Molecular H(2)O in armenite, BaCa(2)Al(6)Si(9)O(30)center dot 2H(2)O, and epididymite, Na(2)Be(2)Si(6)O(15)center dot H(2)O: Heat capacity, entropy and local-bonding behavior of confined H(2)O in microporous

Geiger, C. A., Dachs, E., Dalconi, M. C. and Artioli, G. (2010) Molecular H(2)O in armenite, BaCa(2)Al(6)Si(9)O(30)center dot 2H(2)O, and epididymite, Na(2)Be(2)Si(6)O(15)center dot H(2)O: Heat capacity, entropy and local-bonding behavior of confined H(2)O in microporous Geochimica Et Cosmochimica Acta, 74 (18). pp. 5202-5215. DOI 10.1016/j.gca.2010.05.033.

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Abstract

Armenite, ideal formula BaCa(2)Al(6)Si(9)O(30)center dot 2H(2)O, and its dehydrated analog BaCa(2)Al(6)Si(9)O(30) and epididymite, ideal formula Na(2)Be(2)Si(6)O(15)center dot H(2)O, and its dehydrated analog Na(2)Be(2)Si(6)O(15) were studied by low-temperature relaxation calorimetry between 5 and 300 K to determine the heat capacity, C(p), behavior of their confined H(2)O. Differential thermal analysis and thermogravimetry measurements, FTIR spectroscopy, electron microprobe analysis and powder Rietveld refinements were under-taken to characterize the phases and the local environment around the H(2)O molecule. The determined structural formula for armenite is Ba(0.88(0.01))Ca(1.99(0.02))Na(0.04(0.01))Al(5.89(0.03))Si(9.12(0.02))O( 30)center dot 2H(2)O and for epididymite Na(1.88(0.03))K(0.05(0.004))Na(0.01(0.004))Be(2.02(0.008))Si(6.00(0.01)) O(15)center dot H(2)O. The infrared (IR) spectra give information on the nature of the H(2)O molecules in the natural phases via their H(2)O stretching and bending vibrations, which in the case of epididymite only could be assigned. The powder X-ray diffraction data show that armenite and its dehydrated analog have similar structures, whereas in the case of epididymite there are structural differences between the natural and dehydrated phases. This is also reflected in the lattice IR mode behavior, as observed for the natural phases and the H(2)O-free phases. The standard entropy at 298 K for armenite is S degrees = 795.7 +/- 6.2 J/mol K and its dehydrated analog is S degrees = 737.0 +/- 6.2 J/mol K. For epididymite S degrees = 425.7 +/- 4.1 J/mol K was obtained and its dehydrated analog has S degrees = 372.5 +/- 5.0 J/mol K. The heat capacity and entropy of dehydration at 298 K are Delta C(p)(rxn) = 3.4 J/mol K and Delta S(rxn) = 319.1 J/mol K and Delta C(p)(rxn) = -14.3 J/mol K and Delta S(rxn) = 135.7 J/mol K for armenite and epididymite, respectively. The H(2)O molecules in both phases appear to be ordered. They are held in place via an ion dipole interaction between the H(2)O molecule and a Ca cation in the case of armenite and a Na cation in epididymite and through hydrogen-bonding between the H(2)O molecule and oxygen atoms of the respective silicate frameworks. Of the three different H(2)O phases ice, liquid water and steam, the Cp behavior of confined H(2)O in both armenite and epididymite is most similar to that of ice, but there are differences between the two silicates and from the Cp behavior of ice. Hydrogen-bonding behavior and its relation to the entropy of confined H(2)O at 298 K is analyzed for various microporous silicates. The entropy of confined H(2)O at 298 K in various silicates increases approximately linearly with increasing average wave-number of the OH-stretching vibrations. The interpretation is that decreased hydrogen-bonding strength between a H(2)O molecule and the silicate framework, as well as weak ion dipole interactions, results in increased entropy of H(2)O. This results in increased amplitudes of external H(2)O vibrations, especially translations of the molecule, and they contribute strongly to the entropy of confined H(2)O at T < 298 K. (C) 2010 Elsevier Ltd. All rights reserved.

Document Type: Article
Keywords: pulse calorimetric technique crystal x-ray thermodynamic properties neutron-scattering 3rd law zeolites natrolite minerals water silicates
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1016/j.gca.2010.05.033
ISSN: 0016-7037
Date Deposited: 22 Dec 2011 05:40
Last Modified: 08 Oct 2012 09:59
URI: http://eprints.uni-kiel.de/id/eprint/15680

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