Geochemical interest, aqueous complexes

Langmuir, D., Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symposium, American Chemical Society, Washington, DC, 1979, pp. 353-387. 

In theory, once the activity of an electrolyte in solution is known, the activity of the solvent can be determined by the Gibbs-Duhem integration (see section 2.11). In practice, the calculation is prohibitive, because of the chemical complexity of most aqueous solutions of geochemical interest. Semiempirical approximations are therefore preferred, such as that proposed by Helgeson (1969), consisting of a simulation of the properties of the H20-NaCl system up to a solute... 

Techniques of Estimating Thermodynamic Properties for Some Aqueous Complexes of Geochemical Interest... 

Source Most of the tabulated species are discus,sed by Baes and Mesmer (1976, 1981). Table is modified after Langmuir (1979), Reprinted with permission from Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, D. Langmuir. In Chemical modeling in aqueous systems, ed. E. A. Jenne, Am. Chem. Soc. Symp. Ser. 93. Copyright 1979 American Chemical Society. 

Langmuir, D. (1979), Techniques of Estimating Thermodynamic Properties for Some Aqueous Complexes of Geochemical Interest," in E. Jcnnc, Ed., Chemical Models,... 

However, despite of the great importance of quantum mechanical potentials from the purely theoretical point of view, simple effective two-body potential functions for water seem at present to be preferable for the extensive simulations of complex aqueous systems of geochemical interest. A very promising and powerful method of Car-Parrinello ah initio molecular dynamics, which completely eliminates the need for a potential interaction model in MD simulations (e.g., Fois et al. 1994 Tukerman et al. 1995, 1997) still remains computationally extremely demanding and limited to relatively small systems N < 100 and a total simulation time of a few picoseconds), which also presently limits its application for complex geochemical fluids. On the other hand, it may soon become a method of choice, if the current exponential growth of supercomputing power will continue in the near future. 

In this section, we will review some of the recent applications of both density functional and Hartree-Fock based calculations to metal complexes of geochemical interest. High-level calculations on small clusters representing a metal cation with its first and, and possibly second, coordination shell allow us to predict spectroscopic properties (see Tossell, this volume) and this can be of great utility interpreting experimental data. Moreover, if we are able to adequately model the solvation environment, we can predict the thermodynamic stabilities of different metal complexes. First-principles calculations on small clusters can be used to derive interatomic potentials that can be used in classical molecular dynamical simulations (next section) of aqueous solutions as a function of pressure, temperature and composition. Examples of such simulations will be given below. 

Langmuir D (1979) Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interests. In Jenne EA (ed) Chemical modeling in aqueous systems. Am Chem Soc Symp Ser 93, pp 353-387 Laznicka P (1985) The geological association of coal and metallic ores - a review. In Wolf KH (ed) Handbook of strata-bound and stratiform ore deposits, vol 13. Elsevier, Amsterdam, pp 1-71... 

In a solid-fluid reaction system, the fluid phase may have a chemistry of its own, reactions that go on quite apart from the heterogeneous reaction. This is particularly true of aqueous fluid phases, which can have acid-base, complexation, oxidation-reduction and less common types of reactions. With rapid reversible reactions in the solution and an irreversible heterogeneous reaction, the whole system may be said to be in "partial equilibrium". Systems of this kind have been treated in detail in the geochemical literature (1) but to our knowledge a partial equilibrium model has not previously been applied to problems of interest in engineering or metallurgy. 

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