(330a) Ion Pair Association in Ultra Supercritical Aqueous Environments: Experimental Difficulties, Molecular Simulation Insights, and Modeling Challenges

Ultra-supercritical (USC) aqueous solutions are extreme environments exhibiting relatively small dielectric permittivity that give rise to complex processes whose mechanisms are poorly understood, mainly because system properties at these conditions are exceptionally difficult to access and measure. Understanding the behavior of aqueous solutions at USC is crucial for the design of more efficient electrical power plants, conditions at which trace-levels of ionic species might induce solid deposition, and consequently, affecting the global cycle efficiency. Moreover, interactions of these extreme environments with cladding and metallic surfaces are mostly unknown because their chemistry at these extreme conditions is poorly understood, a direct consequence of our lack of molecular-based understanding of the speciation behavior of electrolytes in steam environments.

Currently used engineering correlations for the thermophysical properties of high temperature/pressure aqueous electrolyte solutions suffer from an inherent inability to capture the unavoidable tendency of ions toward speciation. This scenario points at two relevant implications: (a) the need for reassessing the way we address the modeling of dissociable solutes in USC aqueous solutions, i.e., more realistic, speciation-based thermodynamic descriptions in terms of ion speciation for all solutes; (b) the need for ways to test the reliability of any macroscopic modeling approach, including the validity of proposed hypotheses, as well as to guide the development of alternatives.

In this context, molecular simulation becomes a versatile tool to gain insight into the microscopic behavior of aqueous electrolyte solutions at USC conditions, to interpret the species solvation behavior, and ultimately, to provide a quantitative assessment of the speciation process (CHIALVO et al., 2002). In fact, molecular simulation provides a unique route to quantifying the dielectric behavior of aqueous electrolyte solutions, the complete characterization of the system microstructure, as well as the interpretation and measurement of the ion-pair formation that will ultimately guide the development of successful macroscopic correlations.

Among experimental approaches, the most promising tool currently available to constraint the speciation characterization of USC steam environments is the flow-through electric conductance apparatus (GRUSZKIEWICZ and WOOD, 1997), a technique that allows reliable measurements at ion concentrations as low as 10-6 molal. Electric conductance measurements obtained at these extremely low ionic strengths can be directly used to determine ionic mobilities without the need for questionable extrapolation of the results obtained at higher concentrations.

In this work we study the Na+Cl- pair association process in USC steam solutions over a wide range of densities, including the zero solvent-density limit. Our main goal is to discuss a new integrated approach comprising molecular simulation and ultra-sensitive electric conductance experiments, to provide new crucially needed data for the association constant in USC environments, as well as to address some relevant issues regarding the modeling of ion association in extreme steam environments (CHIALVO et al., 2009).

REFS:

Chialvo A. A. and Simonson J. M. (2003) ?Aqueous Na+Cl- Pair Association from Liquid-like to Steam-like Densities along Near-critical Isotherms?. Journal of Chemical Physics 118, 7921-7929

Chialvo A. A., Gruszkiewicz M. S., Simonson J. M., Palmer D. A., and Cole D. R. (2009) ?Ion Pair Association in Extreme Aqueous Environments: Molecular-based and Electric Conductance Approaches?. Journal of Solution Chemistry (In press)

Gruszkiewicz M. S. and Wood R. H. (1997) Conductance of LiCl, NaCl, NaBr, and CsBr Solutions in Supercritical Water Using a Flow Conductance Cell. Journal of Physical Chemistry B 101, 6549-6559