Rotational Fluctuations of Water inside the Nanopores of SBA-Type Molecular Sieves

Abstract

The rotational molecular dynamics of water confined to nanoporous molecular sieves of a regular hexagonal (SBA-15) and of a foamlike pore structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(9) Hz and in a broad temperature interval. Two relaxation processes were observed: the process at lower frequencies is related to water molecules forming a layer, which is strongly adsorbed at the pore surface, whereas the relaxation process at higher frequencies is assigned to fluctuations of water molecules situated close to the center of the pore. The relaxation times of the low-frequency process for both materials and of the high-frequency process for the SBA-15 material have an unusual saddlelike temperature dependence, reported here for the first time. To describe this temperature dependence, a model developed for water confined to nanoporous glasses by Ryabov et al. [J. Phys. Chem. B 2001, 105, 1845] was applied, which considers two competing effects. The characteristic features of these two competing processes were compared with those reported for other porous systems.

Reply to “Comment on ‘Investigating hydrogen dependence of dynamics of confined water: Monolayer, hydration water and Maxwell-Wagner processes’ [J. Chem. Phys. 133, 037101 (2010)]”

“…The qualitative feature of the relaxation time reflects that observed in experiments. 2,7 The reason why this is strikingly similar to the relaxation map shown in Ref. 1 is that the authors misinterpreted the model such that the volume of the voids changed with temperature as y͑T͒.…”

Water-Triggered Conduction Mediated by Proton Exchange in a Hygroscopic Fulleride and Its Hydrate

“…Our results demonstrate that the strong decrease in the sample's conductivity upon desorption of hydration water can lead to a non-monotonic temperature dependence of the loss feature, although by itself it is not sufficient to explain the simultaneous observation of anomalous temperature dependencies of distinct loss features reported in some samples. 10,23 Below 348 K the loss spectrum does not exhibit any change in line shape but rather undergoes a rigid shift to lower frequency as the temperature is raised with no loss of spectral intensity. The line shape variation and the decrease in strength of the loss feature (by a factor of three) occur only above 348 K (inset to panel d).…”

“…14 The non-monotonic temperature-dependence of σ dc and of the frequency of the loss maximum observable in Figure 2 may be at first surprising, as one expects both the conductivity and the characteristic loss frequency to increase with temperature. 18 However, such behavior is not uncommon in water containing porous systems near the water desorption temperature, 1,2,10,[23][24][25][26] as will be discussed in Section 3.b. Given the complexity of the results and the presence of two modalities of hydration in the as-stored powder (Figure 1c), we have probed separately the effect of the structural and surface hydration water.…”

Comment on “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Chem. Phys. 128, 154503 (2008)]

“…The subject of water in confined porous matrices has been an area of our interests for about a decade. [2][3][4][5][6] Therefore, the study of Sjöström et al 1 attracted our attention and we would like to point out some questionable, from our point of view, aspects of their investigation.…”

“…In general, it is believed that the Johari-Goldstein relaxation is a precursor of glassy dynamics but no glass transition is detected in the system. In literature [2][3][4][5][6]8,9 this low temperature process is usually ascribed to the icelike behavior of water molecules.…”