Water structure, order

S. Sastry, Nature, 409, 300 (2001). Water Structure Order and Oddities. 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

Solvation and especially hydration are rather complex phenomena and little is known about them. Depending on the kind of molecular groups, atoms or ions interacting with the solvent, one can differ between lyo- or hydrophilic and lyo-or hydrophobic solvation or hydration. Due to these interactions the so-called liquid structure is changed. Therefore it seems to be unavoidable to consider, at least very briefly, the intermolecular interactions and the main features of liquids, especially water structure before dealing with solvation/hydration and their effects on the formation of ordered structures in the colloidal systems mentioned above. 

Recently Blake et al.153) made such studies in the case of human (HL) and tortoise egg-white (TEWL) lysozyme based on crystallographic refinements at 1,5 and 1,6 A resolution, respectively. By these investigations they attempted to obtain information on the perturbations of water structure in the hydration shell by neighboured protein molecules and by high salt concentrations as well as on the degree of order of the bound water. The authors came to the conclusion that the number of ordered water molecules are 128 in TEWL and 140 in HL, whereas the overall content is made up of 650 and 350 water molecules per lysozyme molecule. 

This strnctnring of liqnids into discrete layers when confined by a solid surface has been more readily observable in liquid systems other than water [1,55]. In fact, such solvation forces in water, also known as hydration forces, have been notoriously difficult to measure due to the small size of the water molecule and the ease with which trace amounts of contamination can affect the ordering. However, hydration forces are thought to be influential in many adhesive processes. In colloidal and biological systems, the idea that the hydration layer mnst be overcome before two molecules, colloidal particles, or membranes can adhere to each other is prevalent. This implies that factors affecting the water structure, such as the presence of salts, can also control adhesive processes. 

Akiyama, R. and Hirata, F. (1998) Theoretical study for water structure at highly ordered surface Effect of surface structure./. Chem. Phys., 108,4904 911. 

Protein structure (and also the interactions between proteins and small molecules) is a compromise. It may be necessary to sacrifice a hydrogen bond or two in order to gain two or three hydrophobic interactions. In contrast, it may be necessary to place a hydrophobic residue in contact with water in order to pick up a few more hydrogen bonds in... 

As for the intrastrand adduct, however, the platinum atom is forced out of the plane of the bases by 0.3-0.6 A. One study revealed a very well ordered water structure around the site of the... 

Teeter, M. M. Order and disorder in water structure of crystalline proteins. Developments in Biological Standardization, Vol. 74, p. 63-72. Acting Editors Joan C. May - F. Brown. S. Karger AG, CH-4009 Basel (Switzerland), 1992... 

Errington, J.R. and Debenedetti, P.G. 2001. Relationship between structural order and the anomalies of liquid water. Nature 409, 318-321. 

Usually, dissolution of a small amount of one compound in a pure liquid is enthalpically unfavourable and driven by an increase in (mixing) entropy. At room temperature, the opposite is true for the dissolution of a small apolar compound in water. This unexpected behaviour is referred to as the hydrophobic effect [4]. Classically, this effect has been rationalised by ordered water structures around apolar compounds (entropy reduction) and the increase in number... 

Figure 4 The path traversed in structural order-metric space as liquid water (SPC/E) is compressed isothermally at two different temperatures. Filled diamonds represent T = 260 K, and open triangles represent T = 400 K. The arrows indicate the direction of increasing density. A and C are states of maximum tetrahedral order at the respective temperatures, whereas B is a state of minimum translational order. Reprinted with permission from Ref. 29. 

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

A large portion of the calculated entropy change (+ or -) results from the relative ordering or disrupting of water structure during adsorption. 

For a specular rod, the speetmm intensity depends on the structure of both the electrolyte and the eleetrode, whereas for a nonspecular rod, the intensity depends only on the electrode surface structure, because in the plane parallel to the interface, the water is not ordered well. It was eoncluded that water is ordered in a layer extending about three molecular diameters (9-10 A) from the electrode. The extent of the order depends on the potential, and the distance between the electrode and the layer of oxygens is shorter at positive than at negative potentials. The latter result can be regarded as evidence of the reorientation of water molecules within... 

The carboxylic acids were combined with hexylamlne and water in order to study the association structures formed. The hexyl-amine was chosen because it did not by Itself form a liquid crystalline phase with water. Fig. 3A. Water dissolves in the amine to a maximum of 60% to form an isotropic solution. The liquid crystal is formed first at a certain octanoic acid amine ratio, approximately 0.1. The liquid crystalline phase forms a large region reaching to a weight fraction of 0.61 of the acid, corresponding to a 1 1 molar ratio of the two species. 

Confinement of water into regions with dimensions of only a few nanometers, such as typically those found in PEMs, accompanied by a strong electrostatic field due to the anions, will result in a significantly lower dielectric constant for the water than that observed in bulk water. Measurement of this structural ordering of the water has not been accomplished experimentally to date, and this was the motivation to the recent calculation of the dielectric saturation of the water in PEMs with an equilibrium thermodynamical formulation. " In addition to information concerning the state of the water this modeling has provided information concerning the distribution of the dissociation protons in sulfonic acid-based PEMs. 

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