Relaxation time nanopores

Fig. 24 Relaxation time map of bulk PMPS and PMPS films of different thicknesses confined to nanoporous glass, as observed with dielectric spectroscopy, thermal analysis, and INS [60]...

Dielectric spectroscopy and scattering studies on the structural relaxation in many different materials have assumed that the normal T-dependence of the relaxation time of a liquid will closely resemble that of propylene glycol (PG), that is, both bulk water and confined PG relax in the same manner, and with an apparent continuity. The main relaxation time of PG exhibits a thermal behavior that differs from that proposed for bulk and confined water. Confined water relaxation times seem substantiaiiy altered when compared to bulk water (which evidently is not the case in confined EG). It also shows an apparent FSC. In addition, an even more dramatic change in the T-dependence of water confined in nanoporous MCM-41 is clearly evident. These results are not unique in that they simply exhibit the typical behavior of supercooled water in biological materials and in other confined environments. Thus, we consider both bulk and confined ethylene glycol (EG, OHCH2CH2OH). Figure 17 shows the EG dielectric relaxation times studied. 

Recent applications of relaxation dispersion measurements to concrete or cement-based materials are promising for characterizing reactive nanopor-ous materials, the structure of which may evolve over time (75-78). The MRD profiles have provided, for the first time, a direct means for characterizing the specific surface area, Sp, of a hydrated cement-based material (79), without exposing the sample to extremes of temperature or pressure (80-83). The interest in such a surface area is to provide information on the microsctruc-ture and its impact on macroscopic or structural properties. The method is based on a clear separation of surface and bulk contributions of the overall... 

In summary, it appears from spectroscopic studies such as neutron scattering or NMR relaxation measurements which probe rotational water motions on a short time scale, 10 -10 s, and thus over a short distance range that, at the highest water contents, water mobility within the pore of an ionomeric membrane is not drastically different than bulk water mobility. However, as the water content of the membrane decreases, its mobility is increasingly hindered. The nanopore liquid in the membrane is essentially a concentrated acid solution and ion-water (as well as ion-ion) interactions will have significant influences on water motion. Intrusions of sidechains... 

Korb el al. proposed a model for dynamics of water molecules at protein interfaces, characterized by the occurrence of variable-strength water binding sites. They used extreme-value statistics of rare events, which led to a Pareto distribution of the reorientational correlation times and a power law in the Larmor frequency for spin-lattice relaxation in D2O at low magnetic fields. The method was applied to the analysis of multiple-field relaxation measurements on D2O in cross-linked protein systems (see section 3.4). The reorientational dynamics of interfacial water molecules next to surfaces of varying hydrophobicity was investigated by Stirnemann and co-workers. Making use of MD simulations and analytical models, they were able to explain non-monotonous variation of water reorientational dynamics with surface hydrophobicity. In a similar study, Laage and Thompson modelled reorientation dynamics of water confined in hydrophilic and hydrophobic nanopores. 

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