Caffeine exists in two polymorphic forms – form I and form II. Compression of form I prompts its change into form II, resulting in alterations in its properties. Heating with DSC makes sure that caffeine is in the right form.
Since it stimulates the central nervous system, caffeine is used with active ingredients that induce sleepiness. It is a polymorphic substance: which means that it exists under different modifications, that possess different mechanical properties, called forms I and II. Characterization of the present modification can easily and quickly be performed by using a singular DSC heating run.
Heating by DSC Characterizes the Modification of Caffeine
Caffeine in form I was measured with the DSC 214 Nevio using:
- Two heatings to 270 °C at heating and cooling rates of 10 K/min
- Hermetically sealed aluminum crucibles with pierced lid
- 6.92 mg commercial caffeine
The DSC measurement curves are shown in Figure 1.
- Blue curve – first heating: The detected peak at 241 °C is usual for the melting of modification I.
- Green curve – second heating: The endothermal peak at 161 °C is a result of a change in the crystal structure. This demonstrates that during cooling at 10 K/min, the substance crystallized into form II. In the following heating, the form II transforms into form I (Peak at 161 °C) that subsequently melts at 239 °C.
DSC measurements on caffein forms I and II.
Compression of Caffeine Induces Changes in the Crystal Structure!
Next, 35 mg caffeine (form I) is put into a press and compressed with a force of 20 kN for 20 minutes. Following this process, the powder is a tablet which is stored for a week at room temperature before being measured with a DSC. The curve in Figure 2 shows the result.
Figure 2. Caffeine with (green curve) and without compression (pink curve).
It could be perceived that there is no big difference between the curves. Zooming in on the temperature range 120-200 °C shows that an endothermal peak is identified only in the compressed sample (Figure 3). This peak is due to the structure change: modification II to modification I. This means that caffeine transforms its alteration during that compression.
Figure 3. Zoom of figure 2.
Transformations of a polymorphic substance from one modification to another one can be achieved by mechanical treatment such as compression. This can influence its properties. A DSC measurement easily identifies the alteration in just one heating run.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
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