Water molecules cluster

The third step is solvation of the ions by solvent molecules. Water molecules cluster around each ion, oriented to give attractive ion-dipole interactions. This step releases energy. Although each individual ion-dipole interaction is weak, each ion forms from four to eight such interactions, depending on the size of the ion and the concentration and temperature of the solution. Taken together, the vast number of ion-dipole interactions results in a substantial release of energy. 

The different hydration numbers can have important effects on the solution behaviour of ions. For example, the sodium ion in ionic crystals has a mean radius of 0 095 nm, whereas the potassium ion has a mean radius of 0133 nm. In aqueous solution, these relative sizes are reversed, since the three water molecules clustered around the Na ion give it a radius of 0-24 nm, while the two water molecules around give it a radius of only 017 nm (Moore, 1972). The presence of ions dissolved in water alters the translational freedom of certain molecules and has the effect of considerably modifying both the properties and structure of water in these solutions (Robinson Stokes, 1955). 

When an ionic compound is dissolved in a solvent, the crystal lattice is broken apart. As the ions separate, they become strongly attached to solvent molecules by ion-dipole forces. The number of water molecules surrounding an ion is known as its hydration number. However, the water molecules clustered around an ion constitute a shell that is referred to as the primary solvation sphere. The water molecules are in motion and are also attracted to the bulk solvent that surrounds the cluster. Because of this, solvent molecules move into and out of the solvation sphere, giving a hydration number that does not always have a fixed value. Therefore, it is customary to speak of the average hydration number for an ion. 

Finally in this section, we refer to classic studies on gas phase interactions carried out with a pulsed electron beam high ion source mass spectrometer, which have yielded details of hydrogen bonding of substituted pyridinium ions to water in the gas phase (79JA1675). These measurements afford thermodynamic data for the stepwise hydration of pyridinium ions XC6H4NH(OH2)n for values of n varying between 0 and 4. The attenuation of substituent effects is much less than for aqueous solution, because although the water molecules cluster round NH in the gas phase, they cannot provide an overall solvation network, the dielectric constant of which in the liquid phase serves to reduce the influence of the substituent dipole. 

Barium hydroxide octahydrate, Ba(OH)2 8 H20, is a crystalline compound that contains eight water molecules clustered around the barium atom. We ll learn more about hydrates in Section 14.15. 

Makogon (1974) was the first to incorporate the above concepts into a hydrate nucleation mechanism, indicating that water molecules cluster with a decrease in temperature. 

FIGURE 3.3 Stereographic view of water molecules cluster alignment by a dissolved apolar molecule (large circles) from Monte Carlo computer simulation studies. Top figure shown with lines connecting water. (Reproduced from Swaminathan, S., Harrison, S.W., Beveridge, D.L., J. Am. Chem. Soc., 100, 5705 (1978). With permission from the American Chemical Society.)... 

The second assumption consists in the fact, that the diffusion of water molecules clusters without consideration of the molecules OP-7 sizes is considered. As a matter of fact, this means, that in cluster of molecules H20 is assumed the replacement of one of these molecules on molecule OP-7. The clusterization of water molecules at interaction with polymer is a well known fact [5, 6], The estimations show, that in this case the cluster consists of three molecules H20 [6], The schematic representation of water cluster according to the data of paper [5] is shown in Figure 1. This scheme allows to calculate the largest size of cluster <7m 7,8 A allowing, that water molecule diameter is equal to 3,08 A [5],... 

Figure 1. The model of adsorbed by polymer water molecules cluster [5]. dH Q is diameter of water molecule, dm is calculated diameter of water cluster.

Although no hexane molecules were found in the protein s interior for the CTWAT and CTMONO systems, hydrophobic contacts were observed between hexane molecules near the protein surface and hydrophobic side chains in all three systems. Hexane molecules on the protein surface tend to reside in the surface "clefts" formed by the hydrophobic side chains extended into the hexane solvent. At the same time, the hydrophilic residues tended to fold back onto the surface of the protein in order to minimize surface contacts. In our CTMONO simulation, we further observed the water molecules clustered around charged hydrophilic residues, while leaving the hydrophobic residues exposed to the soIvent.(Fig. 1) It has been reported that preferential solvation of the hydrophobic regions of the protein surface by the non-polar solvent is due to the thermodynamically unfavorable formation of a complete monolayer of water in a non-polar solvent. Klibanov and co-workers have also shown that hexane does not strip the water layer - nor does it immobilize the water molecules at the protein/solvent interface. Instead, rearrangements of the water molecules on the protein surface is the more favored process. Our simulations clearly support these experimental observations. 

The base plan unit-cell projection (Figure 2) shows the positions of the water molecules in the unit-cell as deduced from an analysis of the equatorial x-ray diffracted intensities for a fiber diagram recorded at about 50X relative humidity. The water molecules cluster in distinct areas and form a helical column whose symmetry matches that of the xylan chains. In fact, the water of hydration may dictate the symmetry of the xylan chains. The energetically (theoretical) most stable conformation of the xylan chain involves twofold symmetry, whereas in the hydrated crystalline environment, as deduced from x-ray diffraction, the xylan chains possess three-fold symmetry. The water molecules stabilize the three-fold structure by the formation of hydrogen bonds. This structure is an example of columnar hydration which allows a symmetric... 

FIGURE 2-6 Water as solvent. Water dissolves many crystalline salts by hydrating their component ions. The NaCl crystal lattice is disrupted as water molecules cluster about the CC and Na" ions. The ionic... 

The determination of clustered water and water molecule clusters trapped in polyethylene have been described by Baker (209). This water has been related lo a loss in the dielectric properties of polyethylene used in a submarine cable core. When DSC is employed, as shown by the curve in Figure... 

Recall that (aq) means the substance is hydrated—it has water molecules clustered around it. 

In fluid solutions, the resolvation times can be in the subnanosecond time regime. For example, the rapid (<100 ps) relaxation of the excited-state absorption spectra of ruthenium polypyridyl complexes following metal-to-ligand charge-transfer (MLCT) excitation in aqueous solutions have been ascribed to diffusional resolvation of the MLCT excited state. Finally, Robinson and co-workers have provided evidence that the rate of ionization of the singlet excited state of 6-p-toluidine-2-naphthalenesulfonate is determined by the rate at which neighboring solvent fluctuations can form a 3-4 water molecule cluster capable of solvating the electron. 

These two different points of view were explained in Sec. 2.5 in connection with the primitive and cluster-primitive onedimensional models for water. In the primitive model single water molecules are defined through their pair potential. The structure formed by these water molecules — clusters of HBed molecules — is a result of the specific pair potential. On the other hand, in the cluster primitive model, the structures — clusters of HBed molecules — are assumed to be a part of the description of the model, and the HBs are now part of the internal description of the clusters. 

Phenol has been interesting many chemists as a model system to study how photo-excited bases in DNA can deactivate to the ground state without resulting in mutation. Not only in such biology-oriented studies, this molecule shows many other interesting features when put in surrounding molecules, clusters, and solvents. Small ammonia clusters are frequently used in place of water-molecule clusters, because ammonia is more proton-attractive than water, and very extensive studies have been performed... 

Hydration shells form. As an ion separates, water molecules cluster around it in hydration shells. The number of water molecules in the innermost shell depends on the ion s size four fit tetrahedrally around small ions like Li", while the larger Na+ and F have six water molecules surrounding them octahedrally (Figure 13.2). In the innermost shell, normal H bonding is disrupted to form the ion-dipole forces. But these water molecules are H bonded to others in the next shell, and those are H bonded to others stiU farther away. 

When a salt such as NaCl dissolves, ion-dipole forces cause the ions to separate, and many water molecules cluster around each ion in hydration shells. Ion-dipole forces bind the first shell to an ion. The water molecules in that shell form H bonds to others to create the next shell, and so on. 

Moreover, Ben-Naim showed by calculation that one source of the solvent effect on dimerization, and obviously on micellization, is that the nature of the volume available to the solvent molecules is restored in such a way during dimer formation as to increase the entropy of the solvent. By analogy, the formation of micelles reduces the overall area occupied by individual solute molecules and, accordingly, is driven by a strong entropic increase. This approach is actually verging on the well-known hydrophobic effect related to ordered water molecule cluster formation around hydrophobic molecules. 

This catalytic mechanism is consistent with data from X-ray crystallography, site-specific mutagenesis, and various biochemical studies. In crystal structures of E. coli TEM-1 )8-lactamase (34) and other related class A )8-lactamases (35—37), the side chains of the conserved residues (Ser-70, Lys-73, Ser-130, and Glu-166) are, together with a water molecule, clustered around the bound substrate (penicillin G). The presence of a methoxy or hydroxymethyl group on the a face of the j8-lactam ring, e.g., in cefoxitin or 6a-(hydroxymethyl)penicillanic acid, results in a displacement of the water (-712), thereby allowing the mechanism-based inhibitors to form rather stable acyl-enzyme intermediates (38). Substitution of either Lys-73 by Arg or of Ser-130 by Ala or Gly impairs the acylation step. Substitution of Glu-166 by Asn or Ala drastically reduces the deacylation step, leading to the accumulation of the acyl-enzyme intermediate (39,40). 

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