Structure of aqueous electrolyte solutions

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. 

The position of the O-D band maximum has also been used to indicate the structure of fluid electrolyte solutions. Fig. 6.7-12 shows the wavenumber of the O-D band maximum, Vmax, in aqueous (HDO/H2O) lithium chloride solutions at 2800 bar and various temperatures as a function of the LiCl content x (in mol %) (Valyashko et al., 1980). The 25 °C isotherm is measured at 20 bar. If x is small, i> ax is considerably dependent... 

Kunz W, BeUoni L, Bernard O, Ninham BW (2004) Osmotic coefficients and surface tensions of aqueous electrolyte solutions role of dispersion forces. J Phys Chem B 108 2398-2404 Lee L-H (2000) The gap between the measured and calculated liquid-liquid interfacial tensions derived from contact angles. J Adhesion Sci Technol 14 167-185 Li ZX, Lu JR, Styrkas SA, Thomas RK, Rennie AR, Penfold L (1993) The structure of the surface of ethanol/water mixtures. Mol Phys 80 925-939 LoNostro P, Fratoni L, Ninham BW, Baglioni P (2002) Water absorbency by wool fibers hofmeiter effect. Biomacromol 3 1217-1224... 

Since their implementation in the 1970s, NDIS difference methods have been used to investigate ion specific structure in aqueous electrolyte solutions for many monatomic species, and several reviews are already available in the literature. In recent years, the NDIS methods have been applied to polyatomic ions, many of which are used in the denatura-tion of proteins in water. The complexity of these ions means that the neutron data are often difficult to interpret and some sort of modelling is required. Accordingly, as mentioned above we have developed computer simulation codes that provide results which can be direcdy compared with those obtained ftom NDIS. In the second part of this section, we present a few results which illustrate the power of this combined method. 

It is clear from our observations that no one method will be sufficient to resolve structure at the required level of detail around all hydrated species. Instead one must rely on a full complement of diffiaction and other techniques including computer simulation to answer the many and outstanding questions regarding the degree to which ions specifically influence the properties of aqueous electrolyte solutions. 

Results with a certain degree of reliability from MD simulations of aqueous solutions reported up to now are restricted to structural properties of such solutions. In the section on aqueous solutions below very preliminary velocity autocorrelation functions are calculated from an improved simulation of a 9.55 molal NaCl solution. The problem connected with the stability of the system and the different cut-off parameters for ion-ion, ion-water and water-water interactions are discussed. Necessary steps in order to achieve quantitative results for various dynamical properties of aqueous electrolyte solutions are considered. 

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. 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

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