Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

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• 作者：Ligia Frunza ; Andreas Schö ; nhals ; Stefan Frunza ; Vasile I. Parvulescu ; Bogdan Cojocaru ; Daniel Carriazo ; Cristina Martí ; n ; Vicente Rives
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：June 21, 2007
• 年：2007
• 卷：111
• 期：24
• 页码：5166 - 5175
• 全文大小：321K
• 年卷期：v.111,no.24(June 21, 2007)
• ISSN：1520-5215

文摘

The rotational molecular dynamics of water confined to layered oxide materials with brucite structure wasstudied by dielectric spectroscopy in the frequency range from 10^-2 to 10⁷ Hz and in a broad temperatureinterval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuationsof water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence ofthe relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov etal. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules inporous glasses is employed also for the layered materials. This model assumes two competing effects: rotationalfluctuations of water molecules that take place simultaneously with defect formation, allowing the creationof free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy ofdefect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters froma fit of this model to the data. The values of these parameters were compared with those found for waterconfined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed indetail, such as the lower the value of the energy of defect formation, the higher the number of defects. Thepre-exponential factor increases with increasing activation energy, as an expression of the compensation law,and indicates the cooperative nature of the motional process. The involvement of the surface OH groups andof the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. Forthe birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution,which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessiteobeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawarelationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity ₀ indetail gives some hint that ₀(T) has also an unusual saddlelike form.