1841
COM~WNICATIOXS TO THE EDITOR Table I : Calculation of the Molar Absorptivity of the Monomer of Di-t-butyl Carbinol in Carbon Tetrachloride a t 20” Concn Total alcohol rn ill
0.042 0.053 0.034 0.063 0.073 0.078 0.089 0.1n.s
0.0665 0.0839 0.0854 0,0995 0.115 0.123 0.140 0.165
-. Dimera
Monomer molar --absorptivities-$1,
uncorb
M
0.0004 0,0006 0,0007 0.0010 0.0012 0.0014 0.0018 0 0026 I
Av
2.06 2.06 2.05 2.07 2.07 2.08 2.07 2.07 2.07k 0.01
61,
cor‘
2.09 2.09 2.07 2.11 2.10 2.13 2.11 2.13
2.10& 0.02
a Calculated from the absorbance at 3500 cm-l given by Hammaker, et d.,l using the molar absorptivity reported by Patterson and Hammakera3 Using the absorbance values reported by Hammaker, et al., divided by the total molar concenUsing the absorbance values reported tration of the alcohol. by Hammaker, el al., divided by the total molar concentration of the alcohol less twice the molar concentration of the dimer.
these -AH values are below 5 kcal/mole,2i6we interpret them as indicating that the dimer is acyclic (linear) with a single hydrogen bond. The present evidence nom supports the following facts about alcohol self-association. (1) The tetramer is the predominant polymer for unhindered alcohols.2 (2) An acyclic dimer can be seen at 2.86 pm (3500 cm-l),l but its concentration is sufficiently low so that it can usually be ignored in material-balance equations. (3) The dimer overtone band is not readily seen. (4) The alcohol band(s) near 1.53 pm (6536 em-l) are due to a combination of 0-H and C-H stretch modes of the monomer. (3) L. K. Patterson and R. M. Hammaker, Spectrochim. Acta, 23A, 2333 (1967).
(4) C. A. Swenson, J . Phys. Chem., 71, 3108 (1967). ( 5 ) K. H. Illinger and D. E. Freeman, J . Mol. Spectry., 9, 191 (1962). (6) H. C . Van Ness, J. Van Winkle, H. H. Richtol, and H. B. Hollinger, J . Phys. Chem., 7 1 , 1483 (1967).
CHEMISTRY DIVISION RESEARCH DEPARTMENT NAVALWEAPONS CENTER CHISALAKE,CALIFORNIA93555
AARONN.FLETCHER CARLA. HELLER
RECEIVED J.4NUARY 22, 1968
these data do not show any evidence that the dimer is in sufficient concentration so as to affect the materialbalance equation in spite of the high absorbance values found for the dimer at 2.86 pm. Their data for di-tbutyl carbinol (DTBC) are even more conclusive since the tetramer is not formed with this compound due to steric hindrance. We have evaluated their data in Table I. Even with absorbance per unit length values as high as 0.6, their data show no change in El. With DTBC it is possible to calculate the dimer concentration either directly by the equilibrium quotient or through the use of molar absorptivities reported by Patterson and Hammaker.3 Using the dimer concentration ((correction” caimes a perceivable trend in the molar absorptivity in Table I which suggests that the dimer equilibrium quotient of Patterson and Hammaker is too large. A probable source of error in the calculated dimer concentrations is indicated by the molar absorptivity of DTBC being a function of the temperature.1 Swenson4 has presented very convincing evidence that deviations of the molar absorptivity with temperature are due to appreciable amounts of the alcohol existing in the head space above the solution. This causes the calculated concentrations to be too high. These concentration errors would affect the dimer equilibrium study of DTBC performed by Patterson and Hammaker and could explain erroneous equilibrium quotients and molar absorptivities. Since the molar absorptivity should not change with t e m p e r a t ~ r e , ~the J “corrections” to AH performed by Hammaker, et al., should not be made. We consider that the “uncorrected” AH values are correct. Since
A Criticism of the Term cCHydrophobicBond”
Sir: The term “hydrophobic bond,”l that has come into use in the literature of polymers, seems to me to be inappropriate for two reasons. One, the alkyl groups of two polymer chains are not forced together by phobia for water. DuprB,2 as long ago as 1869, pointed out that the excess of the sum of the surface tensions of two immiscible liquids over their interfacial tension is a measure of their work of adhesion: W = y1 yz - ~ 1 , ~Harkins . and Chengj3in 1921, revived this concept and published values for adhesion between water and a number of other liquids. I n the case of water and octane, for example, they give W = 72.80 21.77 - 50.81 = 43.76 ergs cm-2. Furthermore, the energy of evaporating a mole of n-butane from its solution at 1 atm and 25” is 5.46 kcal, greater than it is from its own pure liquid a t the boiling point,4 4.81 kcal. This represents attraction, not phobia. Ice is wet by octane. The fact that octane is nearly insoluble in water is merely the result of the fact that this attraction is not strong enough to penetrate the high cohesion of water.
+
+
(1) For a clear statement of the term, see G. Ku‘Qmethyand H. A. Scheraga, J . Phys. Chem., 6 6 , 1773 (1962). (2) A. DuprB, “Theorie Mechanique de la Chaleur,” Paris, 1869, p 1869. (3) W. D. Harkins and Y . C. Cheng, J . Am. Chem. Soc., 43, 35 (1921). (4) Data from a paper on solutions of inert gases in water, t o be published in Debge Memorial number of J . Amer. Chem. Soc.
Volume 72, Number 6
May 1968
COMMUNICATIOXS TO THE EDITOR
1842 The noun, bond, seems likewise inappropriate because the attraction between the alkyl groups of two polymer chains has none of the characteristics that distinguish chemical bonds from van der Waals forces. Any “simple multiple proportions” between such groups result from regularities in the structures of the two chains, not from any valence forces between alkyl groups. The alkyl chains in micelles of soap are not bonded together by phobia for surrounding mater; they stick together just as strongly in absence of water. Major workers in this field are surely well aware of the fact that there is no true bond between alkyl groups of adjacent polymer chains, and are moreover competent to calculate the thermodynamic quantities involved in the interaction between such chains; why, then, should a terminology continue in use that misleads some into thinking that the “hydrophobic bond” represents a special concept that must be mastered in order to deal with these systems? Why not speak simply of alkyl interaction free energy, energy, or entropy? I do not find it necessary to invent “fluorophobic bonds” in order to handle the thermodynamics of the limited solubility of heptane in perfluoroheptane. I thank NBmethy, Scheraga, and ISautzmann for kindly replying in some detail to my request for their views on this matter. However, I do not agree with their statement that ‘(. . .hydrocarbons actually prefer a nonpolar environment to being surrounded by water.’’ I say rather that molecules of water “prefer” to be hydrogen-bonded together rather than separate to admit alkanes. I n order for these to dissolve, a large amount of water must be present per mole of alkane in order to supply sufficient entropy to offset the unfavorable balance of attractive energies. Gilbert Lewis during a seminar responded to a graduate student who had contradicted him saying, “That is an impertinent remark, but it is also pertinent.” It is in the latter sense only that I offer the above criticism. DEPARTMENT O F CHEMISTRY OF CALIFORRIA URIVERSITY BERKELEY, CALIFORNIA 94720
JOEL
stances, and that they therefore would actually favor mixing. As it was noted repeatedly before12-4 this is indicated by a direct calculation of the relevant energies of interaction, just as it is shown by the data cited by Hildebrand. However, the net free energy of solution, which determines solubility, is dominated by a large negative, i.e., unfavorable excess entropy term.2-6 It has been s h o ~ n that ~ - ~this entropy term arises owing to changes in the state of water and has to be attributed to increased ordering of water molecules, i.e., to an increase in hydrogen bonding. As a result, in spite of the favorable interaction energies, the free energy of solution is positive. This can be expressed by saying that, in over-all terms, ie., in A F ” , hydrocarbons actually prefer a nonpolar environment to being surrounded by water. This is implied in the use of the adjective “hydrophobic.” Because the source of immiscibility is an entropy factor, the water-hydrocarbon system differs qualitatively, and in a unique maimer from most systems of low miscibility. Thus the interactions in this system do represent a special concept. We do not wish to argue about the matter of nomenclature. However, it should be pointed out that the criticism of the use of the term “bond” in the present context, where it does not refer to a chemical bond but to a loose association, has been recognized repeatedly by various worlcers in the past, too. Yevertheless, the term “hydrophobic bond” has proven to be useful as shown by its frequent occurrence in recent physical, chemical, and biochemical nomenclature. H. Hildebrand, J . Phys. Chem., 72, 1841 (1968). (2) W. Kauzmann, Advan. Protein Chem., 14, 1 (1959). (3) G. Nemethy and H. A. Scheraga, J . Phys. Chem., 66, 1773 (1982). (4) G. NQmethy, Angew. Chem., Int. Ed., 6 , 195 (1967). (5) H. S. Frank and M. J. Evans, J . Chem. Phys., 13, 507 (1945). (1) J.
THE ROCKEFELLER UNIVERSITY NEW YORK, NEW YORIC 10021
DEPARTMENT OF CHEMISTRY CORNELL UNIVERSITY H. HILDEBRAND ITHACA, NEW YORK 14850
RECEIVED JANUARY 18, 1968
DEPARTMENT OF CHEMISTRY
GEORGE K~GMETHY HAROLD A. SCHERAGA
WALTER KAUZMANN
PRINCETON UNIVERSITY PRINCETON, NEW JERSEY 08540 RECEIVED FEBRUARY 17, 1968
Comments on the Communication “A Criticism of the Term ‘Hydrophobic Bond’ ”
The Internal Pressure of Simple Liquids
by Joel H. Hildebrand
Sir: Hildebrand’s discussion’ of the nature of the forces of interaction governing the solubility of hydrocarbons is incomplete. It is certainly true, as he states, that van der Waals interactions in themselves are on the balance favorable between the two subThe Journal of Physical Chemistry
Sir: Any general theory of the liquid state must involve expressions for the interaction potentials between the molecules, thermal energy, and volume. Much progress is being made in this field, but the expressions obtained are generally complex and difficult to handle. For many purposes ?t is, therefore,
