HR: 16:30h
AN: V14B-03    [Abstracts]
TI: H2O Inner-Surface Interactions in Micro/Nanoporous Silicates: Thermodynamic Behavior and Low Energy Molecular Vibrations
AU: * Geiger, C A
EM: chg@min.uni-kiel.de
AF: University of Kiel, Institute for Geosciences, Olshausenstr. 40, Kiel, D24098, Germany
AU: Paukov, I E
EM: paukov@che.nsk.su
AF: Institute of Inorganic Chemistry, Russian Academy of Scienes, Lavrentiev prosp. 3, Novosibirsk, RUS 630090,
AU: Kovalevskaya, Y A
EM: kovalevskaya@che.nsk.su
AF: Institute of Inorganic Chemistry, Russian Academy of Scienes, Lavrentiev prosp. 3, Novosibirsk, RUS 630090,
AU: Kolesov, B A
EM: kolesov@che.nsk.su
AF: Institute of Inorganic Chemistry, Russian Academy of Scienes, Lavrentiev prosp. 3, Novosibirsk, RUS 630090,
AB: Macroscopic thermodynamic and molecular-scale behavior related to silicate surface-fluid interactions in nature is complex and poorly understood. The study of confined H₂O at inner surfaces in micro/ nanoporous silicates is helpful for understanding outer-surface interactions, because such phases offer simpler physicochemical systems for investigation. We are investigating the nature of H₂O in various micro/nanoporous silicates. Low temperature calorimetric heat capacity (C_p) determinations have been made to determine thermodynamic behavior. Powder IR and polarized single-crystal Raman spectroscopy are used to investigate local features such as bonding and dynamics. In this report, C_p behavior and low energy external H₂O modes related to van der Waals and/or hydrogen bonding at inner surfaces are emphasized. The first group of microporous silicates that includes cordierite, Mg₂Al₄Si₅O₁₈· xH₂O, and beryl, Be₂Al₃Si₆O₁₈· xH₂O, where x = 0 to 1, can hold single H₂O molecules in small structural microcavities and exchange them with the environment with no change in volume. The C_p behavior of the confined H₂O, which is characterized by weak van der Waals forces to the aluminosilicate framework, is roughly similar to that of steam at T > 100 K up to moderate T's. C_p is greater than that for ice at T < 100 K. Raman, IR and inelastic neutron scattering measurements show that the H₂O molecule is quasi free in both phases. In cordierite, low energy T(H₂O) modes occur at ~80 and ~48 cm^-1 (Winkler and Hennion, 1994), and possibly at ~31 cm^-1 . For beryl, for an external mode T(H2O) at ~ 10 cm^-1 is present. The second type of microporous silicate, namely zeolites (those studied are bikitaite Li₂[Al₂Si₄O₁₂]·2H₂O, natrolite - Na₁₆[Al₁₆Si₂₄O₈₀]·16H₂O, scolecite - Ca₈[Al₁₆Si₂₄O₈₀]·24H₂O, gmelinite - (Na₂,Ca)[Al₂Si₄O₁₂]·6H₂O) are strongly hydrophilic and their intrachannel H₂O molecules are hydrogen bonded. Zeolites show measurable changes in volume with loss or gain of H₂O. The C_p behavior of H₂O in natrolite is similar to that for ice at T < 100 K, but its C_p increases roughly linearly with increasing T and is greater than the C_p of ice at T > 100 K and also for H₂O in cordierite at T > 250 K. At 298 K, T(H₂O) modes between 45 and 180 cm^-1 occur in natrolite and scolecite. Gmelinite shows similar Cp behavior at T < 100 K but anomalously high Cp behavior above 170 K. Conclusions from our investigations are: 1) At T < 200 K, C_p behavior of confined H₂O is controlled by low-energy external T(H₂O) modes. 2) T(H₂O) mode energies reflect weak van-der-Waals to moderately strong hydrogen-bond forces at inner-surfaces. 3) The marked increase in C_p in some zeolites with increasing T indicates a change in hydrogen-bonding behavior from more "ice-like" to increasing "liquid- water-like". It is possible that this "transition" can account for the C_p behavior observed at T > 170 K in gmelinite. 4) C_p and entropy values for confined H₂O in silicates cannot be considered similar, as is done in crystal-chemical-based schemes used for calculating thermodynamic properties of H₂O-bearing silicates.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3939 Physical thermodynamics
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting