Temperature-induced changes, differential scanning calorimetry

Differential scanning calorimetry measures the thermodynamic parameters associated with thermally induced phase transitions. Here, the sample of interest and an inert reference are heated or cooled independently at a programmed rate, and in tandem, such that their temperatures change in unison and the differential temperature is maintained at zero. If the sample undergoes a thermally induced transition, heat must be applied to or withdrawn from the sample in order to maintain the same temperature in both sample and reference compartments. The instrument measures the heat flow into the sample relative to the reference and this dijferential heat flow (or excess specific heat) is recorded as a function of temperature, resulting in a trace, as shown in Fig. 1... 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

When a material is heated or cooled, there is a change in its structure or composition. These transformations are connected with a heat exchange. Differential scanning calorimetry (DSC) is used for measuring the heat flow into and out of the sample, as well as for determining the temperature of the thermal phenomenon during a controlled change of temperature. The first method developed by Le Chatelier in 1887 was differential thermal analysis (DTA), where only the temperature induced in the sample was measured. 

The most extensively used methods for monitoring these heat induced changes in fibers are differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In DTA, both sample and inert reference are heated at the same rate by the same heat source. When a thermally induced transition occurs in the sample, a temperature difference between the sample and reference results can be recognized. A plot of the difference in temperature between the two against increasing applied temperature exhibits deviations from the initial baseline, depending on... 

The most detailed thermodynamic analysis of protein structure stability is based on differential scanning calorimetry (DSC). In a DSC experiment, the heat capacity Cp of a sample is monitored while heating (or cooling) the sample. Figure 13.13 shows a typical DSC thermogram for heat-induced denaturation of a protein in solution. The thermodynamic observables are the temperature of denaturation (the temperature at half-peak area), the enthalpy change An, d (T involved in the denaturation process (the area under the peak), and the change in the heat capacity A,, dC of the solution (the shift of the baseline). 

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