Deciphering magnesium stearate thermotropic behavior.

Area of Science:

• Pharmaceutical Sciences
• Materials Science
• Physical Chemistry

Background:

• Magnesium stearate (MgSt) is a widely used excipient in oral solid dosage forms.
• Commercial MgSt displays significant physicochemical variability, impacting product performance.
• Current quality control methods, like DSC, lack detailed understanding of MgSt's thermotropic behavior.

Purpose of the Study:

• To elucidate the complex thermotropic behavior of magnesium stearate (MgSt).
• To assign physicochemical relevance to Differential Scanning Calorimetry (DSC) thermograms of MgSt.
• To establish a foundation for developing heat-treatment strategies for uniform MgSt.

Main Methods:

• Differential Scanning Calorimetry (DSC) for thermal transitions.
• Thermogravimetric Analysis (TGA) for mass changes.
• Capillary Melting Point and Polarized Hot-Stage Microscopy for visual characterization.
• Temperature-dependent Small-Angle X-ray Scattering (SAXS) for molecular ordering and mesophase identification.

Main Results:

• Multiple DSC thermal transitions were observed, with several being irreversible after the first heating cycle.
• Reproducible cool-heat-cool-heat DSC thermograms revealed 6 reversible exothermic-endothermic conjugate pairs.
• SAXS identified 5 distinct mesophases, with Phase C' stable up to 250°C.
• MgSt maintained molecular ordering above 276°C, indicating complex behavior beyond simple melting.

Conclusions:

• The study provides a detailed physicochemical interpretation of MgSt's DSC thermograms.
• MgSt exhibits complex thermotropic behavior with multiple mesophases, not a simple melting process.
• Understanding these thermal properties is crucial for developing strategies to produce uniform MgSt materials.