title: Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations.
abstract: Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular-scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na-SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K-SO4 show significantly less clustering under equivalent conditions. In mixed systems (Nax K2-x SO4 ), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure-breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K(+) and SO4 (2-) , compared with the analogous Na(+) -SO4 (2-) interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na-K-SO4 . Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium-sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting-in in ternary hydrothermal solutions of Na-K-SO4 reflects the opposing, but complementary, natures of Na-SO4 versus K-SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.
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 Biomasse und Ressourceneffizienz, Fachhochschule Nordwestschweiz, 5210, Windisch, Switzerland.|Laboratory for Bioenergy and Catalysis, Paul Scherrer Institut, 5232, Villigen PSI, Switzerland.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Vogel
authors.affiliation: Department of Geosciences, The University of Arizona, Tucson, AZ, 85721, USA. steelemacinnis@email.arizona.edu.|Institut für Geochemie und Petrologie, ETH Zürich, 8092, Zürich, Switzerland. steelemacinnis@email.arizona.edu.
authors.forename: Matthew
authors.initials: M
authors.lastname: Steele-MacInnis
keywords: molecular dynamics; potassium; salt effect; sodium; structure elucidation