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Hydrophobic Bonding

(Exothermic "bond" breaking?)

Bonding occurs as a result of the attraction between interacting partners, and therefore, an input of energy is required to overcome the attraction, or to break the bond. This aspect of the chemical bond is the central theme of this website.

There are, however, molecular associations, often referred to as "bonding" which are endothermic, so that a release of energy does occur when the "bonding partners" are again separated. This would appear to contradict the central tenant of this website that bond breaking requires energy, but the nature of the "bonding" in these cases has to examined in more detail. The association of non-polar molecules in aqueous solution, or "hydrophobic bonding", represents an example of this kind. Individual non-polar molecules, or groups within a molecule, undergo "hydrophobic hydration" or have water molecules ordered around the non-polar moieties in hydrogen-bonded networks, or "clathrates". This ordered water becomes randomized when these non-polar groups associate, as in lipid bilayer-, or micelle-formation, or globular protein renaturation. The association* is not only entropy driven by virtue of the disruption of the solvent order, but it occurs despite the endothermic nature of the process. The process is endothermic in that the H-bonding between water molecules within the clathrates appears stronger than in the bulk water. In this situation disruption of the hydrophobic association ("bonding") is exothermic but not as the result of breaking an attractive interaction, but due to the formation of the more favourable H-bonding in the clathrates. The references offer greater detail on the nature of the hydrophobic effect, and a review of water, including hydrophobicity can be found in a University College London website.

*From its early history the "bonding" aspect of the term "hydrophobic bonding" has been questioned, and as a consequence the phenomenon is often referred to as the "hydrophobic effect" or "hydrophobic interaction" rather than bonding.

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