The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 Å, respectively, is examined by broadband dielectric spectroscopy ͑10 −2 -10 9 Hz͒ and differential scanning calorimetry for a wide temperature interval ͑110-340 K͒. The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.

The dielectric relaxation of water in glassy aqueous binary mixtures exhibits an Arrhenius behaviour with a nearly universal activation energy. We here demonstrate that its characteristic relaxation time follows a remarkably general functional dependence on the weight fraction of water for a wide range of molecular systems.10

No abstract

The plate thermometer is a device used mainly to measure temperatures in fire resistance tests according to ISO 834-1 and EN 1363-1 and to measure the so-called adiabatic surface temperature. However, it can also be used to measure incident radiant heat flux ( _ q0 inc ) as a simpler, more robust and less-expensive alternative to water-cooled heat flux meters. The accuracy of the measured _ q0 inc is subject to simplifications in the heat transfer analysis model and uncertainties of parameters such as convective heat transfer coefficients, emissivities and ambient gas temperatures. This study investigates the accuracy of the model itself, isolated from the uncertainties of the physical surrounding, by comparing a simple one-dimensional model to the results of finite element modelling. The so-obtained model includes a heat transfer coefficient due to heat losses of the plate thermometer, found to be K PT = 8 W/m 2 K and a heat storage lumped heat capacity C PT = 4200 J/m 2 K for an ISO/EN standard plate thermometer. The model is also compared to real field experiments.

We present the first broadband dielectric spectroscopy (BDS) and differential scanning calorimetry study of supercooled xylitol-water mixtures in the whole concentration range and in wide frequency (10(-2)-10(6) Hz) and temperature (120-365 K) ranges. The calorimetric glass transition, T(g), decreases from 247 K for pure xylitol to about 181 K at a water concentration of approximately 37 wt. %. At water concentrations in the range 29-35 wt. % a plentiful calorimetric behaviour is observed. In addition to the glass transition, almost simultaneous crystallization and melting events occurring around 230-240 K. At higher water concentrations ice is formed during cooling and the glass transition temperature increases to a steady value of about 200 K for all higher water concentrations. This T(g) corresponds to an unfrozen xylitol-water solution containing 20 wt. % water. In addition to the true glass transition we also observed a glass transition-like feature at 220 K for all the ice containing samples. However, this feature is more likely due to ice dissolution [A. Inaba and O. Andersson, Thermochim. Acta, 461, 44 (2007)]. In the case of the BDS measurements the presence of water clearly has an effect on both the cooperative α-relaxation and the secondary β-relaxation. The α-relaxation shows a non-Arrhenius temperature dependence and becomes faster with increasing concentration of water. The fragility of the solutions, determined by the temperature dependence of the α-relaxation close to the dynamic glass transition, decreases with increasing water content up to about 26 wt. % water, where ice starts to form. This decrease in fragility with increasing water content is most likely caused by the increasing density of hydrogen bonds, forming a network-like structure in the deeply supercooled regime. The intensity of the secondary β-relaxation of xylitol decreases noticeably already at a water content of 2 wt. %, and at a water content above 5 wt. % it has been replaced by a considerably stronger water (w) relaxation at about the same frequency. However, the similarities in time scale and activation energy between the w-relaxation and the β-relaxation of xylitol at water contents below 13 wt. % suggest that the w-relaxation is governed, in some way, by the β-relaxation of xylitol, since clusters of water molecules are rare at these water concentrations. At higher water concentrations the intensity and relaxation rate of the w-relaxation increase rapidly with increasing water content (up to the concentration where ice starts to form), most likely due to a rapid increase of small water clusters where an increasing number of water molecules interacting with other water molecules.

Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) were employed to study mixtures of xylitol and water. The results were also related to a previous dielectric relaxation study on the same system. In the temperature range of the DLS measurements the viscosity related structural (α) relaxation is too fast to be observed on the experimental time scale, but a considerably slower exponential and hydrodynamic relaxation process is clearly observable in the polarized light scattering data. A similar ultraslow process has been observed in many other types of binary liquids and commonly assigned to long-range concentration or density fluctuations. In some studies this interpretation has been supported by observations of substantial structural inhomogeneities in static light scattering or SANS experiments. However, in this study we observe such an ultraslow process without any indication of structural inhomogeneities on length-scales above 2 nm. Hence, we suggest that our observed ultraslow process is due to long-range diffusion of single xylitol molecules or small clusters of a few xylitol molecules (and perhaps some associated water molecules) which are randomly dispersed and sufficiently small to not be structurally detected in our SANS study. In the q-range of the DLS measurements this ultraslow relaxation process is around room temperature several orders of magnitude slower than the structural α-relaxation. However, if its 1/q(2)-dependent relaxation time is extrapolated to q-values where relaxation times from dielectric spectroscopy and quasielastic neutron scattering are compatible (about 10 nm(-1)), a relaxation time similar to that of the dielectric α-relaxation is obtained. Thus, the large difference in time scale between the two relaxation processes in the q-range of a DLS study is due to the fact that the α-relaxation is cooperative in nature, rather than caused by long-range single particle diffusion, and thus q-independent at low q-values.

No abstract

The glass transition temperature, T(g), of a binary mixture commonly varies monotonically between the T(g)s of its two components. However, mixtures of strongly associating liquids can instead exhibit a nonmonotonic T(g) variation. The origins of such nonideal mixing behavior have often been correlated with composition dependent structural variations. For binary mixtures between a hydrogen- (H-) bonded liquid and water, however, such behavior is generally not well understood. The ubiquity and importance of aqueous mixtures both in nature and in man-made applications stresses the needed for a better understanding. We here demonstrate nonmonotonic T(g) variations in binary mixtures of n-propylene glycol monomethyl ethers (nPGMEs) and water, where the composition dependent T(g) show maxima within an intermediate composition range. We show that these T(g) maxima correspond to crossovers in the composition dependence of the step amplitude in the isobaric heat capacity at T(g). We further demonstrate that the observed effects are caused by H-bond interactions involving the nPGME hydroxyl group. We can account for our obervations using a simple model based on two effects due to the added water: (i) an H-bond induced formation of effective relaxing entities and (ii) a plasticizing effect at high water contents.