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The combination of differential scanning calorimetry (DSC) and thermomicroscopy analysis were used to characterize the thermal properties of the drug sulfapyridine. According to conventional DSC, a quenched sulfapyridine exhibited a series of transitions on reheating, including a glass transition at 56.9°C, cold crystallization at 101.01°C, and metastable and stable polymorphic melting at 170.74 and 180.08°C, respectively. Thermomicroscopy analysis confirmed the DSC data, showing the formation of spherulitic crystals, which converted to a rhombs plate-shaped morphology upon heating, attributed to metastable and stable polymorphs, respectively.
This study demonstrates that using these two techniques together provides exceptional insights into polymorphic and glass transition behavior of the drug sulfapyridine.
Polymorphism is the ability of a pharmaceutical substance to exist in multiple crystalline forms, which can affect the stability and bioavailability of compounds in formulations. Characterizing different polymorphic forms of drugs is crucial for pharmaceutical development. Differential scanning calorimetry (DSC) is a widely used technique for polymorphism studies, measuring the melting properties of a drug in its received form and after cooling or crystallization.
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DSC can also detect the glass transition temperature (Tg), which impacts formulation stability.
However, limitations in DSC include the inability to differentiate Tg from other transitions and low sensitivity for small amorphous material quantities. Thermal microscopy complements DSC by directly visualizing and recording crystal structures and transitions. Combining these techniques provides a comprehensive understanding of thermal transitions. An example is the analysis of shock-cooled sulfapyridine, where thermomicroscopy reveals spherulites produced through cold crystallization, transitioning into stable rhombic forms. The DSC heating curve reflects these modifications.
This report presents the experimental findings of sulfapyridine's polymorphism using DSC and thermal microscopy.
Sulfapyridine was obtained for thermal analysis on a Mettler Toledo DSC822e equipped with an RP100 refrigerated chiller unit. Temperature accuracy calibration was performed using indium and zinc standards. Heat flow calibration used indium. Samples were encapsulated in standard aluminum crucibles, hermetically sealed, and weighed on an analytical balance. Data analysis and DSC control utilized Mettler Toledo software. Thermal optical analysis (TOA) employed a NIKON Eclipse LV100 hot stage system.
Conventional DSC measurements were conducted at 20°C/min heating rates on fresh and quenched sulfapyridine samples. Quenching involved rapidly cooling the sample from its melting point to room temperature. TOA measurements used quenched samples prepared by melting a small amount on a microscope slide and cooling it on an aluminum block. The samples were heated at 5°C/min.
Heating sulfapyridine from 25 to 200°C at 20°C/min yielded a single melting peak with an onset at 191.54°C, indicating high purity and a single crystal form. This result was consistent with a fresh sample, confirming DSC calibration. Quenched sulfapyridine exhibited multiple thermal events during heating at 10°C/min. These events included a baseline shift (A) at 56.9°C, a strong exothermic peak (B) at 100.01°C, a small exotherm (C) at 170.74°C, and a strong endotherm (D) at 180.08°C.
The events were tentatively attributed as follows: a glass transition at 56.9°C, cold crystallization at 100.01°C, and metastable and stable polymorphic forms melting at 170.74 and 180.08°C, respectively. It's important to note that while two melting peaks were observed in all repeats, their relative sizes varied due to nucleation process unpredictability.
| Event | Temperature (°C) |
|---|---|
| Baseline Shift (A) | 56.9 |
| Strong Exothermic Peak (B) | 100.01 |
| Small Exotherm (C) | 170.74 |
| Strong Endotherm (D) | 180.08 |
Thermal optical analysis revealed the glassy form of sulfapyridine as dark field under crossed polarizing filters. Crystal growth occurred above Tg, forming spherulitic crystals by 109°C. At temperatures between 130 and 150°C, some spherulites transformed into needle-shaped crystals. Spherulites were observed to melt at around 177°C, while the remaining needle crystals melted at 190°C. This transformation suggested that spherulites were metastable and transformed into the stable needle form. The presence or absence of transformation depended on conditions and affected melting points.
Sulfapyridine in its received form demonstrated stability and high purity with a single melting peak. Quench cooling resulted in the formation of a glass, which upon reheating, showed solid/solid transformations, with spherulites converting into stable needle-shaped crystals. Notably, some spherulites remained untransformed with lower melting points. The stable needle crystals melted at the same temperature as the original sulfapyridine sample. DSC detected these processes but provided indirect information about crystalline transformations. TOA characterized the crystals and determined all transition temperatures except Tg. To fully understand the processes and material structures involved, both techniques are essential.
Lab Report: Polymorphic Forms of Sulfapyridine. (2024, Jan 10). Retrieved from https://studymoose.com/document/lab-report-polymorphic-forms-of-sulfapyridine
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