Perspective of solution chemistry of Japan. — The historical background and recent trends —

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The development of solution chemistry in Japan is reviewed in connection with the history of solution chemistry in Europe and in the United States. Studies on solution chemistry from molecular aspects, as well as those by means of thermodynamics and statistical thermodynamics, are discussed from experimental and theoretical sides. Recent trends in solution chemistry, especially those in Japan, are overviewed from static and dynamic points of view.

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In addition to the direct action of the ionic charge on water as a dielectric medium, ions may exert an influence on the equilibrium between the ice-like and non-ice-like forms which are present in room-temperature water. This provides a way of accounting for experimental results in a variety of areas, including entropy, heat capacity, temperature of maximum density, tracer selfdiffusion, thermal conductivity, and dielectric relaxation, as well as viscosity and ionic mobility and their temperature coefficients. The tetrabutyl ammonium cation acts as a structure-promoter in the same way as do non-polar solutes, amino acids and fatty acid anions. These various effects seem explicable in a straightforward manner in terms of a new picture of water as consisting of flickering clusters of hydrogen-bonded molecules, in which the cooperative nature of cluster formation and relaxation is related to the partially covalent character which is postulated for the hydrogen bond. (Author)
The x-ray diffraction intensity-angle distribution for water and its variation for a temperature of 2° to 98° C, are given. Two important periodicities of 3.24 and 2.11A and a third of 1.13A at 21°C are established in satisfactory agreement with Meyer. The first one decreases with increasing temperature and the second increases. It is shown that the conception of molecular complexes explains neither the existence of these periodicities nor their change with temperature. In fact, the description of "association" that involves complexes of two or three or more molecules, should be abandoned in favor of the molecular group conception, (cybotactic condition) emphasized by the author. These groups of molecules containing hundreds and perhaps thousands of molecules in each, have a temporary existence as individuals, have illdefined boundaries, possess an optimum size and an internal regularity determined by the temperature and molecular forces, and expand anisotropically. The experimental facts are in agreement with this view. A detailed description of the molecular arrangement in water from x-ray data is not at present possible yet it simulates the crystal arrangement in ice. A mathematical treatment of the forces within and between the molecules in such groups can probably be studied with much profit only from the quantum viewpoint.
The structure of hydrated copper(II) ion was determined by means of the X-ray diffraction of a 3.55 M copper(II) perchlorate solution. The radial distribution curve showed that the copper(II) ion has a distorted octahedral structure with six water molecules. Four water molecules occupy the equatorial sites of the octahedron at a distance of 1.94±0.02 Å from the Cu(II) ion and the other two water molecules at the axial sites are 2.43±0.03 Å distant from the central ion.
An examination of the action of ions on the translational motion of the water molecules closest to the ions may be offered as a basis for a general approach to the study of ion hydration in aqueous solutions. The exchange of water molecules in the immediate vicinity of the ions is considered. If this exchange occurs relatively rarely the hydration of the ion is considerable. As the frequency of the exchange grows the hydration of the ion weakens. The change (due to the influence of the ion) of the value of activation energy of the exchange of the closest water molecules (ΔE) is the main quantitative characteristic of ion hydration in solutions. A procedure was worked out for calculating the value ΔE for individual ions from experimental data of self-diffusion in water and of the temperature coefficients of the ion mobilities in solutions. For Li+ and Na+, ΔE > 0; for K+, Cs+, Cl-, etc., ΔE < 0. In the latter case the water molecules near the ions become more mobile than in pure water. This phenomenon was called "negative hydration". The possibility of negative hydration is shown in terms of the energy of the interaction of particles in aqueous electrolyte solutions.
X-Ray diffraction measurements were carried out for aqueous solutions of cadmium perchlorate and sodium tetraiodocadmate. X-Ray scattering data showed that the hydrated Cd2+ ion is coordinated to six water molecules, the distance between Cd2+ and H2O being 2.31±0.02 Å, and that the CdI42− complex has a tetrahedral configuration with a Cd–I distance of 2.79±0.01 Å. The tetrahedral structure of the CdI42− complex was confirmed by Raman spectroscopy.
The reaction of Cr++ with a variety of ions of the type Co(NH3)5X++ as oxidizing agent takes place with quantitative transfer of X to the reducing agents. For the system with X- = Cl-, it has been shown that the transfer takes place without there being any exchange with chloride ion in solution. These observations lead to the conclusion that the activated complexes for the electron transfer reactions in question have configurations in which X makes a bond simultaneously to Cr and to Co. Any group which has unpaired electrons available for interaction with Cr++ is found to provide a more accessible pass for electron transfer than does the proton coördination shell of Co(NH3)6+++. A bridged activated complex also explains observations made on reactions of Cr(III) complexes catalyzed by Cr++. Results obtained with other oxidizing agents which are substitution-inert show that although formation of a bridged activated complex does take place, net transfer of the bridging group from oxidizing agent to reducing agent is not an essential feature of the electron transfer process. Strong evidence is presented in support of the view that a bridged activated complex is involved also in the action of substitution-labile oxidizing agents containing Fe(III) on Cr++ (or in the "catalysis" of the reaction of Cr++ and Fe+++ by anions), and in the exchange of electrons between Fe++ and Fe(III) species.
Application of Molecular Dynamics Simulation to the Structural Analysis of X-Ray Diffraction Data on a Concentrated LiCl Aqueous Solution
  • Okada