212, set, $30.15.
Literature Cited 1. Aerts, T.; Clauwaert, J. J. Chem. Educ. 1986, 63, 993. 2. Huque, E. M. J. Chem. Educ. 1989, 66, 581. 3. Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973. 4. Southall, N. T.; Dill, K. A.; Haymet, A. D. J. J. Phys. Chem. B 2002, 106 (3), 521–533. 5. Lambert, F. L. J. Chem. Educ. 1999, 76, 1385. 6. Lambert, F. L. J. Chem. Educ. 2002, 79, 187. 7. Chemistry: A Project of the American Chemical Society; Trial Version, W. H. Freeman & Co., 2002. 8. Silverstein, K. A. T.; Haymet, A. D. J.; Dill, K. A. J. Am. Chem. Soc. 1998, 120 (13), 3166–3175. 9. Marmur, A. J. Am. Chem. Soc. 2000, 122 (9), 2120–2121. 10. Bhushan, N.; Rosenfeld, S. J. Chem. Educ. 1995, 72, 578. 11. MathMol Library. http://www.nyu.edu/pages/mathmol/library/ (accessed June 2002). 12. South Bank University, School of Applied Sciences. http:// www.sbu.ac.uk/water/ (accessed June 2002). —Ed Vitz
JChemEd.chem.wisc.edu • Vol. 79 No. 9 September 2002 • Journal of Chemical Education
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