Application of a Fickian model of diffusion to the dehydration of graded specimens of a precious Australian sedimentary opal derived from Coober Pedy

A model developed for the estimation of the diffusion coefficient based on Fickian diffusion is applied to the dehydration of a Coober Pedy white play of colour (precious) opal using thermogravimetric analysis (TG). The model was originally applied to bulk and powdered opal (opal with no bulk). In this paper the opal was graded prior to TG analysis. The diffusion coefficient was calculated and is reported up to the critical point of water.     

TMA and SEM characterization of the thermal dehydration of australian sedimentary opal

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 200°C 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 210°C 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.     

Isopropanol diffusion and dehydration in zeolite Hzsm-5. Spectrokinetic study

The isopropanol diffusion and dehydration in zeolite HZSM-5 were studied by FTIR spectroscopy. The alcohol dehydration was shown to start at the temperature above 60°C. It was found that at temperatures above 350°C, the isopropanol dehydration was limited by diffusion in the mesopores of zeolite. The method developed allows studying the kinetics of interactions of the reagent molecules with the OH-groups at the external surface of the zeolite and inside its channels.     

Low temperature DSC characterisation of water in opal

A low temperature (?60 to +105 °C) DSC characterisation of opal was carried out to determine the proportion of crystallisable water and to estimate the cavity size in which the crystallisable water is contained. Circa 10 % of the molecular water contained in the opals was found to be crystallisable suggesting that the remaining molecular water is present either trapped in silica cages or surface-adsorbed in micropores. For the opals derived from a sedimentary environment in Australia, the crystallisable water was found to melt in a manner consistent with the melting of bulk water, suggesting that the water is contained in cavities in the opal. The lack of depressed melting temperatures suggested little or no mesoporosity. A volcanic opal specimen of Mexican origin was found to contain both mesoporous and cavity water, while a Tintenbar opal, also of volcanic origin, was found to contain only mesoporous water due to the melting of the crystallisable water, with an estimated pore diameter size range 4–7 nm. The differences in mesoporosity observed between the volcanic and sedimentary opals are consistent with the demarcation in the physical properties observed between these types of opals in previous studies.     

Kinetics of the dehydration of boehmites prepared under different hydrothermal conditions

The kinetics of the dehydration of five boehmites, which were prepared under different hydrothermal conditions (300°C, 85 atm, 20 h — 150°C, 4.5 atm, 5 h), were studied by means of isothermal TG at an air pressure of 150 mm Hg and at a constant flow rate of nitrogen (30 ml min^?1) containing water vapor of partial pressures between 10^?4 and 23.8 mm Hg. It was found that the dehydration temperature of boehmites was lowered as the preparation conditions became more mild. Moreover, the rate-controlling step of the dehydration of boehmites varied with their preparation conditions. The water vapor pressure led to decreases in the rate of dehydration.     

Effect of neutralization and cross-linking on the thermal degradation of chitosan electrospun membranes

Thermal degradation of as-electrospun chitosan membranes and samples subsequently treated with ethanol and cross-linked with glutaraldehyde has been studied by thermogravimetry (TG) coupled with an infrared spectrometer. The influence of the electrospinning process and cross-linking in the electrospun chitosan thermal stability was evaluated. Up to three degradation steps were observed in the TG data, corresponding to water dehydration reaction at temperatures below 100 °C, loss of side groups formed between the amine groups of chitosan and trifluoroacetic acid between 150 and 270 °C and chitosan thermal degradation that starts around 250 °C and goes up to 400 °C. The Kissinger model was employed to evaluate the activation energies of the electrospun membranes during isothermal experiments and revealed that thermal degradation activation energy increases for the samples processed by electrospinning and subsequent neutralization and cross-linking treatments with respect to the neat chitosan powder.     

TG and DSC investigation of the dehydration of the extra large pore aluminophosphate molecular sieve VPI-5 and related materials

The dehydration of a series of VPI-5 and H3 samples, synthesized under various conditions, as well as the solid state transformation of VPI-5 to AlPO₄-8 have been investigated using combined TG-DTG-DSC and high-resolution solid state³¹P-NMR. The TG curves show a quasi-continuous release of water, the total loss being characteristic for each sample. Complete dehydration is achieved when the samples are heated from 20°C to about 150°C at various beating rates. Besides the main dehydration effect, several weak endothermic peaks are observed. These generally non-reproducible modulated peaks, recorded at high heating rates, are presumably due to the interactions of the water molecules leaving the channels of VPI-5 with the randomly positioned fragments stemming from the destruction of the water triple helix assemblage. The non-isothermal kinetic parameters of the dehydration have been evaluated from the TG and DTG curves recorded at low heating rates. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

The influence of temperature and different storage conditions on the stability of supersulphated cement

The stability of Supersulphated Cement (SSC) is investigated at 95°C when subjected to relative humidities of 100, 53 and 11% of water vapour. Previously [1] investigations at 25, 50, 75°C under the same conditions of humidity reported the stability of ettringite, one of the initial hydration products. At 95°C, decomposition of ettringite, is found at all humidities and is rapid at 100% relative humidity. The hydration products of cement pastes at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). The formation of the hydragarnet, plazolite is recorded during the decomposition/dehydration process enhanced by possible carbonation. Rehydration studies on the products after storage for up to 9 months were carried out using distilled water and the samples tested for ettringite content. It is concluded that ettringite in SSC is inherently unstable at 95°C.     

Thermal evolution of microporous nitroprussides on their dehydration process

Divalent transition metal nitroprussides form a family of microporous materials which lose their crystallization water (coordinated and zeolitic) below 100°C and then remain stable up to above 150°C. The dehydration process of representative samples in their stable phases was studied by thermo-gravimetry (TG) and differential scanning calorimetry (DSC). The copper complex dehydrates in a single step through a practically irreversible process. For cadmium and cobalt complexes the water evolution on heating takes place in two stages. The first one, where only zeolitic waters are removed, is dominated by a diffusion mechanism while, during the loss of the strongly bonded waters (second stage) the material framework effect is added. The involved activation energy and its dependence on the conversion degree were estimated evaluating the thermo-gravimetric data according to an isoconversion model.     

Kinetics and mechanism of non-isothermal dehydration of nickel acetate tetrahydrate in air

The non-isothermal decomposition of nickel acetate tetrahydrate in air was studied using thermogravimetry (TG)–DTG, differential scanning calorimetry (DSC) and XRD techniques. The decomposition occurs in two major steps and the final product is NiO. The dependence of mass loss on heating rates in TG measurements imply that the dehydration and hydrolysis concur at temperature below 240 °C; the apparent activation energies calculated by Flynn, Wall and Ozawa (FWO) isoconversional method indicate the existence of a consecutive process. The kinetics of the first major decomposition step (below 240 °C) was obtained with multivariate non-linear regression of four measurements at different heating rates. According to the kinetics results from non-linear regression, the dehydration reaction (F1 type with an activation energy E of 167.7 kJ/mol) goes first. After the loss of almost half of water, the retained water and acetate are linked to each other by hydrogen bonding, so dehydration and hydrolysis concur. The pathway with a lower E is related to the hydrolysis process and the other is corresponding to the dehydration process. The simulations of reactants at different heating rates were used to verify the correctness of the reaction model. With the kinetics results, the dehydration mechanism was discussed for the first time.     

Thermal behavior of a bentonite

The mineralogical composition of the Kütahya calcium bentonite (CaB) from Turkey was obtained as mass% of 60% calcium rich smectite (CaS), 30% opal-CT (OCT), trace amount illite (I), and some non-clay impurities by using chemical analysis (CA), X-ray diffraction (XRD), and thermal analysis (TG-DTA) data. The crystallinity, porosity, and surface area of the samples heated between 25–1300°C for 2 h were examined by using XRD, TG, DTA and N₂-adsorption-desorption data. The position of the 001 reflection which is the most characteristic for CaS does not affect from heating between 25–600°C and then disappeared. The decrease in relative intensity (I/I ₀) from 1.0 to zero and the increase in full width at half-maximum peak height (FWHM) from 0.25 to 1.0° of the 001 reflection show that the crystallinity of the CaS decreased continuously by rising the heating temperature from 25 to 900°C and then collapsed. The most characteristic 101 reflection for opals intensifies greatly between 900 and 1100°C with the opal becoming more crystalline. The total water content of the natural bentonite after dried at 25, 105 and 150°C for 48 h were determined as 8.8, 5.0 and 2.5%, respectively. The mass loss occurs between 25 and 400°C over two steps with the maximum rate at 80 and 150°C, respectively. The exact distinction of the dehydration temperatures for the adsorbed water and interlayer water is seen almost impossible. The temperature interval, maximum rate temperature, and mass loss during dehydroxylation are 400–800°C, 670°C and 4.6–5.0%, respectively. The maximum rate temperatures for decrystallization and recrystallization are 980 and 1030°C, respectively. The changes in specific micropore volume (V _mi), specific mesopore volume (V _me), specific surface area (S) were discussed according to the dehydration and dehydroxylation of the CaS. The V _mi, V _me and S reach to their maxima at around 400°C with the values of 0.045, 0.115 cm³ g⁻¹ and 90 m² g⁻¹, respectively. The radii of mesopores for the bentonite heated at 400°C are distributed between 1–10 nm and intensified approximately at 1.5 nm.     

Thermal studies on Zirconium hydroxide gel formed by aqueous gelation

Zirconium hydroxide gel has been prepared by a novel aqueous gelation process by the controlled hydrolysis of zirconium oxychloride in the presence of sodium acetate. The gel thus formed has been subjected to thermal analysis: TG, DTG, and DSC. Thermal analysis shows that the gel is continuously dehydrated in the temperature range between room temperature and 500?°C. The total mass loss relative to the initial mass is about 44.1%. Thermal analysis shows that the decomposition takes place in three stages. The gel contains absorbed and coordinated water. In the second stage of dehydration, dehydration of the Zr(OH)₄ gel also takes place along with the removal of the coordinated water. The DSC analysis coupled with TG and structural information, indicate that the exothermic processes between 349 and 460?°C can be attributed to the nucleation process of the formation of tetragonal zirconia, with phase transformation at 460?°C.     

The Conductance Percolation and Droplets Dimension of AOT in Alkanol Systems

The conductance behaviors of AOT in alkanol (hexanol, heptanol, octanol, and decanol) reverse microemulsions have been investigated. The percolation phenomenon induced by water is observed in the water/AOT/decanol system at 15°C and 30°C, and the water/AOT/octanol system at 15°C. The percolation phenomenon of water/AOT/alkanol systems is discussed from the interaction between the hydroxy group of alkanol and the polar group of AOT, droplets diffusion coefficient, and the rate constant for droplets collision. The droplets size and diffusion coefficient of the water/AOT/alkanol systems have also been studied by modifying the water concentration. The results show that hydrodynamic diameter of droplets decreases and diffusion coefficient increases with the increasing of water content, which may be explained by the polarity of alkanol phase.     

Phase Transitions of MBBA Confined in Porous Solids

The diffusive and dynamic mechanical behaviour of an epoxy system containing tetraglycidyl-4,4′-diaminodiphenylmethane and a multifunctional Novolac glycidyl ether cured with 4,4′-diaminodiphenylsulfone was studied after water sorption. The diffusion of water was performed at 100% relative humidity, by immersion of specimens in water at 20, 40, 70 and 100°C. In all sorption experiments, the water diffusion followed Fick's law. Diffusion coefficients and saturated water concentrations were estimated for these temperatures. The activation energy for diffusion was determined from the relationship linking the diffusion coefficient and the reciprocal of the absolute temperature. The value obtained was 45.7 kJ mol^-1. Dynamic mechanical analysis of samples immersed in water at 25 and 100°C, and with various water contents, showed a shift in T_g> (defined by the tanδ peak) to lower temperatures over the glass transition region, and a slight decrease in the dynamic storage modulus in the presence of water as a result of a plasticization effect. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of water on the transport of oxygen through nylon-6 films

The effect of presence of water on the transport of oxygen through films of Nylon-6 was evaluated at 5, 23, and 40°C by permeation experiments. Through the oxygen permeability experiments it was found that the diffusion of oxygen through Nylon-6 is not a simple Fickian process and the total diffusion process can be expressed by a bimodal diffusion mechanism. Permeability, solubility, and diffusion coefficients were determined as a function of water activity for both mechanisms. The effect of sorbed water on the oxygen diffusion and solubility in the polymer is presented as a function of the state of water in the polymer and as a result of the molecular competition between water and oxygen. © 1994 John Wiley & Sons, Inc.     

Viscosity effects in cobaloxime‐mediated catalytic chain‐transfer polymerization of methacrylates

The effect of bulk viscosity on the cobaloxime‐mediated catalytic chain‐transfer polymerization of methacrylates at 60 °C was investigated by both the addition of high molecular weight poly(methyl methacrylate) to methyl methacrylate polymerization and the dilution of benzyl methacrylate polymerization by toluene. The results indicate that the bulk viscosity is not directly linked to the chain‐transfer activity. The previously measured relationship between chain‐transfer‐rate coefficient and monomer viscosity therefore probably reflects changes at the molecular level. However, the results in this article do not necessarily disprove a diffusion‐controlled reaction rate because cobaloxime diffusion is expected to scale with the monomer friction coefficient rather than bulk viscosity. Considering the published data, to date we are not able to distinguish between a diffusion‐controlled reaction rate or a mechanism directly affected by the methacrylate substituent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 782–792, 2002; DOI 10.1002/pola.10152     

Phase equilibrium and mutual diffusion in the poly(sulfone)–dimethylsulfoxide system

Phase equilibrium and mutual diffusion in the poly(sulfone) (PSF)–dimethylsulfoxide (DMSO) system have been studied at temperatures from 20 to 110°C over a wide solution composition range. The phase diagram for this system has been obtained and the effect of water on boundary concentrations has been studied. It is shown that the presence of water in DMSO has a considerable effect on the binodal curve. Thus, increasing the water content up to 1.3% by weight results in the displacement of the upper critical solution temperature by 30°C and the widening of the two-phase region. The kinetic regularities in mutual dissolution of components have been investigated. The concentration dependencies of the mutual diffusion coefficients in the systems studied are presented. The effect of moisture in DMSO on the mutual diffusion coefficients appears near the phase transition in the system. © 1994 John Wiley & Sons, Inc.     

TG and DTA Study of the Thermal Dehydration of Metal-exchanged Zeolite-4A Samples

Zeolite-4A is a hydrated aluminosilicate which becomes more hydrated when exchanged with transition metals. In this work, the dehydration kinetics of cobalt, nickel and copper(II)-exchanged zeolite-4A were studied by means of TG and DTA over the temperature range from 20 to 500°C, and the numbers of water molecules in the metal-exchanged zeolite samples were calculated. It was observed that, as the ionic radius of the hydrated metal increased, the number of water molecules also increased. The loss of water from the zeolite samples generally occurred in the temperature range 100–300°C and was manifested in the DTA graphs by an extended endothermic effect. The DTA curves demonstrated that the peak position shifted towards lower temperatures as the metal concentration increased or, in other words, the water of hydration increased. The kinetic parameters (order of reaction and activation energy) were calculated via the Coats and Redfern method. The process of dehydration was found to follow first-order kinetics. This revised version was published online in August 2006 with corrections to the Cover Date.     

TG,DTA and electrical conductance properties of some Cu(II) AND Mn(II) bisazo- dianils complexes

The TG and DTA of a new series of Mn(II) and Cu(II) complexes with a number of newly prepared bisazo-dianil ligands were studied in the temperature range (20-700°C). The TG and DTG curves display to main steps, the first one within the temperature range (25-330°C) correspond to the elimination of water or and ethanol from the complexes. The second step within the range (350-625°C) is due to the decomposition of the complexes yielding the metal oxides as the final product. The rate constants of the dehydration and decomposition reactions were determined, from which some kinetic parameters were evaluated. The DTA curves show that the dehydration of the metal complexes is an endothermic reaction. In all cases the anhydrous metal complexes undergo exothermic decomposition reactions to give the metal oxide. The thermodynamic parameters (ΔE, ΔH, ΔS, ΔG) for the occurring processes are calculated. The electrical conductivities of the solid complexes were measured and the activation energy of the complex and its free ligand are also calculated. This revised version was published online in August 2006 with corrections to the Cover Date.