title: Ion Association in Hydrothermal Sodium Sulfate Solutions Studied by Modulated FT-IR-Raman Spectroscopy and Molecular Dynamics.
abstract: Saline aqueous solutions at elevated pressures and temperatures play an important role in processes such as supercritical water oxidation (SCWO) and supercritical water gasification (SCWG), as well as in natural geochemical processes in Earth and planetary interiors. Some solutions exhibit a negative temperature coefficient of solubility at high temperatures, thereby leading to salt precipitation with increasing temperature. Using modulated FT-IR Raman spectroscopy and classical molecular dynamics simulations (MD), we studied the solute speciation in solutions of 10 wt % Na2SO4, at conditions close to the saturation limit. Our experiments reveal that ion pairing and cluster formation are favored as solid saturation is approached, and ionic clusters form prior to the precipitation of solid sulfate. The proportion of such clusters increases as the phase boundary is approached either by decreasing pressure or by increasing temperature in the vicinity of the three-phase (vapor-liquid-solid) curve.
authors.affiliation: †Laboratory for Bioenergy and Catalysis, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
authors.forename: Joachim
authors.initials: J
authors.lastname: Reimer
authors.affiliation: ‡Institut für Geochemie und Petrologie, ETH Zürich NW F 82.4, Clausiusstrasse 25, 8092 Zürich, Switzerland.
authors.forename: Matthew
authors.initials: M
authors.lastname: Steele-MacInnis
authors.affiliation: †Laboratory for Bioenergy and Catalysis, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
authors.forename: Jörg M
authors.initials: JM
authors.lastname: Wambach
authors.affiliation: †Laboratory for Bioenergy and Catalysis, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.|§Institut für Biomasse und Ressourceneffizienz Fachhochschule Nordwestschweiz 5210 Windisch, Switzerland.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Vogel
mesh_terms: D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D012965:Sodium Chloride / Q000737:chemistry; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D013431:Sulfates / Q000737:chemistry*; D013696:Temperature
chemical_list: D012996:Solutions; D013431:Sulfates; C012036:sodium sulfate; D012965:Sodium Chloride