The solvent structure

The structural stability and the biological activity of protein molecules are dependent upon the interactions of the protein with solvent. Protein crystals typically contain 50% solvent and this component needs to be accounted for in the calculation of structure factors. In turn, the refinement of crystal structures and their solvent content leads to a description of the ordered water molecules and the bulk solvent continuum. 

An alternative representation of the bulk solvent has been developed by D.I. Stuart and P.J. Artymiuk and A. Leslie (unpublished results) following methods used in fibre 

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For a substance to dissolve in a liquid, it must be capable of disrupting the solvent structure and permit the bonding of solvent molecules to the solute or its component ions. The forces binding the ions, atoms or molecules in the lattice oppose the tendency of a crystalline solid to enter solution. The solubility of a solid is thus determined by the resultant of these opposing effects. The solubility of a solute in a given solvent is defined as the concentration of that solute in its saturated solution. A saturated solution is one that is in equilibrium with excess solute present. The solution is still referred to as saturated, even... 

W. R. Fawcett. Molecular models for the solvent structure at polarizable interfaces. Israeli J Chem 75 3-16, 1979. 

Similar observations hold for solubility. Predominandy ionic halides tend to dissolve in polar, coordinating solvents of high dielectric constant, the precise solubility being dictated by the balance between lattice energies and solvation energies of the ions, on the one hand, and on entropy changes involved in dissolution of the crystal lattice, solvation of the ions and modification of the solvent structure, on the other [AG(cryst->-saturated soln) = 0 = A/7 -TA5]. For a given cation (e.g. K, Ca +) solubility in water typically follows the sequence... 

This subroutine calculates the three radial distribution functions for the solvent. The radial distribution functions provide information on the solvent structure. Specially, the function g-AB(r) is die average number of type B atoms within a spherical shell at a radius r centered on an aibitaiy type A atom, divided by the number of type B atoms that one would expect to find in the shell based cm the hulk solvent density. 

In concentrated NaOH solutions, however, the deviations of the experimental data from the Parsons-Zobel plot are quite noticeable.72 These deviations can be used290 to find the derivative of the chemical potential of a single ion with respect to both the concentration of the given ion and the concentration of the ion of opposite sign. However, in concentrated electrolyte solutions, the deviations of the Parsons-Zobel plot can be caused by other effects,126 279"284 e.g., interferences between the solvent structure and the Debye length. Thus various effects may compensate each other for distances of molecular dimensions, and the Parsons-Zobel plot can appear more straight than it could be for an ideally flat interface. 

Guidelli and co-workers336-338 measured the potential of zero charge by chronocoulometry. They found that the pzc was independent of the electrolyte concentration in both NaC104 and Na2S04. However, Ea=0 in the presence of sulfates was ca. 40 mV more negative. These authors have explained this apparent discrepancy in terms of the perturbation of the solvent structure at the interface by the ions at the electrode surface, which are, however, nonspecifically adsorbed. 

Considerable progress has been made in the last decade in the development of more analytical methods for studying the structural and thermodynamic properties of liquids. One particularly successful theoretical approach is. based on an Ornstein-Zernike type integral equation for determining the solvent structure of polar liquids as well as the solvation of solutes.Although the theory provides a powerful tool for elucidating the structure of liquids in... 

Three types of methods are used to study solvation in molecular solvents. These are primarily the methods commonly used in studying the structures of molecules. However, optical spectroscopy (IR and Raman) yields results that are difficult to interpret from the point of view of solvation and are thus not often used to measure solvation numbers. NMR is more successful, as the chemical shifts are chiefly affected by solvation. Measurement of solvation-dependent kinetic quantities is often used (<electrolytic mobility, diffusion coefficients, etc). These methods supply data on the region in the immediate vicinity of the ion, i.e. the primary solvation sphere, closely connected to the ion and moving together with it. By means of the third type of methods some static quantities entropy and compressibility as well as some non-thermodynamic quantities such as the dielectric constant) are measured. These methods also pertain to the secondary solvation-sphere, in which the solvent structure is affected by the presence of ions, but the... 

In addition to thermal and viscosity changes, a third and perhaps the most important physical consideration in UHPLC is the influence of pressure on the solvent structure of the hydroorganic... 

Certain aspects of the solvent structural changes in regions near to the solute have received specialized attention and even inspired their own nomenclature. Two examples, each with a long and distinguished theoretical history, are the hydrophobic effect and dielectric saturation. ... 

Contributing to Ajj are, in addition to the solvent structural effects explicitly considered, contributions from dielectric saturation, from the liquid structure effects one has even in simple fluids, from solvent-mediated dispersion interactions of the ions, from charge-polarizability interactions of the ions, and so on. It is difficult to tell a-priori which effects are dominant or how big they are. However the collection of A 5 coefficients has characteristics that are consistent with the first named effect being dominant. 

It was shown earlier that the acceptor number can be treated as a measure of disorder of the solvent structure around the ion, reflecting the changes of caused by the solvent-solvent interaction. The contact... 

The activation parameters of the hydrolysis in aqueous dioxane of j -toluenesulfonyl bromide (336) pass through maxima at dioxane mole fractions of 0.01 and 0.12, which correspond to the range of stabilization of the solvent structure. 

Hagler, A.T. Moult, j. Computer simulation of the solvent structure around biological macromolecules. Nature 1978, 272, 222-226. 

We see that after a rapid initial decay the components evolve slowly in time and that Sbft) falls to a negative value and stays negative over much of the interval depicted. Study of the time evolution of the solvent structure aroimd the solute can help explain the behavior of S f) and of its components. We have examined solute-solvent pair correlations involving the solute C sites that change their partial charges. Fig. 11 shows some of these results, specifically the pair correlations g+jvW and g+c (r) for the solute site that increases its charge by e/2 with an acetonitrile N site and a benzene C site, respectively. 

What is next The above results give only a particular view of one part of the interface, i.e., the solvent structure. It would be good to End how this solvent and its changes in configuration affect—if at all—the total interfacial properties, for example, properties that we are already familiar with, such as the surface potential or the capacity. Thus, what would be the expression for the surface potential due to a layer... 

On the other hand, although the reorganization energy is a construct (like the Fermi energy of electrons in an intrinsic semiconductor in the middle of a region with no electrons), it is easy to imagine. Thus, in Fig. 9.24 at D, the ferrous ion would just have been formed by a vertical electronic transition and be with all the solvent structure of the ferric ion. But not C the ferrous ion has its solvation shell, teoiganized from that of the ferric ion. 

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