Water hydration shells

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 interaction between a solute and a solid phase is also influenced by water. Hydration shells or icebergs associated with one or the other phase are destroyed or created in this interaction and often contribute to conformational changes in macromolecular structures — and ultimately to changes in biological and functional properties important in food processing. 

In several systems, interfacial water, which is associated with the hydrophilic surfaces (polar groups and counterions) of surfactant microstructures, is present. This kind of water is also called bound water, hydration shell, hydration water, solvent shell [182], or vicinal water [171]. This water can be operationally defined as water detected by a certain technique as it had been influenced by the surface of the substrate in contact with the water [177]. The presence of the microstructure surface may alter the thermodynamic properties (such as melting point, melting enthalpy and entropy, and heat capacity) and the spectroscopic properties (such as IR absorption frequencies and band shapes) of water [61,214]. The chemical potential of bound water is different from that of bulk water [216]. Properties of bound water (viscosity, density, fl-eezing point, etc.) adsorbed on different surfaces of adsorbents differ from those of bulk water [216-223]. 

In general, anions are less strongly hydrated than cations, but recent neutron diffraction data have indicated that even around the halide ions there is a well defined primary hydration shell of water molecules, which, in... 

The complete hydration shell of the proton consists of both the central FI O unit and fiirther associated water molecules mass spectrometric evidence would suggest that a total of four water molecules fomr the actual FIgOj unit, givmg a hydration number of four for the proton. Of course, the measurement of this number by... 

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Taking into account the hydration shell of the NA and the possibility of the water content changing we are forced to consider the water -I- nucleic acid as an open system. In the present study a phenomenological model taking into account the interdependence of hydration and the NA conformation transition processes is offered. In accordance with the algorithm described above we consider two types of the basic processes in the system and thus two time intervals the water adsorption and the conformational transitions of the NA, times of the conformational transitions being much more greater... 

The Langevin model has been employed extensively in the literature for various numerical and physical reasons. For example, the Langevin framework has been used to eliminate explicit representation of water molecules [22], treat droplet surface effects [23, 24], represent hydration shell models in large systems [25, 26, 27], or enhance sampling [28, 29, 30]. See Pastor s comprehensive review [22]. 

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. 

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first 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 theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

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... 

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