Hydrophobic Effect

Themis Lazaridis, City College of New York, New York, New York, USA

Published online: January 2013

DOI: 10.1002/9780470015902.a0002974.pub2

The term ‘hydrophobic effect’ refers to the poor solubility of nonpolar substances in water compared to organic solvents or to polar substances. The transfer of small nonpolar molecules from the gas phase or organic solvents to water has a characteristic thermodynamic signature: positive free energy, negative enthalpy, large negative entropy and positive heat capacity. This thermodynamic signature can be explained by considering the structure of water around nonpolar substances, which depends on the size and shape of the nonpolar solute. The poor solubility of nonpolar groups in water leads to aggregation of these groups (hydrophobic interaction) and the formation of self-assembled structures such as miscelles and lipid bilayers. The hydrophobic interaction is also the major contributor to protein folding. The origin of the hydrophobic effect lies in the fact that water interacts with itself much more strongly than it does with nonpolar groups.

Key Concepts:

  • The origin of hydrophobicity lies in the strong water–water interactions.
  • The hydrophobic effect is entropic or enthalpic depending on temperature and the geometry of the associating solutes.
  • The characteristics of hydrophobic hydration differ depending on the length scale of the solute.
  • The hydrophobic effect is responsible for the formation of lipid bilayers and the folding of proteins.
  • Interaction between small hydrophobic solutes is in many ways different from transfer to a bulk phase.
  • Most studies find anticooperativity in the interaction between three small nonpolar solutes.
  • Very long-range attractions between hydrophobic surfaces are probably due to macroscopic phenomena, such as air bubbles.

Keywords: water; hydrophobic; nonpolar; solvation; hydration; hydrogen bonding

Figure 1. The thermodynamics of transfer of ethane from carbon tetrachloride to water. Th and Ts represent the temperatures at which H° and S° of transfer are zero, respectively. At Ts, G° is at its maximum and at Th, G°/T is at its maximum and the partition coefficient Kd=exp (–G°/RT) is at its minimum.
Figure 2. Representation of a single water molecule and the structure of ice I. (a) Water's two hydrogen atoms and two electron pairs are oriented tetrahedrally. (b) In ice I, the tetrahedra are oriented so each water molecule is hydrogen bonded to its four neighbours.
Figure 3. Favourable orientations of water at the surface of a solute that is (a) nonpolar, (b) hydrogen bonding and (c) polar but not hydrogen bonding (the dashed line indicates the dipole axis).
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Related Sub-Topics:

Protein Folding, Evolution and Degradation | Biomolecular Interactions
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Article Title: Hydrophobic Effect
 
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Lazaridis, Themis(Jan 2013) Hydrophobic Effect. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002974.pub2]