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J. Phys. Chem. B 2008, 112, 5878
Reply to “Comment on ‘The Kirkwood-Buff Theory of Solutions and the Local Composition of Liquid Mixtures’” Enrico Matteoli* and Luciano Lepori Institute for Physical-Chemical Processes of National Research Council, Research Campus, Pisa, Via Moruzzi, 1, I-56124 Pisa, Italy ReceiVed: October 19, 2007; In Final Form: February 14, 2008 Although the Comment article refers to other Authors’ work, in the text there are two or three passages where the Author critically hints to our work. For the benefit of the readers, a reply is in our opinion due, in addition to our previous discussion on this matter.1 (1) It is said that “quotation (2) contains a similar error as in the Matteoli & Lepori papers”, and that somewhere in refs 2 and 3 we have claimed that “a negative KBI is not plausible”. In those articles (as well as in other papers of ours) such sentence never appears. In ref 2 are reported examples of actual mixtures whose KBI have negative values in the full mole fraction range (see Figures 3a, 5, and 8a) without any hints on those values being not plausible. In refs 2 and 3, our discussion referred to the values, signs and meaning of ∆nij of ideal mixtures, reaching the conclusion that these systems should be taken as reference in the calculation of ∆nij and derived quantities (local composition, preferential solvation) and defining valid equations for that. In doing that, we have never said or meant that KBI and ∆nBN ij are wrong and should be corrected, we simply showed that our procedure is more helpful for the interpretation of the KBI. (2) As to the “misunderstanding of the KBI meaning ... in view of the subtle difference in the long range behavior of gij * To whom correspondence should be addressed. E-mail: matteoli@ ipcf.cnr.it.
in open and closed systems”, that the author implicitly attributes to us too, it is easy to realize that it is a gratuitous speculation. It is known that the behavior of gij(r) at very short distances, r, is such that gij values in the range starting from the center of the central particle up to the distance of maximum approach, dma, are always practically zero for all mixtures in open as well closed systems. In that range, gij gives a contribution, KBI* ) ∫ d0ma (gij - 1)4πr2 dr, to the whole KBI that for most mixture is not negligible, is always negative and independent of the chemical nature of the components. This means that a fraction of the KBI value, and therefore of ∆nij, more or less large depending on the molecular volumes, and not exactly computable because dma is not known, is not relevant for the correlation of the local composition to molecular interactions, and may produce wrong or biased indications. In our papers, (a) we have shown that this contribution is also evident from the behavior of KBI of ideal mixtures, (b) we have provided a new definition of ∆nij and a related procedure to get rid of the unwanted contribution, and (c) we have shown the usefulness and advantages of using it. (3) Finally, we want to point out that there is nothing ironic in our appreciation of Shulgin and Ruckenstein’s work. Their effort to propose other ways to calculate ∆nij has had the same objective as ours, that is, to provide a procedure, based on a new definition of ∆nij, capable to guarantee an easier identification of the correlations between local composition and molecular interactions, and not to claim the incorrectness of ∆nBN ij nor the “correction” of the KBI. References and Notes (1) Matteoli, E.; Lepori, L. J. Phys. Chem. B 2007, 111, 3069-3072 (ref 5 of the Comment). (2) Matteoli, E. J. Phys. Chem. B 1997, 101, 9800 (ref 1 of the Comment). (3) Matteoli, E.; Lepori, L. J. Chem. Soc. Faraday Trans. 1995, 91, (3), 431 (ref 2 of the Comment).
10.1021/jp710158h CCC: $40.75 © 2008 American Chemical Society Published on Web 04/12/2008
