Hydrophobic hydration changes

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 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... 

It is well known that the transfer of nonpolar molecules from nonpolar to polar surroundings results in a decrease in the partial molar volume of the solute. The dimerization studies also show that there is a similar volume decrease when two monomers form a dimer. This volume decrease is of the order of 20 cm3 mol-1. It is difficult to understand how there can be first a volume decrease when the nonpolar molecules are transferred from the nonpolar to the polar environment and then a further volume decrease when two molecules come together and partly reverse the first transfer. It is a little dangerous to speak of the partial reversal of a process we know so little about. It is believed that the hydrophobic hydration is a cooperative phenomenon, in which the exact microstructure of water is very important for the occupied volume. How this microstructure changes when two molecules associate in a hydrophobic interaction is not par-... 

The behavior of hA in real micellar systems is more complex as seen in Fig. 2.12. Similar data have been obtained for several other amphiphiles148,149). The deviations in hA from the standard value at infinite dilution appear clearly below the CMC, but at these concentrations one has a compensating change in the partial molar entropy. This effect might be due to a repulsive interaction between the hydrophobically hydrated alkyl chains leading to a breakdown of the water structure with a concomitant increase in entropy. 

When the surfaces of the particle and the sorbent are predominantly hydrophilic it is probable that in the adhered state some hydration water is retained at their surfaces preventing intimate contact. However, if the surfaces are hydrophobic dehydration would stimulate adsorption and adhesion. The contribution from changes in hydration to AGad may be estimated from the partitioning of model compounds between water and a non-aqueous medium.18 Hydration changes involve only a few layers of water molecules and hence are effective only over a separation distance of less than a nm. 

Desnoyers, J.E. and Arel, M. Apparent molal volumes of n-alkylamine hydrobromides in water at 25°C hydrophobic hydration and volume changes. Can. J. Chem. 1967, 45, 359-366. 

In this equation R4N+. . . (H20)n denotes the resulting hydrophobic entity and K is an equilibrium constant. The enthalpic effect of hydro-phobic hydration then can be considered as the result of the formation of this hydration complex. In both models the choice of the cosolvent ought to be rather unimportant as long as this solvent does not show specific interactions like hydrogen bonding. As a consequence n and HbW should not vary with the different cosolvents. Besides that, the basic assumption in the concepts is that in the absence of hydrophobic hydration AH° would change proportionally to the solvent composition. In this chapter we will investigate more systematically both aspects. 

Water is well known for its unusual properties, which are the so-called "anomalies" of the pure liquid, as well as for its special behavior as solvent, such as the hydrophobic hydration effects. During the past few years, a wealth of new insights into the origin of these features has been obtained by various experimental approaches and from computer simulation studies. In this review, we discuss points of special interest in the current water research. These points comprise the unusual properties of supercooled water, including the occurrence of liquid-liquid phase transitions, the related structural changes, and the onset of the unusual temperature dependence of the dynamics of the water molecules. The problem of the hydrogen-bond network in the pure liquid, in aqueous mixtures and in solutions, can be approached by percolation theory. The properties of ionic and hydrophobic solvation are discussed in detail. 

Lee B, Graziano G. A two-state model of hydrophobic hydration that produces compensating enthalpy and entropy changes. J. Am. Chem. Soc. 1996 118 5163-5168. 

If e milliliters of free water are bound electrostrlctlonally to the ionic groups whereupon they occupy a volume of e milliliters, the volume change due to the electrostrlction, Ep, is expressed as Ep= e - e. In the same way, Hp= h - h and Pp" p - p, where h and p represent the volume of free water which are changed to volumes h and p, respectively, by the hydrophobic hydration... 

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