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Speciation in aqueous MgSO(4) fluids at high pressures and high temperatures from ab initio molecular dynamics and Raman spectroscopy.
MedLine Citation:
PMID:  21049967     Owner:  NLM     Status:  In-Process    
Abstract/OtherAbstract:
Ab initio molecular dynamics simulations and in situ Raman spectroscopy are used to study the speciation in two molal aqueous MgSO4 solutions at high pressures, P, and temperatures, T. While at ambient conditions the fluid is dominated by dissociated SO42−(aq) ions and solvent-separated ion pairs, ion association strongly increases with increasing temperature and pressure along a 1.33 g/cm3 isochore. At T = 450 °C and P = 1.4GPa, the ν1(SO42−) Raman band is well described by three Gaussian + Lorentzian components of about equal intensity with peaks at about 980, 995, and 1005 cm−1. Analysis of the simulations, however, indicates the coexistence of more than three species, including dissociated SO42−(aq) ions, and contact and triple ion pairs as well as larger complexes. In addition, the sulfate groups may be bonded to Mg as monodentate or bidentate ligands. The frequencies of the associated species seem to depend mainly on the type and number of Mg−SO4 bonds. We interpret the two rather broad high-frequency Raman components as a single “Mg−SO4 contact” component with variable frequency distribution. As a consequence, the ν1(SO42−) Raman band provides only information on the molecular environment of the sulfate group; i.e., individual species cannot be resolved. At fluid densities less than about 1.2 g/cm3 and temperatures above 400 °C, the formation of HSO4−(aq)-containing species is observed in both simulations and experiments, which may be accompanied by a change in pH and electrical conductivity.
Authors:
Sandro Jahn; Christian Schmidt
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Publication Detail:
Type:  Journal Article     Date:  2010-11-04
Journal Detail:
Title:  The journal of physical chemistry. B     Volume:  114     ISSN:  1520-5207     ISO Abbreviation:  J Phys Chem B     Publication Date:  2010 Dec 
Date Detail:
Created Date:  2010-11-25     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101157530     Medline TA:  J Phys Chem B     Country:  United States    
Other Details:
Languages:  eng     Pagination:  15565-72     Citation Subset:  -    
Affiliation:
GFZ German Research Centre for Geosciences, Section 3.3, Telegrafenberg, 14473 Potsdam, Germany. jahn@gfz-potsdam.de
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