Structure and hydrogen bonding in hydrothermal H2O-NaCl solutions: Are IR and Raman vibrational spectra telling us the whole story?

GEOC 7

Andrey G. Kalinichev1, Galina V. Bondarenko2, Yuri E. Gorbaty2, and R. James Kirkpatrick1. (1) Department of Geology and NSF WaterCAMPwS, University of Illinois at Urbana-Champaign, Urbana, IL 61801, (2) Russian Academy of Sciences, Institute of Experimental Mineralogy, Chernogolovka, Moscow Region, 142432, Russia
H2O stretching vibrations are very sensitive to the local H-bonding environment in aqueous systems, and IR and Raman spectroscopy is often used to quantify the effects of temperature, pressure, and ionic concentration on the structural and thermodynamic properties of electrolyte solutions. Surprisingly however, at 1 kbar between 25 and 500oC we observe vibrational spectra of aqueous 1.1 M NaCl solutions, which are very similar to those of pure water. Molecular dynamics computer simulations help to resolve this controversy. The average number of H-bonds between water molecules is always lower in the solution than in pure water, but the H-bonds donated by H2O to Cl- ions almost exactly compensate for this deficit. Thus, despite quite significant structural differences between both systems, the average number and strength of H-bonds formed by H2O molecules remain nearly the same in the solution as in pure water, leading to their nearly identical spectroscopic signatures.
 

Frontiers in Geochemistry: Commemorating the 25th Anniversary of the ACS-Geochemistry Division
9:00 AM-11:55 AM, Sunday, 10 September 2006 Moscone Center -- Room 262, Oral

Division of Geochemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006