Structure of Aqueous Solutions

More complicated and less known than the structure of pure water is the structure of aqueous solutions. In all cases, the structure of water is changed, more or less, by dissolved substances. A quantitative measure for the influence of solutes on the structure of water was given in 1933 by Bernal and Fowler 23), introducing the terminus structure temperature, Tsl . This is the temperature at which any property of pure water has the same value as the solution at 20 °C. If a solute increases Tst, the number of hydrogen bonded water molecules is decreased and therefore it is called a water structure breaker . Vice versa, a Tsl decreasing solute is called a water structure maker . Concomitantly the mobility of water molecules becomes higher or lower, respectively. 

The structure of aqueous solutions depends on the following parameters 1. content of H-bonds of water molecules, 2. interactions water-solute, 3. orientations of H-bonds, 4. H-bond distances, 5. life-time of H-bonds, 6. movement of protons in acids or defect-protons in bases. There are some computer experiments with water and a model of ions78. They show a separation of the ions in 1 psec but that the distance of ions remains about 5 A78. This gives a simple interpretation of the optical constancy of absorbing ions independantly from the concentration79. ... 

O. Ya. Samoilov, Structure of Aqueous Solutions of Electrolytes and Hydration of Ions, Izdat. Akademii Nauk SSSR, Moscow, 1957 (in Russian). 

Attempts to determine < 0L by the NMR method have not so far produced results remotely in line with those considered above (Kresge and Allred, 1963 Gold, 1963). This perhaps underlines the need for considering the effect of hydroxide ions on the structure of aqueous solutions. 

The other major realm of formation of minerals and rocks, and the most important medium of transport and redistribution of the chemical elements at the Earth s surface, is the aqueous solution. The molecular and electronic structures of aqueous solutions, their behavior at elevated temperatures, formation and stabilities of complexes in solution, and the mechanisms of reactions in solution are all considered in the second section of this chapter. 

Cerreta, M.K. Berglund, K.A. The structure of aqueous-solutions of some dihydrogen ortho-phosphates by laser... 

It is speculated that the effect of temperature on the critical electrolyte concentration is similarly related to the effect of temperature on the structure of aqueous solutions. An increase in temperature has been shown to extend the range of micellar solutions to a higher salinity in anionic surfactant systems (31). Hence, polymer-aggregate incompatibility would be less when the temperature is increased. However, addition of alcohol or change in temperature... 

Benton, W. J. Fort, Jr. T. Miller C. A. "Structures of Aqueous Solutions of Petroleum Sulfonates", SPE 7579, 53rd Ann. Fall Tech. Conf. and Exhib. of SPE-AIME, Houston, Tx. October 1978. 

Silicate Modifications. A method has been described in which silicate minerals are simultaneously acid-leached and trimethylsilyl end-blocked to yield specific trimethylsilyl silicates having the same silicate structure as the mineral from which these were derived (12). Olivine, hemimorphite, sodalite, natrolite, laumontite, and sodium silicates are converted to TMS derivatives of orthosilicates, pyrosilicates, cyclic polysilicates, etc, making it possible to classify the minerals according to their silicate structure. The same technique is used to analyze the siloxanol structure of aqueous solutions of vinyltrimethoxysilane (13). Certain anionic siliconates stabilize solutions of alkali silicates to give stable solutions in water or alcohols at any pH (14). Such silicate—siliconate mixtures are used as corrosion inhibitors in glycol antifreeze (15) (see Antifreezes and deicing fluids Corrosion and CORROSION CONTROL). 

Temperature significantly influences on stability of complexes. The structure of aqueous solution changes according to the change in temperature. (Zhloro-complexes change from octahedral coordination to tetrahedral coordination... 

While it is possible to use neutron scattering results to appraise and refine MD simulations, molecular dynamics can also be used as a heuristic test to yield the non-intuitive relationship between structures of aqueous solutions and the experimental results those structures would produce, and hence allow interpretation of the NDIS results. In this role the physical validity of the MD used to generate a three-dimensional structure is largely a secondary concern, as the significant test of this method is the comparison of the neutron scattering result to the formally correct mathematical prediction of the experimental result from the disordered structure produced by the modelling. 

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