Authors:A. Smallwood, P. Thomas, A. Ray, and P. Šimon
The dehydration of samples of a Coober Pedy, South Australian sedimentary
white opal, displaying play of colour, was investigated using TMA by heating
the samples of the specimen to a range of temperatures between room temperature
and 1000 at 200C intervals followed by cooling to room temperature. Etched
fracture surfaces of the samples were then examined using SEM. The samples
showed the typical expansion at low temperature up to 210C before contraction
was observed. The contraction of the opals was ascribed to both sintering,
supported by morphological change observed in the SEM micrographs, and dehydroxylation
of the silanol groups producing silicon-oxygen-silicon bridges resulting in
a more dense silica network.
Authors:B. Berton, V. Dupray, H. Atmani, and G. Coquerel
Dehydration of trehalose dihydrate implemented by slow heating (1 K min−1), has been monitored by Raman microspectroscopy from 25 to 110�C directly on single crystals. Between 90 and 120�C, gas initially
trapped in irregular macroscopic defects, reorganizes to form spherical vacuoles. The Raman analysis of these vacuoles highlights
that the areas in vicinity of the defects are the first affected by the dehydration mechanisms. Indeed, the progressive amorphization
of the crystal starts around these defects.
Drought stress is often the most limiting factor for maize and sorghum production in the semi-arid areas. This study evaluates the enzymatic antioxidant protection mechanism response of maize (cv Melkassa-2) and sorghum (cv. Macia) after exposure to and recovery from pre- and post-flowering dehydration.The response of enzymatic antioxidant protection systems revealed that in both test crops dehydration during both the pre- and post-flowering stages resulted in increased activities of enzymatic antioxidant protection mechanisms (SOD, GR, CAT and APX). There were, however, differences between the species in the type and extent of enhanced developmentally-induced and dehydration-induced antioxidant activities. Differences were also noticed in the relative water contents at which changes in enzymatic antioxidant activities occurred. Under dehydration conditions, sorghum was generally found to have relatively higher enzymatic antioxidant activities, providing it better protection against oxidative stress by minimizing the level of lipid peroxidation.Lipid peroxidation, measured as MDA content, was increased in both species during pre- and post-flowering dehydration, but the increase was greater in maize than in sorghum during both developmental stages. Sorghum appeared to be able to reduce MDA on rehydration, but maize contained only 85% less MDA after rehydration as compared to the control following pre-flowering rehydration. During post-flowering rehydration, neither species was able to decrease the MDA content to the control level.The results indicated that tolerance to drought in sorghum is well associated with the consistent enhanced capacity of the enzymatic antioxidant system under both pre- and post-flowering dehydration conditions, and that the sensitivity of maize to drought is linearly correlated to the decreased capacity of the antioxidant system. It may be concluded that, since differences were observed between the species in the response of enzymatic antioxidants to pre- and post-flowering dehydration/rehydration, with sorghum exhibiting comparatively higher overall activities of enzymatic antioxidants and a lower level of MDA than maize during both pre- and post-flowering dehydration, selection based on these criteria may help in the development of genotypes tolerant to dehydration.
Investigations were made on the seed viability (standard germination test and vigour after accelerated ageing) and seed quality (starch, protein, lipid, sucrose, glucose and fructose) of seeds of maize (cv Melkassa-2) and sorghum (cv Macia) harvested from plants after exposure to and recovery from pre- and post-flowering dehydration. The objectives of the study were to achieve a better understanding of 1) the effects of water deficit during the pre- and post-flowering stages on the seed viability and food (storage reserves) quality, and 2) the effects of dehydration and rehydration cycles at critical growth stages on subsequent seed performance and production, which could lead to the development of cultivars more able to efficiently partition assimilates to the reproductive organs in the field. The experiment was conducted in a controlled environment growth chamber under constant environmental conditions (12/12 h day/night, 28–32/17°C day/night temperature, 60–80% RH and 1200–1400 μmol m
PPDF). The seed viability and vigour tests were done on air-dried seeds and the seed quality analysis on freeze-dried seeds of both species.The results of the standard germination test indicated that sorghum seeds harvested after both pre- and post-flowering dehydration were not affected by the treatment, while maize seeds had reduced % germination. Sorghum seeds harvested after post-flowering dehydration had significantly decreased vigour after accelerated ageing. Dehydration during both the pre- and post-flowering stages resulted in reduced contents of protein, lipid and soluble carbohydrates (sucrose, glucose and fructose) in both species as compared to the control seeds. The species differed in the extent to which these reserves were reduced.
Authors:Karen L. Koster, Catherine L. Sommervold, and Yao Ping Lei
We studied the effects of storage temperature on the stability of dehydrated POPC (1-palmitoyl-2-oleoyl-phosphatidylcholine) mixed with sucrose, trehalose, or a sucrose/raffinose mixture. We used DSC to measure the gel-to-fluid phase transition temperature (Tm) of POPC after incubation either below or near the glass transition temperature (Tg) of the sugars in the mixture. Glass formation by the sugars around fluid-phase POPC led to the lowering ofTm below that of the fully hydrated lipid. Phospholipid phase behavior did not change during storage belowTg. In some samples stored aboveTg, trehalose crystallized completely; in these samples, theTg of POPC increased to that of the partially dehydrated phospholipid. Melting the crystalline sugar re-established its ability to lower POPC'sTm. We conclude that prevention of complete sugar crystallization was important for stability in the dry state, and that storage belowTg conferred long-term stability to the dehydrated sugar-lipid mixtures.
Authors:C. Bhongale, A. Ghule, R. Murugan, and H. Chang
In this work, dehydration of sodium diphosphate decahydrate Na4P2O7⋅10H2O and phase transformations of Na4P2O7 in open air have been studied in detail by thermo-Raman spectroscopy. The spectra were measured continuously in a temperature
range from room temperature up to 600C for the bands of P2O74- and H2O. The spectral variation showed one step of dehydration and four-phase transformations. The thermo-Raman intensity(TRI) and
differential thermo-Raman intensity (DTRI) curves calculated from the characteristic bands of H2O also showed one step of dehydration with the loss of all hydrated water in the temperature interval from 45 to 69C. Thermogravimetric
measurements supported this result. The thermo-Raman investigation indicated the transformation of Na4P2O7 from low temperature phase to high temperature phase proceed through pre-transitional region from 75 to 410C before the
major orientational disorder at 418C and minor structural modifications at 511,540 and 560C. The results from differential
scanning calorimetry and differential thermal analysis on Na4P2O7 showed endotherms at 407,517, 523, 548, 557C and 426, 528, 534, 555, 565C, respectively.
Authors:A. Coetzee, M. Brown, D. Eve, and C. Strydom
Both isothermal and programmed temperature experiments have been used to obtain kinetic parameters for the dehydrations and
the decompositions in nitrogen of the mixed metal oxalates: FeCu(ox)2·3H2O, CoCu(ox)2·3H2O and NiCu(ox)2·3.5H2O, [ox=C2O4]. Results are compared with those reported for the thermal decompositions of the individual metal oxalates, Cuox, Coox·2H2O, Niox·2H2O and Feox·2H2O. X-ray photoelectron spectroscopy (XPS) was also used to examinee the individual and the mixed oxalates.
Dehydrations of the mixed oxalates were mainly deceleratory processes with activation energies (80 to 90 kJ·mol−1), similar to those reported for the individual hydrated oxalates. Temperature ranges for dehydration were broadly similar
for all the hydrates studied here (130 to 180°C).
Decompositions of the mixed oxalates were all complex endothermic processes with no obvious resemblance to the exothermic
reaction of Cuox, or the reactions of physical mixtures of the corresponding individual oxalates.
The order of decreasing stability, as indicated by the temperature ranges giving comparable decomposition rates, was NiCu(ox)2>CoCu(ox)2>FeCu(ox)2, which also corresponds to the order of increasing covalency of the Cu−O bonds as shown by XPS.
Authors:P. Thomas, P. Šimon, A. Smallwood, and A. Ray
The dehydration of an opal specimen was investigated
by thermogravimetric analysis (TG) in powder and bulk forms. The change in
geometry resulted in a significant difference in the temperature range in
which dehydration occurred with peak temperatures in the differential TG (DTG)
curve for the hand ground opal at 203°C and for the bulk opal at 340°C.
This difference was attributed to time taken for diffusion of free water in
the bulk opal to the specimen surface prior to evolution as a registered mass
loss. A model was proposed to account for the diffusion of water and was used
to estimate the diffusion coefficient.
Non-isothermal dehydration of copper chloride dihydrate and nickel chloride hexahydrate were studied by using TG, DTG, DTA
and DSC measurements. The copper chloride salt loses its two water molecules in one step while nickel chloride salt dehydrates
in three consecutive steps. The first two steps involve the loss of 4 water molecules in two overlapped steps while the third
step involves the dehydration of the dihydrate salt to give the anhydrous NiCl2.
Activation energies (ΔE) and the frequency factor (A) were calculated from DTG and DTA results. We have also calculated the different thermodynamic parameters, e.g. enthalpy
change (ΔH), heat capacity (Cp) and the entropy change (ΔS) from DSC measurements for both reactants.
The isothermal rehydration of the completely dehydrated salts was studied in air and under saturated vapour pressure of water.
Anhydrous nickel chloride was found to rehydrate in three consecutive steps while the copper salt rehydrated in one step.
Authors:M. Ezahri, M. El Hadek, G. Coffy, and B. Mentzen
The study of the dehydration/rehydration of ammonium tris-oxalato aluminate(III) (NH4)3Al(C2O4)3⋅3H2O in flowing dinitrogen saturated with water vapor at room temperature, using thermogravimetric analysis and X-ray diffraction
techniques, allowed the determination of the temperature stability domains of (NH4)3Al(C2O4)3⋅3H2O, (NH4)3Al(C2O4)3⋅2H2O and the anhydrous salt. The X-ray powder diffraction profiles are reported for each of the three phases.