Chemical Education Today
Letters Cohesive and Adhesive Forces versus Surface Tension Gradients—Revisited Marcos Gugliotti of the Institute of Chemistry at the University of São Paulo, Brazil has pointed out to me that several statements included in two articles I wrote require clarification (1, 2). Here I address the most salient points. In both papers the relationship between hydrogen bonding, viscosity, surface tension, and cohesive and adhesive forces bears further explanation. Although hydrogen bonding capacity varies in the order: glycerol >> water > ethanol, surface tension (in dyne/cm) varies in the order: water > glycerol >> ethanol, and viscosity (in cP) varies in the order: glycerol >> ethanol > water (3). The fact is that macroscopic parameters like viscosity and surface tension are rarely controlled by a single microscopic factor like hydrogen bonding. Gugliotti has pointed out (personal communication) that in addition to hydrogen bonding, viscosity depends on London forces and also on the size and structure of the molecules involved, which can become “entangled”. A good discussion of this topic can be found in Chemistry Comes Alive, Vol. 2 (4). Similarly, surface tension depends not only on hydrogen bonding capacity, but also on molecular structure, and on the orientation of molecules at the liquid’s surface. Furthermore, the best explanation for the rising film of wine in a glass (‘legs’ or ‘tears’ of wine) involves changes in surface tension as ethanol evaporates from the beverage at the surface near the glass. Adamson and Gast wrote that “evaporation of alcohol produces a surface tension gradient
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driving a thin film up along the side of a wine glass, where the liquid accumulates and forms drops or tears” (5). The importance of surface tension gradients in explaining liquid motion has been discussed in a few recent papers in this Journal (6, 7). Furthermore, Gugliotti has submitted a manuscript (8) that expands on this explanation and I wish to take this opportunity to thank him for highlighting these clarifications. [Editor’s note: Gugliotti’s article on this topic is published in this issue on p 67.] Literature Cited 1. Silverstein, T. P. J. Chem. Educ. 1998, 75, 723–724. 2. Silverstein, T. P. J. Chem. Educ. 1993, 70, 253. 3. Handbook of Chemistry and Physics, 52nd ed.; CRC Press: Cleveland, 1971; pp F30–40. 4. Jacobsen, J. J.; Moore, J. W. Chemistry Comes Alive!, Vol. 2, 2nd ed. [CD-ROM]; J. Chem. Educ. Software 2000, SP 21; http://www.jce.divched.org/JCESoft/CCA/CCA2/main/VISCLIQ/ CD2R1.htm (accessed Oct 2003). 5. Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley: NY, 1997; p 371. 6. Ahmad, J. J. Chem. Educ. 2000, 77, 1182. 7. Jasien, P. G.; Barnett, G. J. Chem. Educ. 1993, 70, 251. 8. Gugliotti, M. J. Chem. Educ. 2004, 81, 67–68. Todd P. Silverstein Department of Chemistry, Willamette University Salem, OR 97301
[email protected] Vol. 81 No. 1 January 2004
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