Aqueous electrolyte solutions structure

Aqueous electrolyte solutions have been a subject of determined studies for over a century. Numerous attempts were made to construct theories that could link the general properties of solutions to their internal structure and predict properties as yet nnknown. Modem theories of electrolyte solutions are most intimately related to many branches of physics and chemistry. The electrochemistry of electrolyte solutions is a large branch of electrochemistry sometimes regarded as an independent science. 

Samoilov, O. Ya., Structure of Aqueous Electrolyte Solutions and Hydration of Ions, Consultants Bureau, New York, 1965. 

The pressure-volume-temperature (PVT) properties of aqueous electrolyte and mixed electrolyte solutions are frequently needed to make practical engineering calculations. For example precise PVT properties of natural waters like seawater are required to determine the vertical stability, the circulation, and the mixing of waters in the oceans. Besides the practical interest, the PVT properties of aqueous electrolyte solutions can also yield information on the structure of solutions and the ionic interactions that occur in solution. The derived partial molal volumes of electrolytes yield information on ion-water and ion-ion interactions (1,2 ). The effect of pressure on chemical equilibria can also be derived from partial molal volume data (3). 

An understanding of equilibrium phenomena in naturally occurring aqueous systems must, in the final analysis, involve understanding the interaction between solutes and water, both in bulk and in interfacial systems. To achieve this goal, it is reasonable to attempt to describe the structure of water, and when and if this can be achieved, to proceed to the problems of water structure in aqueous solutions and solvent-solute interactions for both electrolytes and nonelectrolytes. This paper is particularly concerned with two aspects of these problems—current views of the structure of water and solute-solvent interactions (primarily ion hydration). It is not possible here to give an exhaustive account of all the current structural models of water instead, we shall describe only those which may concern the nature of some reported thermal anomalies in the properties of water and aqueous solutions. Hence, the discussion begins with a brief presentation of these anomalies, followed by a review of current water structure models, and a discussion of some properties of aqueous electrolyte solutions. Finally, solute-solvent interactions in such solutions are discussed in terms of our present understanding of the structural properties of water. 

For measurements between crossed mica cylinders coated with phospholipid bilayers in water, see J. Marra andj. Israelachvili, "Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions," Biochemistry, 24, 4608-18 (1985). Interpretation in terms of expressions for layered structures and the connection to direct measurements between bilayers in water is given in V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayers in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). The bilayer-bilayer interactions are reported in E. A. Evans and M. Metcalfe, "Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by van der Waals attraction," Biophys. J., 46, 423-6 (1984). 

The knowledge of the surface potential for the dispersed systems, such as metal oxide-aqueous electrolyte solution, is based on the model calculations or approximations derived from zeta potential measurements. The direct measurement of this potential with application of field-effect transistor (MOSFET) was proposed by Schenk [108]. These measurements showed that potential is changing far less, then the potential calculated from the Nernst equation with changes of the pH by unit. On the other hand, the pHpzc value obtained for this system, happened to be unexpectedly high for Si02. These experiments ought to be treated cautiously, as the flat structure of the transistor surface differs much from the structure of the surface of dispersed particle. The next problem may be caused by possible contaminants and the surface property changes made by their presence. 

Wolf B,- Hanlon S (1975) Structural transitions of deoxyribonucleic acid in aqueous electrolyte solutions. I. Reference spectra of conformational limits. Biochemistry 14 1648-1660... 

J.E.B. Randles, Structure at the Free Surface of Water and Aqueous Electrolyte Solutions, Phys. Chem. Liq. 7 (1977) 107. (Review, mainly of the items considered In our secs. 3.9 and 3.10f.)... 

These two effects are essential for pressure effect of the B in aqueous electrolyte solution and the observation in Fig. 2 is a balance of them. The larger increment of the B of CsCl up to the maximum pressure than that of NaCl may be ascribed to the fact that the former effect for CsCl is stronger than that for NaCl because the breaking-effect of CsCl on the water structure is larger at atmospheric pressure. 

First electrochemical studies on structuring and modification of different high superconductor surfaces have recently been started [6.190]. One of the main problems is the instability of oxide ceramic material in neutral and acidic aqueous electrolyte solutions at room temperature [6.196-6.198]. HTSC surfaces corrode, and superconductivity was found to decrease within the topmost layers of IfTSC samples after water contact. This aging effect decreases in alkaline media [6.197]. However, sufficient long term stability of HTSC samples was only found in aprotic solvents such as acetonitrile. Therefore, experiments were carried out in acetonitrile-containing... 

Janusz, W.. The structure of the electrical double layer at the Lichrospher-type adsorbent/aqueous electrolyte solution interface, Adsorption Sci. Technol.. 14, 151, 1996. 

The fit of the MSA to activity coefficient data for aqueous electrolyte solutions can be considerably improved if one takes into consideration the decrease in solvent permittivity which accompanies the increase in electrolyte concentration. This phenomenon is clearly related to the effect that ions have on solvent structure and was studied originally in aqueous solutions by Hasted et al. [21, 22]. More recently, data have been collected for a large number of electrolytes by Barthel and coworkers [23]. In the case of NaCl solutions, the change in dielectric permittivity with electrolyte concentrations up to 2 M is given by... 

X-ray diffraetion studies have also been earned out in non-aqueous electrolyte solutions. In the case of methanol, there are two atoms which scatter X-rays, namely, carbon and oxygen. When a monoatomic electrolyte is added the number of scattering atoms increases to four. As a result, such a system has ten partial structure factors. If the ion ion correlations are neglected this reduces to seven. A system which has been analyzed in some detail is MgCl2 in methanol [6, 7]. Analysis of the data gives a Mg-O distance of 207 pm and a Cl-O distance of 318 pm for coordination numbers of six [6]. 

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