Search Results
Abstract
In this work two aluminic pozzolans (metakaolins) and a non-pozzolan were added to two Portland cements with very different mineral composition, to determine the effect on the rate of heat release and the mechanisms involved. The main analytical techniques deployed were: conduction calorimetry, pozzolanicity and XRD. The results showed that the two metakaolins induced stimulation of the hydration reactions due to the generation of pozzolanic activity at very early stage, because of their reactive alumina, Al2O3 r− contents, mainly. Such stimulation was found to be more specific than generic for more intense C3A hydration than C3S, at least at very early on into the reaction, and more so when 7.0% SO3 was added, and for this reason, such stimulation is described as ‘indirect’ to differentiate it from the ‘direct’ variety. As a result of both stimulations, the heat of hydration released is easy to assimilate to a Synergistic Calorific Effect.
Abstract
This study examines the polymerization of dental monomers catalyzed by synthesized acylphosphine oxides in a differential scanning calorimetry (DSC) cell. This research focuses on establishing a relationship between radicals generated by the acylphosphine oxide photoinitiators and the kinetic reaction rates of methyl methacrylate (MMA) and acrylamide (ACM), a model monomer. The thermal stability of mono- and di-acylphosphine oxides was examined by DSC. Endothermic melting and exothermic polymerization reactions initiated with the two initiators were recorded. The acrylamide model system laid the ground work for the critical evaluation of the synthesized new initiators of mono (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, and bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide. The bis(acyl) phosphine oxide photoinitiator was more reactive than the mono-(acyl) phosphine oxide with methyl methacrylates under laboratory conditions. In exothermic reactions, temperatures rose higher and more rapidly for bis(acyl) phosphine oxide initiated reactions than mono-(acyl) phosphine oxide initiated reactions.
Abstract
Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs.
Abstract
Heat divided by ligand concentration vs. heat, similar to the Scatchard plot, was introduced to obtain the equilibrium constant (K) and the enthalpy of binding (DH) using isothermal titration calorimetry data. Values of K and DH obtained by this linear pseudo-Scatchard plot for a system with a set of independent binding sites (such as binding fluoride ions on urease and monosaccharide methyl a-D-mannopyranoside on concavalin A) were remarkably like that obtained from a normal fitting Wiseman method and other our technical methods. On applying this graphical method to study the binding of copper ion on myelin basic protein (MBP), a concave downward curve obtained was consistent with the positive cooperativity in the binding. A graphical fitting by simple method for determination of thermodynamic parameters was also introduced. This method is general, without any assumption and restriction made in previous method. This general method was applied to the product inhibition study of adenosine deaminase.
Abstract
Materials with high surface areas and small particle size (nanophases), metastable polymorphs, and hydrated oxides are increasingly important in both materials and environmental science. Using modifications of oxide melt solution calorimetry, we have developed techniques to study the energetics of such oxides and oxyhydroxides, and to separate the effects of polymorphism, chemical variation, high surface area, and hydration. Several generalizations begin to emerge from these studies. The energy differences among different polymorphs (e.g., various zeolite frameworks, the 
- and 
-alumina polymorphs, manganese and iron oxides and oxyhydroxides) tend to be small, often barely more than thermal energy under conditions of synthesis. Much larger contributions to the energetics come from oxidation-reduction reactions and charge-coupled substitutions involving the ions of basic oxides (e.g., K and Ba). The thermodynamics of hydration involve closely balanced negative enthalpies and negative entropies and are very dependent on the particular framework and cage or tunnel geometry.
Abstract
The quality of measurement of heat capacity by differential scanning calorimetry (DSC) is based on strict symmetry of the twin calorimeter. This symmetry is of particular importance for temperature-modulated DSC (TMDSC) since positive and negative deviations from symmetry cannot be distinguished in the most popular analysis methods. The heat capacities for sapphire-filled and empty aluminum calorimeters (pans) under designed cell imbalance caused by different pan-masses were measured. In addition, the positive and negative signs of asymmetry have been explored by analyzing the phase-shift between temperature and heat flow for sapphire and empty runs. The phase shifts change by more than 180° depending on the sign of the asymmetry. Once the sign of asymmetry is determined, the asymmetry correction for temperature-modulated DSC can be made.
Abstract
Kinetics of polyurethane formation between several polyols and isocyanates with dibutyltin dilaurate (DBTDL) as the curing catalyst, were studied in the bulk state by differential scanning calorimetry (DSC) using an improved method of interpretation. The molar enthalpy of urethane formation from secondary hydroxyl groups and aliphatic isocyanates is 723 kJ mol-1 and for aromatic isocyanates it is 552 kJ mol-1 . In the case of a single second order reaction for aliphatic isocyanates reaction, activation energy is 705 kJ mol-1 with oxypropylated polyols and 503 kJ mol-1 with Castor oil. For aromatic isocyanates and oxypropylated polyols the activation energy is higher around 77 kJ mol-1 . In the case of two parallel reactions (situation for IPDI and TDI 2-4) best fits are observed considering two different activation energies.
Solubility diagrams in solvent-antisolvent systems by titration calorimetry
Application to some pharmaceutical compounds in water-ethanol mixtures
Abstract
Isothermal titration calorimetry (ITC) has been used to develop a method to construct the solid-liquid equilibrium line in ternary systems containing the solute to precipitate and an aqueous mixed solvent. The method consists in measuring the heat of dissolution of a solid component (the solute) during successive additions of the liquid solvent. The cumulated heat, resulting from the successive heat peaks obtained for the different injections of known volumes of solvent, plotted vs. the ratio of the numbers of moles n solvent/n solute is represented by two nearly straight lines. The intercept of the two lines gives the solubility limit and the corresponding enthalpy of dissolution of the solute in the solvent. Solubility diagrams have been established at 303.15 K in binary mixed solvents ethanol-water over the whole concentration range for seven compounds of pharmaceutical interest, namely: urea, phenylurea, l-valine, dl-valine, l-valine ethyl ester hydrochloride, tris(hydroxymethyl)amino methane.
The thermal polymerization of inhibited styrene monomer is investigated by Accelerating Rate Calorimetry (ARC). The time-temperature-pressure data generated by this technique are utilized in evaluating the thermal hazards associated with the industrial processing of styrene monomer. Several examples are given on the interpretation and application of ARC data to environments ranging from lab to plant-scale conditions including discussions concerning the similarities and dissimilarities between the ARC and large-scale equipment. The polymerization of styrene monomer is also used to evaluate the performance of the ARC over a broad temperature range, 80–300°C. The data indicate that removal of the radiant heater assembly yields better agreement between the heat of polymerization of styrene as measured by the ARC and corresponding values from the literature. This effect is believed to be observable only under conditions of low reaction rates for long periods of time such as in the case of styrene monomer.
A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C20H42). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.
