HR: 17:00h
AN: V34A-05    [Abstracts]
TI: Hydrogen Bonding, Hydration of Species, Ion Pairing and Clusterization in H$_2$O-NaCl-CaCl$_2$-CO$_2$-NaHCO$_3$-Na$_2$CO$_3$ Fluids: Molecular Dynamics Simulation of the Effects of Temperature, Pressure and Composition
AU: * Kalinichev, A G
EM: kalinich@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AU: * Kalinichev, A G
EM: kalinich@uiuc.edu
AF: NSF Science and Technology Center on Advanced Materials for Water Purification with Systems (NSF WaterCAMPWS),University of Illinois at Urbana-Champaign, 3254 Digital Computer Laboratory, Urbana, IL 61801
AU: Kirkpatrick, R J
EM: kirkpat@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AU: Kirkpatrick, R J
EM: kirkpat@uiuc.edu
AF: NSF Science and Technology Center on Advanced Materials for Water Purification with Systems (NSF WaterCAMPWS),University of Illinois at Urbana-Champaign, 3254 Digital Computer Laboratory, Urbana, IL 61801
AU: Wang, J
EM: jianwei7@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 1301 W Green St, Urbana, IL 61801
AB: Molecular computer simulation is an especially valuable tool to study the structural and dynamic properties of carbon-bearing aqueous fluids on the fundamental atomic time- and length- scale because these fluids are not readily studied experimentally using conventional X-ray or EXAFS methods. In this case, experimental methods can produce ambiguous results, because the carbon and oxygen atoms of the solute species are not easily distinguishable from the oxygen atoms of solvent water. Systematic molecular dynamics (MD) computer simulation studies of several fluid compositions $-$ H$_2$O-CO$_2$, H$_2$O-CO$_2$-NaCl, H$_2$O-NaHCO$_3$, and H$_2$O-Na$_2$CO$_3$ $-$ were performed to study the effects of temperature, pressure (fluid density) and concentration on the structural, energetic, spectroscopic and dynamic properties of these solutions characterized on the atomic scale via the statistical parameters of individual hydrogen bonds and H-bonding networks, local hydration structures of dissolved species, and ion pair formation. Similar molecular-level characteristics of pure water and H$_2$O-NaCl and H$_2$O-CaCl$_2$ solutions are calculated and used for comparison with the properties of carbon-bearing fluids. H$_2$O-rich and CO$_2$-rich compositions of the ternary H$_2$O-CO$_2$-NaCl system demonstrate strikingly different structural and dynamic behavior at about the same average density. In dense CO$_2$-rich fluids, dissolved H$_2$O molecules exhibit a high degree of hydrogen bonding and form relatively stable H-bonded clusters structurally similar to those observed in supercritical water at a much lower density. In contrast, CO$_2$ molecules dissolved in water-rich fluids occur in clathrate-like cages formed by surrounding H-bonded water molecules. The hydration shells of carbonate and bicarbonate ions both contain approximately 10 water molecules, but the water structure around the carbonate ion is much more pronounced due to the higher anion charge. This also leads to the formation of very stable ion pairs and larger ionic clusters of Na$^+$ and CO$_{3}^{2-}$ even under ambient conditions. Due to the ion cluster formation, the diffusion rates of both Na$^+$ and CO$_{3}^{2-}$ are $\sim$ 3-6 times lower than in similar NaCl solutions. The effect of dissolved Na$_2$CO$_3$ on the water structure is comparable to that of CaCl$_2$. In contrast, the structure and dynamics of NaHCO$_3$ solutions is very similar to that of H$_2$O-NaCl, where ion pairing is non-existent at lower temperatures, and the diffusion rates of HCO$_{3}^{-}$ are about the same as those of Cl$^-$. The molecular simulations also predict that a high-density fluid ($\rho$ $\sim$ 1.1 g/cm$^3$) of the composition H$_2$O/CO$_2$/NaCl=60/28/12 mol%, homogeneous at 1000$\deg$C, should experience a phase separation upon cooling at approximately 700$\deg$C by forming a low-salinity CO$_2$-rich phase (H$_2$O/CO$_2$/NaCl $\sim$ 50/48/2 mol%) and a concentrated brine (H$_2$O/CO$_2$/NaCl $\sim$ 66/20/14 mol%) in good agreement with available experimental data and thermodynamic calculations. The structural and dynamic properties of both fluid phases are rationalized on the molecular level in terms of the electrostatic and H-bonding interactions between the fluid species in order to understand physical mechanisms driving this phase separation process.
DE: 3919 Equations of state
DE: 3939 Physical thermodynamics
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting