Thermodynamics analysis melt curve

Thermal melting curves Thermodynamic analysis Hydrodynam. properties... 

Bi- and tetramolecular structures also give quite comparable enthalpies. However, the validity of the van t Hoff analysis for some of these quadruplexes is questionable, as the melting curve used to determine thermodynamic parameters rarely corresponds to a true equilibrium curve. We will address this point later in the manuscript. An enthalpy of formation of —21 to —26 kcal mol per quartet is found for the G2T2NT2G2 sequence in comparison to —22.5 kcal mol per quartet measured for the G4T4G4 sequence. Concerning parallel quadruplexes, analysis of the calorimetric or thermokinetic data gives a value of —72 kcal moD (ref. 27) and —76.5 kcal moD (ref. 28) for the TG4T sample... 

Experimental thermodynamic parameters have been obtained with three methods (1) optical melting curves, (2) signal amplitudes from temperature-jump experiments, and (3) calorimetry. Methods (1) and (2) rely on a van t Hoff analysis of absorbance changes as a function of temperature. Calorimetry measures the heat released by a reaction. Only a limited amount of calorimetric data is contained in this chapter to avoid overlap with chapter 4.2. 

Meyer returned to sound thermodynamic analysis in the discussion of the dependence of the melting point on tension for lightly vulcanized gutta-percha. The classic curve is shown in Fig. 3.12. 

Many techniques have been used to characterize the physical nature of solid dispersions. These include thermal analysis (e.g., cooling-curve, thaw-melt, differential scanning calorimetry and X-ray diffraction, microscopic, spectroscopic, dissolution rate, and thermodynamic methods) Usually, a combination of two or more methods is required to obtain a complete picture of the solid dispersion system. 

Roberts ( 1 1) surveyed the superconductive properties of the elements and recommended a critical temperature of 1.175 0.002 K for Al(cr). Since this temperature is so low, the effects of superconductivity on the thermodynamic functions are not considered. The entropy contribution due to superconductivity will be less than 0.002 J X mol . The data of Giauque and Meads (j ) and Downie and Martin (3) agree at temperatures up to 150 K but drift apart by 0.2 J X mol at 200 X and 0.17 J X mol at 300 K, with the Downie and Martin study being lower. The Takahashi (4, 5) study is even lower at 298 X. The high temperature heat capacity values are derived from the enthalpy study of Ditmars et al. (9). Their curve is intermediate between those derived from previous studies (4, 5, 6, 7, 8) and implies a flatter Cp curve near the melting point (in comparison to previous interpretations). Numerous other heat capacity and enthalpy studies are available but were omitted in this analysis. A detailed discussion of the Group IIIA metals (B, Al, and Ga) is in preparation by the JANAF staff. 

When the solvent-isolated losartan was subjected to DSC characterization, the DSC curve (Fig. 3-5, curve A) showed a minor endotherm at an extrapolation onset temperature of about 229°C (10°C/min) and a major melting endotherm at an extrapolation onset temperature of about 273°C (10°C/min). When a sample of Form I was heated to 255°C and then cooled back to room temperature, the subsequent DSC curve showed only the high-temperature endotherm (Fig. 3-5, curve B). Chemical analysis (HPLC) and solution NMR showed no change in the material heated to 255°C and cooled back to room temperature. However, XRPD indicated a change in the crystal structure. Therefore, it was concluded that the minor endotherm corresponded to a kinetically irreversible enantiotropic polymorphic transition and that the losartan system was not under complete thermodynamic control. Form I is the low-temperature stable form, up to the transition point, and the high-temperature stable form was... 

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