In his 1774 article, Franklin observed that “when [a drop of oil] is put on water it spreads instantly…as if mutual repulsion between its particles took place as soon as it touched the water”. Gugliotti argues that Franklin, with his “repulsion between particles” explanation, “took the right direction towards another impressive conclusion”. Citing Irving Langmuir’s famous 1917 JACS article (2), Gugliotti writes that “the repulsion between surfactant molecules on the water surface that leads to the spreading of a film is due to the secondary valence of atoms of the polar head group of the molecules.…This explains not only why ionic surfactants spread on the water surface (in this case due to primary valence too), but also why nonionic surfactants do it.”

I found Gugliotti’s explanation confusing in two ways: First, it is unlikely that nonionic surfactant molecules would repel each other and second, the exact meaning of the outdated terms primary versus secondary valence was not clear to me. With the encouragement of Gugliotti, I consulted Langmuir’s classic 1917 article¹ (2). Langmuir explains the spreading of oleic acid on water thusly: “[the charged carboxylate head group] portion of the oil² molecule is attracted to the water, while the [non-polar hydrocarbon chain] remainder is more attracted to other oil molecule [chains] than to the water” (2, p 1863). Note that Langmuir does not invoke repulsion among molecules, but attraction, between head groups and water and among hydrocarbon chains. Although to Franklin the oil spreading on water behaved “as if mutual repulsion between its particles took place”, Langmuir and others have shown that the spreading is due to the maximization of attractive forces, rather than the minimization of repulsion. Charles Tanford, in his book³ Ben Franklin Stilled the Waves (4), devoted a chapter to this problem of repulsion versus attraction. Tanford argued that although 18th century scientists lacked the molecular knowledge necessary to choose between repulsion and attraction, Franklin’s invocation of repulsion was “internally inconsistent”: His observation of the cohesiveness of a drop of oil on a solid surface shows that oil molecules do not repel, but attract one another. The misuse of repulsion in explanations of hydrophobicity was also recently discussed by Goss and Schwarzenbach in this Journal (5).

As for secondary and primary valence, these vague terms were employed by Langmuir, Werner, and others to describe a wide variety of interactions. Most of these interactions had nothing to do with the primary valence value assigned to an element based on its group in the periodic table. As Smith has argued in this Journal (6), this is why these valency terms, “whose definition has been the source of much confusion since they were introduced—were superceded [by terms currently in common usage]….Werner’s terminology should by now be regarded as of purely historic interest.” For a detailed discussion of the historical complexities of the secondary or auxiliary valence concept, see the monograph by Russell (7). Regarding the spreading of oil on water, Langmuir’s use of secondary valence refers to what we would now call the hydrogen bonding that occurs between a surfactant’s polar head group and surface water molecules. These interactions, similar to most intermolecular forces, are attractive, not repulsive.

The pedagogical use of historical scientific literature has its advantages and its drawbacks. The advantages include the many tie-ins that Gugliotti was able to make between Franklin’s marvelous observations in a 1774 article and the work of scientists who came before and after him, for example, Democritus, Pliny, Newton, Dalton, Avogadro, Rayleigh, and Langmuir. The disadvantages include misreadings (repulsion vs attraction) and obsolete terminology (primary and secondary valence) that appear now and again. Needless to say, teachers must be aware of these potential pitfalls as they plan their classroom discussions.

Acknowledgments

I wish to thank Marcos Gugliotti as well as the other two reviewers of this letter; their suggestions were greatly appreciated and served to improve this piece.

Notes

1. This article was a major part of Langmuir’s citation for the 1932 Nobel Prize in Chemistry. Langmuir showed that molecular size and shape could be determined with remarkably simple tools. “To this day, his set of experiments remains one of the simplest and most elegant demonstrations of the reality and properties of molecules” (3).
2. Langmuir’s reference to oleic acid as an “oil” differs from modern usage of the term.
3. Gugliotti based a portion of his article (1) on Tanford’s book (4), citing it as ref 13.

Literature Cited

1. Gugliotti, M. J. Chem. Educ. 2007, 84, 941–943.
2. Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848–1906.
3. Nobel Laureates in Chemistry, 1901–1992; James, L. K., Ed.; ACS/CHF Press: Philadelphia, 1993; p 207.
4. Tanford, C. Ben Franklin Stilled the Waves: An Informal History of Pouring Oil on Water, with Reflections on the Ups and Downs of Scientific Life in General; Duke University Press: Durham, NC, 1989; Chapter 13, pp 144–148.
5. Goss, K. U.; Schwarzenbach, R. P. J. Chem. Educ. 2003, 80, 450–455.
6. Smith, D. W. J. Chem. Educ. 2005, 82, 1202–1204.
7. Russell, C. A. The History of Valency; Leicester University Press: Leicester, U.K., 1971.

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