Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.5 The phenomenological theory of solvent effects 303
greater than sucrose-ethanol energies. We may expect that the sucrose-water interfacial
tension is very low.
Now, gA turned out to be negative because gA(γ2-γ1), a positive quantity as generated
by eq. [5.5.23], was divided by (γ2-γ1), a difference of surface tensions that is negative. Inevitably
gA was found to be negative. The interfacial tension argument, however, leads to the
conclusion that division should have been by the difference in interfacial tensions. We have
seen that the interfacial tension between sucrose and water may be unusually low. Thus the
factor (γ2-γ1), when replaced by a difference of interfacial tensions, namely [γ(sucrose/ethanol)
- γ(sucrose/water)], is of uncertain magnitude and sign. We therefore do not know the
sign of gA; we only know that the quantity we label gA(γ2-γ1) is positive. This real example
demonstrates the soundness of the advice that products of the form gAγ not be separated into
their factors. 27,28
5.5.5 NOTES AND REFERENCES
1 D. Khossravi and K.A. Connors, J. Pharm. Sci., 81, 371 (1992).
2 R.R. Pfeiffer, K.S. Yang, and M.A. Tucker, J. Pharm. Sci., 59, 1809 (1970).
3 J.B. Bogardus, J. Pharm. Sci., 72, 837 (1983).
4 P.L. Gould, J.R. Howard, and G.A. Oldershaw, Int. J. Pharm., 51, 195 (1989).
5 Also, when K1 = 1 and K2 = 1, eq. [5.5.13] shows that ΔGsolv = ΔGWW; in this special case the solvation
energy is composition-independent.
6 H.H. Uhlig, J. Phys. Chem., 41, 1215 (1937).
7 J.E. Leffler and E. Grunwald, Rates and Equilibria of Organic Reactions, J. Wiley & Sons, New York,
1963, p. 22.
8 J.M. LePree, M.J. Mulski, and K.A. Connors, J. Chem. Soc., Perkin Trans. 2, 1491 (1994).
9 The curvature correction factor g is dimensionless, as are the solvation constants K1 and K2. The parameter
gA is expressed in Å 2 molecule -1 by giving the surface tension the units J Å -2 (where 1 erg cm -2 =1x10 -23 J
Å -2 ).
10 D. Khossravi and K.A. Connors, J. Pharm. Sci., 82, 817 (1993).
11 J.M. LePree, Ph.D. Dissertation, University of Wisconsin-Madison, 1995, p. 29.
12 A. Leo, C. Hansch, and D. Elkins, Chem. Revs., 71, 525 (1971).
13 C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 1988.
14 D. Khossravi and K.A. Connors, J. Solution Chem., 22, 321 (1993).
15 K.A. Connors and J.L. Wright, Anal. Chem., 61, 194 (1989).
16 K.A. Connors, Binding Constants, Wiley-Interscience, New York, 1987, pp. 51, 78.
17 R.D. Skwierczynski and K.A. Connors, J. Chem. Soc., Perkin Trans. 2, 467 (1994).
18 K.A. Connors and D. Khossravi, J. Solution Chem., 22, 677 (1993).
19 M.J. Mulski and K.A. Connors, Supramol, Chem., 4, 271 (1995).
20 K.A. Connors, M.J. Mulski, and A. Paulson, J. Org. Chem., 57, 1794 (1992).
21 J.M. LePree and K.A. Connors, J. Pharm. Sci., 85, 560 (1996).
22 M.C. Brown, J.M. LePree, and K.A. Connors, Int. J. Chem. Kinetics, 28, 791 (1996).
23 J.M. LePree and M.E. Cancino, J. Chromatogr. A, 829, 41 (1998).
24 The validity of this approximation can be assessed. The free energy of hydration of benzene is given as -0.77
kJ mol -1 (E. Grunwald, Thermodynamics of Molecular Species, Wiley-Interscience, New York, 1997,
p. 290). Doubling this to -1.5 kJ mol -1 because of the greater surface area of naphthalene and repeating the
calculation gives g1A1γ1 =4.88x10 -20 J molecule -1 , not sufficiently different from the value given in the text
to change any conclusions.
25 D. Khossravi, Ph.D. Dissertation, University of Wisconsin-Madison, 1992, p. 141.
26 F.M. Fowkes, Chemistry and Physics of Interfaces; American Chemical Society: Washington, D.C., 1965,
Chap. 1.
27 The introduction of the interfacial tension into the cavity term was first done by Yalkowsky et al., 28 who also
argue that a separate solute-solvent interaction term is unneeded, as the solute-solvent interaction is already
embodied in the interfacial tension. In our theory we explicitly show the coupling between the solute-solvent
and solvent-solvent interactions (eq. [5.5.19]), but this is in addition to the solute-solvent interaction (eq.
[5.5.13]). This difference between the two theories is a subtle issue that requires clarification.
28 S.H. Yalkowsky, G.L. Amidon, G. Zografi, and G.L. Flynn, J. Pharm. Sci., 64, 48 (1975).