Thermodynamic functions of paraffins

The heat capacity for a longer alkane, pentacontane, C50H102, is shown in Fig. 4.50 and integrated to the thermodynamic functions H, G, and S in Fig. 4.51. This paraffin approaches the vibrational characteristics and thermodynamic behavior of polyethylene (see Sect. 2.3). The skeletal vibrations continuously approach those of polyethylene, while the group vibrations are practically the same for CH2-groups in polyethylene and alkanes. 

The use of the thermodynamic data for a discussion of stability of crystals is demonstrated in Fig. 4.52. The differences in crystal structure and melting temperatures of successive n-alkanes have been linked to the symmetries of odd and even CHj-sequences. Note, that planar zig-zag chains of odd-numbered sequences of carbon atoms in a molecular backbone point with their final bonds into the same direction, while even ones point into opposite directions. Odd paraffins have an orthorhombic, rectangular layer-stmcture, while the even ones up to C24H50 are triclinic with oblique layers. From structural analyses, orthorhombic crystals can accommodate even and odd chains without difference in packing density. The triclinic 

Differential scanning calorimetry, DSC, is a technique which combines the ease of measurement of heating and cooling curves as displayed in Fig. 4.9 with the quantitative features of calorimetry (see Sect. 4.2). Temperature is measured continuously, and a differential technique is used to assess the heat flow into the sample and to equalize incidental heat gains and losses between reference and sample. Calorimetry is never a direct determination of the heat content. Measuring heat is different from volume or mass determinations, for example. In the latter cases the total amount can be established with a single measurement. The heat content, in contrast, must be measured by beginning at zero kelvin where the heat content is zero, and add all heat increments up to the temperature of interest. 

In Fig. 4.53, a brief look is taken at the history of the DSC. Both, heating curves and calorimetry had their beginning in the middle of the nineteenth century. Progress toward a DSC became possible as soon as continuous temperature monitoring with thermocouples was possible (see Fig. 4.8), and automatic temperature recording was invented (see also Sect. 4.1). These developments led to the invention of differential thermal analysis, DTA. In Sect. 2.1.3 an introduction to thermal analysis and its instrumentations is given. 

DevelopThOTit continuous temperature measurement and recording LeChatelier 1887 time - AT recording  

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Thermodynamic Functions of Paraffins [20]. Another example of heat capacity measurements by adiabatic calorimetry and DSC is given by the paraffins. Figure 4.49 illustrates heat capacities of some normal paraffins of short chain length CxH +2-The drawn-out lines represent the computed heat capacities based on vibrational contributions fitted using the ATHAS system (see Sect. 2.3). Two observations can be made from these data. First, at higher temperature there is a constant increment in... 

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