Frank and Evans

The solvation thermodynamics have been interpreted in a classical study by Frank and Evans in terms of the iceberg model . This model states that the water molecules around an nonpolar solute show an increased quasi-solid structuring. This pattern would account for the strongly negative... 

As shown by Frank and Evans 41 , solutions of apolar substances in water are characterized by a large entropy of mixing, leading to a high positive free energy of dissolving. 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

The most popular explanation of such changes in solubility stems from the hypothesis of Frank and Evans (1945), that the presence of the solute molecule causes an increase in the order of bulk water molecules, with the formation of water hydration shells around solute molecules. This is with the caution that such ordering... 

The crystallinity was improved by the presence of such electrolytes as NaCl and CaS04 even though they did not enter into the reaction. The role of these electrolytes is not understood. Frank and Evans (1945), Bradley and Krause (1957) and others have shown that large anions such as CP and S042- tend to breakdown the water structure. This would presumably increase the mobility of the small clay building cations (which are water structure-formers) and facilitate their organization. 

The interest in hydrophobic interactions was stimulated by their unusual thermodynamic properties it was argued and believed that they are governed, not by enthalpic, but by entropic features, characterized by the undesirable entropy decrease of water in the vicinity of nonpolar groups (Frank and Evans, 1945 Kauzmann, 1959 Franks, 1975 Tanford, 1980). This conclusion was reached largely from consideration of solvation effects at room temperature. 

The enthalpy of dissolution of gaseous nonpolar molecules into water is always negative at room temperature, and its absolute value is proportional to the accessible surface area of the solute molecule (Frank and Evans, 1945 Tanford, 1980 Dec and Gill, 1984, 1985a,b Olofsson et al., 1984). 

This hydrophobic hydration was first postulated by Frank and Evans in 1945. They wrote The nature of deviation found for non-polar solutes in water leads to the idea that the water forms frozen patches or microscopic icebergs around such solute molecules. The word iceberg represents a microscopic region, surrounding the solute molecule, in which water molecules are tied together in some sort of quasi-solid structure [226]. 

Bockris in 1949 and by Frank and Evans in 1957. According to these workers, there are two regions. One is in the first (and for polyvalent cations, the second) layer near the ions, where water molecules are tightly bound and give rise to new frequencies. Such waters accompany the ion in its movements in the solution. 

Frank and Evans, in studying the numbers for the hydrational entropies of ordinary monatomic ions, found them insufficiently negative, indicating that, due to structure breaking, the entropy of the ion itself should be larger than that expected if only the ordering effect of the ion is considered. 

These thermodynamic approaches to hydrophobic effects are complemented by spectroscopic studies. Tanabe (1993) has studied the Raman spectra manifested during the rotational diffusion of cyclohexane in water. The values of the diffusion coefficients are approximately half those expected from data for other solvents of the same viscosity, and the interpretations made are in terms of hindered rotation arising from the icebergs presumably formed (c/. Frank and Evans) around the cyclohexane. 

The correlation volume was not accounted in the Butler s scheme. Butler s scheme for dissolution in water accoimted only for the formation of a cavity, introduction of the solute molecule in that cavity, and its interactions with the nearest-neighbor water molecules. He assumed, however, that the water molecules are distributed around a solute molecule as randomly as in its absence. One more step should be added, namely, the formation of a hydrophobic layer of volume around the cavity, in which the water molecules are reorganized and are no longer randomly distributed (Figure 2). While this layer is similar to that suggested by Frank and Evans in their iceberg ... 

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