Enthalpic hydrophobic effect

Hydrophobic effects arise from the exclusion of non-polar groups or molecules from aqueous solution. This situation is more energetically favourable because water molecules interact with themselves or with other polar groups or molecules preferentially. This phenomenon can be observed between dichloromethane and water which are immiscible. The organic solvent is forced away as the intersolvent interactions between the water molecules themselves are more favourable than the hole created by the dichloromethane. Hydrophobic interactions play an important role in some supramolecular chemistry, for example, the binding of organic molecules by cyclophanes and cyclodextrins in water (see Chapter 2, Sections 2.7.1 and 2.7.5, respectively). Hydrophobic effects can be split into two energetic components, namely an enthalpic hydrophobic effect and an entropic hydrophobic effect. 

Figure 1.22 The displacement of water molecules from a hydrophobic cavity is responsible for the enthalpic hydrophobic effect.

Usually, dissolution of a small amount of one compound in a pure liquid is enthalpically unfavourable and driven by an increase in (mixing) entropy. At room temperature, the opposite is true for the dissolution of a small apolar compound in water. This unexpected behaviour is referred to as the hydrophobic effect [4]. Classically, this effect has been rationalised by ordered water structures around apolar compounds (entropy reduction) and the increase in number... 

Hydrophobic binding. The hydrophobic effect can have both enthalpic and entropic components, although the classical hydrophobic effect is entropic in origin (Section 1.9.1). Studies on the associations between planar aromatic molecules show an approximately linear relationship between the interaction energy and their mutual contact surface area with slope 64 dyn cm-1, very close to the macroscopic surface tension of water (72 dyn cm-1). Hence, in the absence of specific host or guest interactions with the solvent the hydrophobic effect can be calculated solely from the energy required to create a free surface of 1 A2 which amounts to 7.2 X 10 12 J or 0.43 kjA 2 mol. ... 

Earlier literature has used the term hydrophobic bond, but it is clear from the above discussion that no special hydrophobic force exists. Nonpolar groups self-associate in water because their dispersal throughout the solvent would be entropically unfavorable. Once they come together and water is largely excluded, enthalpically favorable interactions are possible, but these are just (for nonaromatic hydrocarbons) the normal weak London forces between any polarizable groups. There is no bonding that is specifically hydrophobic. The correct term is hydrophobic effect. 

The primary contribution to the hydrophobic effect comes from the favorable increase in system entropy upon the release of ordered first-shell waters from nonpolar surface areas on both the macromolecule and the ligand upon ligand binding there are also secondary enthalpic contributions. 

In fact, hydrophobic molecules such as hydrocarbons are excluded from water through the same mechanisms as outlined above. Because they cannot dissolve in water they must form an interface. The same forces that defy gravity to minimize the surface area of a water droplet drive the segregation of hydrophobic compounds to minimize their interfacial contact with water. This is the macroscopic manifestation of the hydrophobic effect. It is a complex phenomenon with both enthalpic and entropic components, but under physiological conditions the... 

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