The kinetics and mechanism of the dehydration and decomposition of heteropolyacids of molybdenum, tungsten and vanadium (H3+xYx+M12O40mH2O;
Y=Si, P; M=Mo, W) were studied. The data obtained on the dehydration kinetic parameters correlate with the expected structures,
of these crystal hydrates, the IR data and X-ray phase analysis.
The thermal dehydration of copper(II) acetate hydrate has been studied between 353 and 406 K, over a range of humidities.
The dehydration is controlled by nucleation-and-growth kinetics at low temperatures, with an activation energy of 154 kJ·mol−1, which changes to contracting-disc kinetics at higher temperatures with a lower activation energy of 76 kJ·mol−1. Frequency factors have also been derived; the value for the high temperature process is low (107s−1) and that for the low temperature step is high (1017s−1). Optical microscopy has been used to clarify the bulk kinetics; there is evidence for a reactive layer at the surface of
the decomposing solid.
The kinetic parameters of dehydration were determined under non-isothermal conditions for different polystyrenedivinylbenzene
sulfonic acid type cationites (DVB) and their dependence on the degree of cross-linking granulation, porosity, specific surface,
content of SO3M groups (M=Li, Na, K, Rb), nature of the alkali metal in partially neutralized -SO3H groups and heating rate was investigated.
Authors:L. M. Yarisheva, L. Yu. Kabal'nova, A. A. Pedy, and A. L. Volynskii
The thermophysical behaviour of Nylon-6 with various moisture contents was studied. It was shown that the thermal effects occurring below the Nylon-6 melting temperature are due to the dehydration process. The temperature of the heat flow maximum is a function of the state of the water molecules in the polymer. It was found that the thermophysical study of Nylon-6 in the temperature region below the polymer melting temperature allows a more precise value of its melting heat to be obtained.
Authors:M. Sikorska-Iwan, Renata Mrozek, A. Ostasz, and Z. Rzączyńska
The 5-nitro-2-anthranilates of lanthanum(III), samarium(III), terbium(III), erbium(III) and lutetium(III) were obtained as
hydrates having 2.5 mol of water molecules per 1 mol of compound. The compounds are isostructural. The processes of dehydration
and rehydration were investigated. The first step of dehydration does not cause the change of crystal structure. The entire
dehydration gives anhydrous compounds with different structure than the structure of hydrates. However, the dehydration of
La, Sm, Tb and Er is reversible - the rehydration process gives the complexes having the same crystal structure as the initial
compounds. Only the anhydrous lutetium complex under the influence of moisture does not give the starting compound.
A procedure for measurement of the heat of zeolite dehydration by scanning heating has been designed. Simultaneous data on heat flow (DSC) and mass loss (TG) are required for evaluation. The heating rate depends on the experimental conditions (point-spread function, sample mass, crucible design, and calorimetric reproducibility). Dehydration measurements have three advantages as compared with the sorption procedure: i) one can investigate samples with irreversible dehydration; ii) no approximation model is needed for calculation of the partial molar heat of dehydration; and iii) the procedure is not labor-consuming.The procedure was tested on the natural zeolites heulandite, chabazite and mordenite. The results are close to those measured by the sorption procedure. The partial molar heat of dehydration was found to depend on the water content. It increases from 50 to 87 J mol–1 K–1 for heulandite, from 53 to 81 J mol–1 K–1 for chabazite, and from 51 to 71 J mol–1 K–1 for mordenite.The approximation of the heat of sorption by linear regression was found to be wrong. Detection of a phase transitioN after this approximation has no meaning.
Authors:B. Janković, B. Adnađević, and J. Jovanović
An isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel in the temperature range from 306 to 361 K was
investigated. The specific parameters connected with shape of the conversion curves were defined. The activation parameters
(E, lnA) of the isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel were calculated, using Johnson-Mehl-Avrami
(JMA), ‘initial rate’ and ’stationary point’ methods. The reaction models for the investigated dehydration are determined
using the ‘model-fitting’ method. It was established that both, the reaction model and activation parameters of the hydrogel
dehydration were completely different for the isothermal process than for the non-isothermal one. It was found that the increase
in dehydration temperature lead to the changes in isothermal kinetic model for the investigated hydrogel dehydration. It was
established that the apparent activation energy (E) of hydrogel dehydration is similar to the value of the molar enthalpy
of water evaporation.
Authors:A. Kochanowski, R. Dziembaj, M. Molenda, A. Izak, and E. Bortel
Key issue in the gelcasting method is the way water is released from
the ceramic–hydrogel system. It is the first step to the formation of
ceramic materials called green body.
The purpose of the presented
investigations is to establish the range of temperatures in which dehydration
of the various hydrogels takes place, and at what temperatures the eight prepared
hydrogels are disintegrated.
The set of hydrogels polymers was
obtained by radical polymerization from ionic and non-ionic monomers. The
polymers were solved in water causing formation of clear gels. The dehydration
and thermal decomposition of the obtained hydrogel samples was studied using
thermal analysis techniques. The amount of water contained in hydrogels was
determined as well as the temperature and products of polymer disintegration.
Enthalpies of dewatering were also determined.
Hydrated inclusion complexes of the hosts β-CD (CD=cyclodextrin), γ-CD and permethylated β-CD with the guest clofibric acid
were analysed by TG and DSC methods to characterise their dehydration behaviours. Activation energies for dehydration of the
β- and γ-CD clofibric acid complexes, determined by isothermal thermogravimetry, are significantly lower (∼20-25%) than those
for the corresponding uncomplexed hydrated CDs. These data can be reconciled with X-ray structural data which show that H2O molecules in the complexes occupy different crystal sites from those occupied in the parent CDs.
Authors:S. de Castelnuovo, J. Harness, and I. McColm
Simultaneous Differential Thermal Analysis/Thermogravimetric experiments carried out on one large single crystal, several small single crystals and powdered crystals of pentahydrate copper sulphate have been used to demonstrate the role that retained liquid water plays in maintaining crystal morphology during dehydration. Measured activation energies for stepwise dehydration in the system show the presence of solution-based transformations provide lower energy paths for the dehydration steps and stress relieving mechanisms. Skeletal anhydrous crystals from large-sized pentahydrate copper sulphate have the same morphology as the starting crystal on complete dehydration at controlled heating rates as long as a solution phase is maintained within the crystal during decomposition. The athermal activation energies, in this work, are in agreement with those obtained by isothermal methods as long as coincident reaction paths for the two techniques are maintained. The literature has been reviewed in the light of this work and a three-stage process is presented to rationalise the conflicting information obtained by workers using a variety of different experimental techniques.