Is There a "Hydrophobic Effect"? JOEL H
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Proc. Natl. Acad. Sci. USA Vol. 76, No. 1, p. 194, January 1979 Biochemistry Is there a "hydrophobic effect"? JOEL H. HILDEBRAND Department of Chemistry, University of California, Berkeley, California 94720 Contributed by Joel H. Hildebrand, October 17,1978 ABSTRACT This paper presents facts that disagree with at 250C in CC14 it is 2.89 cm2-sec-1. If the molecules of methane the hypothesis of "a hydrophobic effect." were encased in "icebergs," they could not diffuse 0.6 as rapidly in water as in CCI4. In 1968 I published "A Criticism of the Term 'Hydrophobic Miller and I (7) in 1968 published a paper on "Solutions of Bond' " (1) in which I gave evidence that the alkyl groups in Inert Gases in Water" in which the following was stated in the polymer chains in water are not forced together by "phobia" abstract: for water. In subsequent papers I gave further evidence that throws light upon the forces acting between water and alkanes. Data for the solubility in water of gases ranging from These facts have been largely overlooked by biologists, as il- Ne to n-C4HX0 are reviewed and compared with their lustrated by a recent paper on "The Hydrophobic Effect and solubilities in c-C6HI2. Entropies in the two solvents are the Organization of Living Matter" (2). Alfred North White- very different in amount and origin. A variety of expla- head has written: "An unflinchible determination to take the nations have been offered to account for the losses of en- whole evidence into account is the only method of preservation tropy caused by dissolving neutral molecules in this al- against the fluctuating extremes of fashionable opinion." In- ready highly structured solvent. Most of them assume ei- deed, no scientist can afford, in the long run, to overlook any ther that water molecules form more rigid structures fact that is pertinent to understanding the phenomenon that around solute molecules, or else that water is a labile he is studying. In this paper I set forth such facts. mixture of different structures. We regard such models Dupre (3) pointed out in 1869 that the "work of adhesion" as open to question and propose instead an explanation between two immiscible liquids can be calculated from the sum based upon the Pople model of water molecules all of the surface energies of the pure liquids less the energy of their bonded together by the maximum number of flexible interface. In 1921, Harkins and Cheng (4) published values of hydrogen bonds, all participating equally in thermal en- the adhesion between a large number of pairs of immiscible ergy. When inert molecules are introduced, we suggest liquids; in the case of water and octane at 200C, for example, that H-bonds are deactivated or destroyed to an extent they give 72.80 + 21.77 - 50.81 = 43.76 erg-cm-2. This is at- depending upon the total surface of the solute. The en- traction, not hydrophobia. tropy of solution of nine gases to the same mole fraction Linford et al. (5), in 1970, reported values of the entropy and accordingly varies linearly with the two-thirds power of Gibbs energy of interfaces of water and n-hexane and of water their molal volumes at their boiling points. The losses of and perfluorotributylamine between 15 and 450C. Their entropy that occur when equal surfaces of water and liq- values give the "work of adhesion" between water and hexane uid alkanes unite to form interfaces show virtually the at 250C to be 72.0 + 17.9 - 50.4 = 39.5 erg-cm-2. The adhesion same dependence upon the molal surfaces of the alkanes between water and the fluorocarbon was found to be 72.0 + as is shown by the gaseous alkanes. 16.35 - 40.7 = 47.7 erg-cm-2. The alkane interface -gained entropy with temperature to the amount of 0.089 erg-cm-2 In conclusion, there is no hydrophobia between water and deg'1; the fluorochemical interface lost entropy, -0.086 erg- alkanes; there only is not enough hydrophilia to pry apart the cm 2-deg'1. Not surprisingly, more hydrogen bonds are de- hydrogen bonds of water so that the alkanes can go into solution stroyed by contact with a fluorochemical than with an al- without assistance from attached polar groups. kane. The loss of entropy when an alkane gas dissolves in water has 1. Hildebrand, J. H. (1968) J. Phys. Chem. 72, 1841. been explained by the formation of what has been facetiously 2. Tanford, C. (1978) Science 200, 1012-1018. called an "iceberg" around the solute molecules. Any such as- 3. Dupre, A. (1869) Theorie de la Chaleur (Paris). sumption is not tenable (6); the viscosities of water and carbon 4. Harkins, W. D. & Cheng, Y. C. (1921) J. Am. Chem. Soc. 43, tetrachloride at 250C are nearly the same, 0.880 and 0.895 cp; 35-53. and the diffusivity of methane in water 105D is 1.72 cm2-sec-1; 5. Linford, R. E., Powell, R. J. & Hildebrand, J. H. (1970) J. Phys. Chem. 74,3024-3025. The publication costs of this article were defrayed in part by page 6. Hildebrand, J. H. (1969) Proc. Natl. Acad. Sci. USA 64,1329- charge payment. This article must therefore be hereby marked "ad- 1330. vertisement" in accordance with 18 U. S. C. §1734 solely to indicate 7. Miller, K. & Hildebrand, J. H. (1968) J. Am. Chem. Soc. 90, this fact. 3001-3004. 194 Downloaded by guest on September 29, 2021.