Hydrophobic hydration shell

The ideas of Frank, Evans and Kauzmann had a profound influence on the way chemists thought about hydrophobic effects in the decades that followed However, after the study of the hydrophobic hydration shell through computer simulations became feasible, the ideas about the hydrophobic hydration gradually changed. It became apparent that the hydrogen bonds in the hydrophobic hydration shell are nof or only to a minor extent, stronger than in normal water which is not compatible with an iceberg character of the hydration shell. 

In the traditional view hydrophobic interactions are assumed to be driven by the release of water molecules from the hydrophobic hydration shells upon the approach of one nonpolar solute to another. Although the ideas about the structure of the hydrophobic hydration shell have changed, this view is essentially unaltered... 

The distinction between pairwise and bulk hydrophobic interactions is often made, although some authors doubt the existence of an intrinsic difference between the two ". Pairwise hydrophobic interactions denote the interactions behveen two isolated nonpolar solutes in aqueous solution. They occur in the regime where no aggregation takes place, hence below the critical aggregation concentration or solubility limit of the particular solute. If any breakdown of the hydrophobic hydration shell occurs, it will be only transient. 

Finally, also size and shape of the nonpolar solute seem to influence the formation of hydrophobic hydration shells. Particularly the curvature of the nonpolar surface has been suggested to be... 

The observation that in the activated complex the reaction centre has lost its hydrophobic character, can have important consequences. The retro Diels-Alder reaction, for instance, will also benefit from the breakdown of the hydrophobic hydration shell during the activation process. The initial state of this reaction has a nonpolar character. Due to the principle of microscopic reversibility, the activated complex of the retro Diels-Alder reaction is identical to that of the bimoleciilar Diels-Alder reaction which means this complex has a negligible nonpolar character near the reaction centre. O nsequently, also in the activation process of the retro Diels-Alder reaction a significant breakdown of hydrophobic hydration takes placed Note that for this process the volume of activation is small, which implies that the number of water molecules involved in hydration of the reacting system does not change significantly in the activation process. 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

The chemical outcome of an organic synthesis performed in water depends on the effective concentration of reactants which addresses the problem of hetero-genity. In addition, the hydration sphere must accommodate the product for as long as it forms during the progression of the reaction since the solubility of apolar compounds is attributed to the formation of a hydrophobic hydration shell. 

For larger ions such as Cs+ ions, loose hydrophobic hydration shells are formed around these ions. However, water molecules try to keep their integrity as pure water clusters. Because the direct cation-anion interaction, which will lead to fewer water molecules participating in the hydration shells, is energetically more favorable, the primary coordination number increases faster as a function of salt concentration. For the intermediate ion combination (e.g., Rb+ and Cl"), the ion-water and the ion-ion interactions are in close competition. 

In the previous section we have seen that the formation of hydrophobic hydration shells aids the dissolution of apolar solutes in water. Upon increasing concentration and/or size of the solute it is inevitable that, at a critical concentration, the large hydrophobic hydration shells start to overlap, leading to mutually destructive breakdown of these water arrangements (Fig. 2.6). This sacrifice of H-bonding interactions results in a solvent-induced sticking... 

Figure 2.6 Hydrophobic interactions. Cartoon of the destructive overlap of two hydrophobic hydration shells (striped areas) resulting in a release of water to the bulk solution.

It should be emphasized here that hydrophobic hydration shells are quite voluminous. Computer simulations can be used for estimating hydration numbers. For example, Jorgensen has reported hydration numbers of 20 and 34 for, respectively, methane and n-pentane. A C-NMR study gave a hydration number of 20 for aqueous methane. 

Fig. 6.4 (a) The iceberg modei of hydrophobic hydration, with a hydrophobic particle light grey) surrounded by an ordered shell of water molecules, (b) Kauzmaim s explanatirai of the hydrophobic interaction as an entropic effect due to the liberation of structured water in the hydrophobes hydration shells [28]... 

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