Hydrophobic hydration concept

Diamond was the first to focus on the concept of hydrophobic association and demonstrated that, at variance with the Bjerrum theory, ion-pairing of univalent organic electrolytes in water is possible [12]. He capitalized on the hydrophobic hydration concept [11,12] typical of large organic ions (yide supra) that increase the water structure via the formation of ice-like cages, thereby decreasing the system... 

In order to understand the influence of alcohol on the zeolitization process, it is useful to summarize the structural aspects of alcohol-water mixtures. Considerable work has been done in this area. It is well-recognized that at low alcohol concentrations the viscosity, reciprocal self-diffusion coefficient, the dielectric relaxation time and NMR relaxation times of the water molecules are all greater than that of pure water.(21-241 These observations indicate that addition of alcohol to water at low levels leads to an increased structure of water.(25) This concept is also supported by X-ray diffraction studies(26) and is commonly referred to as hydrophobic hydration.(27) On a molecular level, this effect... 

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. 

The knowledge of reliable thermodynamic data of saturated and aromatic hydrocarbons in water is very useful in several practical and theoretical fields, f.i. it is essential for the developments of the concepts of the hydrophobic hydration and of the hydrophobic interaction largely used for the understanding of some important biological phenomena. So new experimental data of high quality are necessary, having the elaborations of the existing material reached their limits. In the last years instead new solubility data in pure water have been reported only for methane(10,53a) ethane(53), cyclo-propane(54), n-butane(9,53a) and benzene(17). Enthalpies of hydration have been furthermore determined by direct calorimetric measurements only for benzene and some derivatives, cyclohexane, pentane and hexane(29). 

Here we present and discuss an example calculation to make some of the concepts discussed above more definite. We treat a model for methane (CH4) solute at infinite dilution in liquid under conventional conditions. This model would be of interest to conceptual issues of hydrophobic effects, and general hydration effects in molecular biosciences [1,9], but the specific calculation here serves only as an illustration of these methods. An important element of this method is that nothing depends restric-tively on the representation of the mechanical potential energy function. In contrast, the problem of methane dissolved in liquid water would typically be treated from the perspective of the van der Waals model of liquids, adopting a reference system characterized by the pairwise-additive repulsive forces between the methane and water molecules, and then correcting for methane-water molecule attractive interactions. In the present circumstance this should be satisfactory in fact. Nevertheless, the question frequently arises whether the attractive interactions substantially affect the statistical problems [60-62], and the present methods avoid such a limitation. 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

The existence of clathrate-like water structure adjacent to the hydrophobic surfaces of macromolecules is an attractive hypothesis. Models have been proposed which have received some support from thermodynamical arguments [808]. However, this concept has proved ineffective as a basis for the interpretation of the structure associated with the many electron density solvent peaks, separated by 2.8 A to 3.0 A, which are frequently observed on the Fourier X-ray maps close to the surface of a protein [809, 810], Recently, however, some local clathrate-like water has been observed in special cases in the high-resolution studies of the small plant protein, crambin [811], in a hydrated deoxydinucleoside-phosphate drug complex [812], in (Phe4Val6) antamanide hydrate [8131 and in an oligodeoxy-nucleotide duplex [814],... 

Thomlinson [78] was the first chromatographer to point out that the classical electrostatic ion-pair concept did not hold for IPRs that were usually bulky hydrophobic ions he also emphasized that in the interfacial region between the mobile and the stationary phases, the dielectric constant of the medium is far lower than that of the aqueous phase. Chaotropes that break the water structure around them and lipophilic ions that produce cages around their alkyl chains, thereby disturbing the ordinary water structnre, are both amenable to hydrophobic ion-pairing since they are both scarcely hydrated. The practical proof of such ion-pairing mode can be found in References 80 and 81 many examples of such pairing modes are reported in the literature [79-86],... 

The main conclusion from the above experimental facts and theoretical concepts to be emphasized is that the hydration energy or the hydrophobicity (hydrophilicity) of a solute depends upon the chemical composition of a given aqueous medium. It seems reasonable to assume that this dependence plays a regulating role in living systems and therefore it should be taken into account while studying the hydropobicity of biological molecules. 

Surfactant self-assembly is a delicate balance between hydrophobic and hydrophilic interactions, and the interactions between the headgroups and the solvent are decisive both for the onset of self-assembly and for the curvature of the surfactant films and thus for aggregate shape and phase behavior. H, H, and NMR have been used successfully to study the hydration of surfactant aggregates. The three by far most used approaches are H (or H) self-diffusion, O quadrupole relaxation, and quadrupole splittings. We stress at the outset that a division into free and bound water molecules on which the concepts of hydration and hydration number are based is far from unambiguous, and furthermore this division is dependent on the physicochemical parameter monitored. 

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