Endothermal peaks hydration

The first two peaks involve the loss of 2 waters of hydration. The broad endothermic peak following in the above diagram is similar to that we might see for a change in composition such as ... 

The thermal decomposition of hydrated rare earth oxalates, M2(Ox)3 nSLsO, has attracted considerable attention [397—400]. Wendlandt and his coworkers [397, 39S] have used both thermogravimetric (TGA) and differential thermal analysis (DTA) for studying the thermal decomposition of the rare earth, Th4+ and U4+ oxalates. The DTA curve for Eu2(Ox)3 IOH2O consists of three endothermic peaks centered at about 160°, 200° and 285° C respectively. The thermogravimetric analysis [397] shows the presence of unstable intermediate hydrates on going from the decahydrated to the anhydrous oxalate. The thermal decomposition of Eu2(Ox)3 IOH2O can be summarized as... 

Figure 10 shows the differential thermograms. A mild endothermic peak at 100-200°, which was observed for the samples of any composition, is presumed to be the result of evaporation of adsorbed water or the removal of hydration water. The peak of the binary mixture shifted... 

Irrespective of whether the crystallization response should be taken as Parts n and III or just Part II for lactose (see Figure 8.4, and earlier discussion), it is clear that the crystallization response is reduced in size due to the endotherm for water desorption. Buckton and Darcy (27) have shown that the measured calorimetric crystallization peak, plus the enthalpy of vaporization for an estimate of the amount of water desorbed (i.e., the quantity of water needed to plasticize Tg below T minus the quantity of water that is retained as a hydrate) approximates to the combined endotherms for hydrate loss and melting, which are measured in a DSC for the crystallized lactose. This balance of heats would indicate that the contributions within the saturated salt solution are relatively small during the crystallization response. It... 

The DTA curve for oxalic acid dihydraie. the only acid studied containing water of hydration, had dehydration peaks with A7 n values ofl 10.120, and 125 C, respectively. All other curve peaks for the organic acids were caused by fusion and decomposition reactions. For example, the second endothermic peak in the succinic acid curve was probably caused by dehydration reaction. 

Three endothermic peaks occur when the cured paste samples are heated. These peaks are a result of evaporation of the bonded water from the paste, which is incorporated in different structures. The sharp peak at 330 °C is related to melting of the metallic lead in the paste. When the plates are soaked in H2SO4 solution of 1.05 rel. den., the peak C disappears and the energy of the endothermic peak A increases, i.e. this type of hydration increases. The height of peak B diminishes slightly. 

The thermograms of DMPC and longer chain lipid bilayers show a small peak at temperatures well below that of the sharp peak of the main phase transition. The general interpretation is that, when the bUayers are fully hydrated and sufficiently incubated at low temperature, this small endothermic peak (about 1 kcal mol ) is related to a pretransition between a Lc gel state and a rippled gel state (Pp )> both characterized by all trans chain conformers. This pretransition state is thus not related to molecular structural changes, but to a different packing arrangement of... 

After a PEM is cooled at very low temperatures (e.g., less than -50°C) differential scanning calorimetry (DSC) shows an endothermic peak at around 0°C as the temperature scans up. This seems to indicate that the PEM contains water that freezes af the water freezing temperature. From the peak area the amount of such "freezable" water can be determined, and its difference from the total amount of water within the membrane that is pre-determined by weighing is used to represent the amount of water that is not freezable (called non-freezable water). For example, Hou et al. found that fully hydrated Nation 212 membrane in liquid water at 25°C contains 5.3 and 15.2 freezable and non-freezable water molecules per -SOjH group, respectively while the same membrane hydrated in water vapor at 75% RH and 25°C contains 0 and 6.2 freezable and non-freezable water per -SO3H group, respectively (Hou 2008). 

It is important to acquire information on the hydration processes that occur when cement is subjected to low temperatures. Even at temperatures below zero cement hydrates slowly, as water does not freeze due to the presence of soluble materials in the pores. The products that form at low temperatures are calcium aluminate hydrates at earlier periods and after several months, the products of hydration of tricalcium silicate. A weak endothermal peak at about 330°C could be due to the presence of silica gel and this is preceded by the appearance of calcium hydroxide and C-S-H gel.t l... 

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