The author replies to Silverstein.
While observing the spreading of oil on water, Benjamin Franklin (1) stated: “It seems as if a mutual repulsion between its particles took place as soon as it touched the water.” Later, Langmuir (2) concluded that the cause of spreading of oils (fatty acids) is the affinity of the polar head groups (PHGs) of the molecules for water, which “is due to the strong secondary valence” of atoms in the PHGs. In my article (3), I compared Franklin’s observations with Langmuir’s conclusions and wrote: “According to Langmuir, the repulsion between surfactant molecules…that leads to the spreading of a film is due to the secondary valence”. I also stated that secondary valence explains the spreading of nonionic surfactants and that for ionic ones primary valence is important too. In his letter (4), Silverstein finds my statements confusing, probably because of the use of the word “repulsion” in connection to attractive forces such as primary and secondary valences, but I hope the following explanations can avoid any confusion.
Surfactants spread on water as a film because of attraction of their PHGs for water (2). For nonionic surfactants, this attraction is due to secondary (residual) valence, while for ionic surfactants it is due to both primary and secondary valences. The electrostatic repulsion among ionic PHGs also contributes to spreading, whereas a steric repulsion due to hydration of the PHGs can influence both charged and uncharged films (5). Langmuir’s theory is based on attraction between PHGs and water molecules and not on repulsion. However, according to Tanford (6, cited in my article as ref 13), Franklin’s observation “contains the uncomfortable word repulsion”, but “The trouble with this statement, however, is not that it invokes a repulsion force.” Thus, Franklin’s usage of “repulsion” was not accurate, but his observation was remarkable, and I still think “he took the right direction”. Accordingly, my usage of that word may sound as a wrong interpretation of previous articles (1, 2), but it should not be understood this way, for it was used to link Franklin’s to Langmuir’s explanations, and to me it was not a fatal flaw. Besides, I disagree on many points in Silverstein’s letter (4), which are commented below.
In no part of my article (3) did I state that primary or secondary valences are repulsive forces or that nonionic molecules repel each other. To me, Langmuir’s usage of the word oil as a reference to fatty acids is acceptable, and his explanations on primary and secondary valences are clear (2), so it is not necessary to reproduce them here. Langmuir’s definitions of such terms were based on Werner’s concepts (7), and if this terminology is not clear to Silverstein I encourage him to check other sources. Hydrogen bonding may be one kind of manifestation of secondary valence, but this discussion as well as the possible confusion involving the valence terms are not the subject of my article. The expression “internally inconsistent” used by Tanford in his book (6) was related to the spreading on solids, but I believe Silverstein misread that passage or maybe he is not familiar with surface phenomena, as suggested by a previous letter (8).
Langmuir’s article (2) is the basis of the Nobel Prize in Chemistry he received in 1932. To me, Langmuir made proper use of the valence concepts, for he knew them well. In fact, Langmuir defined covalence and electrovalence; he introduced the concept of isosterism and formulated the electroneutrality principle; and he is a co-author of the “octet rule” and coined terms such as covalent bond and polar union, nowadays called ionic bond (9). Unfortunately, the only citation for Langmuir in chemistry textbooks is related to his adsorption isotherm. For these and other reasons, I believe it is useful to discuss historical articles with students and also to cite them, thus giving the pioneers the proper credit.
Finally, I believe Silverstein misinterpreted my explanations and misread or did not correctly check the references cited in my article (3). Some points raised by him are not the subject of my article, and he did not answer the questions that motivated his communication (“How can a modern reader of historical literature be led astray, and why does certain terminology become obsolete?”), but unfortunately the two reviewers he thanks in his letter (4) did not realize that.
Literature Cited
- Franklin, B. Philos. Trans. 1774, 64, 445–460.
- Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848–1906.
- Gugliotti, M. J. Chem. Educ. 2007, 84, 941–943.
- Silverstein, T. P. J. Chem. Educ. 2009, 86, 807–808.
- Gaines, G. L., Jr. Insoluble Monolayers at Liquid–Gas Interfaces; John Wiley and Sons: New York, 1966; pp 166, 193.
- 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; Oxford University Press: New York, 2004; pp 170–171.
- Werner, A. On the Constitution and Configuration of Higher-Order Compounds. Nobel Lecture, 1913 (accessed Apr 2009).
- Silverstein, T. P. J. Chem. Educ. 2004, 81, 35.
- Gugliotti, M. Quim. Nova, 2001, 24, 568–572 (and ref therein).
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