Differential scanning calorimetry first-order transitions

Method employed to establish first-order character of transition cal = calorimetry DSC = differential scanning calorimetry X-ray = X-ray diffraction ME = Mossbauer effect... 

Similarly, if a quantity such as the volume exhibits an abrupt change in slope, which occurs at the T, then there is a discontinuity in quantities associated with first derivatives of this parameter, or second derivatives of the free energy (with respect to appropriate thermodynamic variables), such as the specific heat (Figure 10-19). Accordingly, the Tg may be related to a second-order phase transition, but this remains in dispute. The experimentally observed transition is clearly governed by kinetics and the standard method of measuring this transition is by differential scanning calorimetry (DSC), which measures the specific heat. 

Synchrotron-x>ray powder-diffraction and differential-scanning-calorimetry measurements on solid Cso reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature. 

Differential scanning calorimetry was used by Murrill and co-workers (43-45) to elucidate solid - solid phase transitions in a large number of organic compounds. First-order transitions were reported for tetrahedral compounds of the type CR1R2R2R4, where R is methyl, methylol, amino, nitro, and carboxy, as well as for octahedral-type compounds. This technique was also used to detect phase transitions in alkali metal stearates (46), some dibenzazepines, carbazoles, and phenothiazines (16), and the half esters of O-phthalic acid (31). The solid-state decomposition kinetics and activation parameters of N-aryl-N -tosyl-oxydt-imide N-oxides were determined using DSC by Dorko et al. (49). 

In this paper, a thermodynamic phase transition is studied using Differential Scanning Calorimetry (DSC). This phase transition, which will be described according to the current thermodynamic theories as a first order or a second order one, is recorded on the DSC trace as an anomalous change in the differential power ZP, different from the normal IP variation only due to the heat capacity of the material. This variation, sharp or smooth, will be called the "transition peak". We define the height h of the peak as the distance between the heat capacity trace, or baseline, and the maximum during the course of the phase transition. In the case of a pure second order phase transition, this height is the diffe-... 

Differential scanning calorimetry is not an absolute measuring technique, calibrations are thus of prime importance. Calibrations are necessary for the measurement of temperature, T (in K) amplitude, expressed as temperature difference, AT (in K) or as heat-flow rate, dQ/dt (in J s or W) peak area AH (in J) and time, t (in s or min). Figure 4.62 shows the analysis of a typical first-order transition, a melting transition. 

The TMDSC with Fourier analysis of the melting pentacontane and the calculations using saw-tooth analysis methods are given in the publication Wunderlich B, Boiler A, Okazaki I, Ishikiriyama K, Chen W, Pyda W, Pak J, Moon, 1, Androsch R (1999) Temperature-modulated Differential Scanning Calorimetry of Reversible and Irreversible First-order Transitions. Thermochim Acta 330 21-38. 

Differential scanning calorimetry can be used to provide an insight into the thermal behavior of a gel. On heating, it is possible to observe an endotherm associated with the gel-sol transition, and on cooling, an exotherm can be observed as the sol reforms the gel. However, it should be noted that the conversion of a gel to a sol is not a first-order phase transition, as it is a multistep hierarchical process, and for this reason, the observed endotherms/exotherms can be very broad. However, if such peaks are observed, then integration can provide data about the enthalpy of the transition (A//gei-soi on heating and A//soi-gei on cooling). This method can also provide detailed information on the reversibility and repeatability of the gel-sol-gel transitions. 

---