Water relaxation enthalpy

An Arrhenius-type analysis of temperature dependence can be used to calculate the enthalpy and entropy of activation for the relaxation process. For liquid water, the enthalpy of activation is 19 kjmol-1, which corresponds approximately to the energy required to break one hydrogen bond. For ice, the equivalent enthalpy is 54 kj mol-1,... 

The reliable estimation of residual water in dried solids is of importance but is beset by several problems, mainly related to the shape and interpretation of DSC heating traces, as illustrated in Figure 11 for a typical aqueous mixture, maximally frozen, from which any relaxation enthalpy contribution has been removed by annealing." The drawn-out DSC heating trace represents the superposition of several distinct processes Tg of the mixture, the heat of dilution, produced by ice... 

In measurements of the dielectric relaxation of water adsorbed on acetylated wood, a large change in the activation enthalpy and entropy of dielectric relaxation was found to occur at 6 % moisture content (Zhao etal., 1994), this presumably being attributable to the onset of formation of capillary water in the cell wall. 

The ionization potentials of some of the bipyridines have been investigated. Solubility data for 2,2 -bipyridine in aqueous solution, in aqueous solvent mixtures, and in various aqueous salt solutions have been obtained, whereas the heat of solution, heat capacities, and related data for 2,2 - and 4,4 -bipyridines in water have been measured. The enthalpies of solution of 2,2 -bipyridine in water and aqueous solvent mixtures have also been obtained. Dielectric relaxation studies of 2,2 -bipyri-dine in carbon tetrachloride have been reported in connection with hindered internal rotation. Partition coefficients for 2,2 -bipyridine between water and various organic solvents have been measured. ... 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

Enthalpy Relaxation in Freeze-Concentrated Sucrose-Water Glasses... 

The objective of the present study is to investigate enthalpy relaxation of a very common model food system, sucrose-water, by using DSC. In accordance with the previous study, the Kohlraush-William-Watts (KWW) equation is applied to calculate the relaxation time and the activation energy of the enthalpy relaxation. 

Kim, Y.J., Hagiwara, T., Kawai, K., Suzuki, T., and Takai, R. Kinetic process of enthalpy relaxation of glassy starch and effect of physical aging upon its water vapor permeability property, Carbohydr. Polym., 53, 289, 2003. 

Water and low temperature (20-35 C) aging influence the dynamic mechanical properties of a poly(amide-imide). At concentrations below 2 weight percent, water contributes to a low temperature relaxation between -120 and -50 C. Above 2 weight percent the water influences the beta transition. The enthalpy of activation for the beta relaxation is dependent upon aging temperature and time. Aging temperatures closer to the beta transition temperature result in higher activation enthalpies for that dispersion. 

Based on the dielectric and dynamic mechanical data, it appears that water and small polar molecules contribute to three dispersions in this poly(amide-imide). One is the low temperature relaxation between -100 and 0°C. This may be a hydrogen bonded relaxation since the activation enthalpy was 30 kJ/mol. This occurs at concentrations of water ranging between 0 to 4 weight percent. Two, the dielectric relaxation between 0 and 70 C can probably be attributed to conductive contaminants whose mobility is dependent upon a minimum amount of water. Three, at high water concentrations, greater than 2 weight percent, the water/NMP contributes to the beta relaxations observed between 50 and 150 C. 

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