College Park, Maryland      June 6 - 10 , 2004

M4-C1 (3:45 PM): Quasielastic neutron scattering investigation of the translational and rotational dynamics of supercooled water confined in nanoporous silica matrices

Sow-Hsin Chen, Li Liu, Antonio Faraone (Massachusetts Institute of Technology)

Using incoherent quasielastic neutron scattering we investigated the dynamics of water confined in two nanoporous alumino-silicate matrices, from ambient temperature down to deeply supercooled states in a temperature range inaccessible in bulk water. We collected data using three instruments with widely different resolutions, the Fermi chopper, the disk chopper, and the backscattering spectrometers, at the NIST Center for Neutron Research. The confining systems were lab synthesized nanoporous silica glasses MCM-41-S and MCM-48-S. The former matrix (25 Ĺ pore diameter) has 1-D cylindrical tubes arranged in a hexagonal structure whereas the latter (22 Ĺ pore diameter) has a 3-D bicontinous morphology. Inside the pores of these matrices the freezing process of water is strongly suppressed, taking place at a temperature about 50 K less than in bulk water. Thus, with the combined use of different instruments, we were able to study an interesting temperature range, 200 K < T < 300 K. The data from all the three spectrometers were analyzed using a single consistent intermediate scattering function of water based on the relaxing cage models for the translational and rotational motions. An exponential slowing down of the translational and rotational relaxation times, as temperature is lowered, has been observed, indicating the existence of a kenetic glass transition temperature at about 220 K.

[1] Liao, Ciya, F. Sciortino and S.H. Chen, “Model for Dynamics in Supercooled Water,” Phys. Rev. E 60, 6776-6787 (1999).
[2] Liu, Li, Antonio Faraone and S.H. Chen, “A Model For the Rotational Contribution to Quasi-elastic Neutron Scattering Spectra From Supercooled Water,” Phys. Rev. E. 65, 041506-8 (2002).
[3] Fraone, A., S.H. Chen, E. Fratini, P. Baglioni, L. Liu and C. Brown, “Rotational Dynamics of Hydration Water in Dicalcium Silicate by Quasi-Elastic Neutron Scattering,” Phys. Rev. E. 65, 040501-4 (2002).
[4] Faraone, A., Li Liu, Chung-Yuan Mou, Pei-Chun Shih, John R.D. Copley, and Sow-Hsin Chen, “Translational and rotational dynamics of water in mesoporous silica materials: MCM-41-S and MCM-48-S,” J. Chem. Phys. 119, 3963-3971(2003).
[5] Faraone, A., Li Liu, and Sow-Hsin Chen, “Model for translation-rotation coupling of molecular motion in water,” J. Chem. Phys. 119, 6302-6313 (2003).

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