Case Study 614 Indomethacin Processing Route
This case study investigates how three processing routes—unaltered crystalline, ball‑milled, and quench‑cooled—affect the surface energetics of the pharmaceutical compound indomethacin. Indomethacin (>98% purity, γ‑form) was prepared in three forms: as‑received crystals, a milled powder (ceramic ball mill, 2 h, 1:40 mass ratio), and a quench‑cooled amorphous powder (heated above melting point, then rapidly cooled). All samples were sieved to 90‑106 µm to isolate surface effects. Structural verification employed powder X‑ray diffraction (PXRD) and differential scanning calorimetry (DSC), confirming complete amorphisation for the quench‑cooled sample and only surface‑localized disorder for the milled sample. Surface properties were probed using inverse gas chromatography with surface energetics analysis (iGC SEA), allowing measurement of dispersive surface energy (γs^D) across a range of surface coverages (n/nm) and determination of specific acid‑base components via the Good‑Van Oss‑Chadury approach and Gutmann Ka/Kb numbers. Results show: (i) milled indomethacin exhibits the highest average dispersive surface energy (≈44 mJ m⁻²) with a narrow distribution, indicating homogeneous high‑energy sites likely from defect formation or exposure of high‑energy crystal facets; (ii) quench‑cooled indomethacin has slightly lower average dispersive energy (≈41 mJ m⁻²) but a broader distribution, reflecting a more heterogeneous amorphous surface; (iii) crystalline indomethacin displays the lowest dispersive energy (≈39 mJ m⁻²) and modest heterogeneity. Acid‑base analysis reveals all forms are basic (Kb > Ka), with the milled sample showing the highest electron‑donor (base) character (lowest Ka/Kb ratio), followed by quench‑cooled, then crystalline. The study concludes that processing route, not just final solid state, critically influences surface energetics, which can affect interactions with excipients, devices, and processing equipment, underscoring the need for controlled manufacturing to maintain desired pharmaceutical performance.
Topics
Processing Routes Explored: Crystalline, Ball‑Milled, and Quench‑Cooled Indomethacin
Definition and preparation of the three indomethacin forms used in the study, including specific milling parameters and quench‑cooling protocol.
Structural Verification via PXRD and DSC
Use of powder X‑ray diffraction and differential scanning calorimetry to confirm amorphous conversion, detect residual crystallinity, and identify polymorphic transitions.
Inverse Gas Chromatography (iGC SEA) Methodology for Surface Energy Profiling
Explanation of the iGC SEA technique, injection size control, surface coverage concept (n/n_m), and how dispersive and specific surface energy distributions are derived.
Dispersive Surface Energy Profiles Reveal Milling‑Induced Surface Heterogeneity
Quantitative comparison of γ_s^D values across the three samples, highlighting the higher average and narrower distribution for milled indomethacin versus quench‑cooled and crystalline forms.
Acid‑Base Surface Energy and Electron Donor‑Base Characteristics Across Routes
Presentation of Gutmann Ka and Kb numbers, Ka/Kb ratios, and specific acid‑base surface energy components, showing all samples are basic with the milled form most electron‑donating.
Implications of Surface Energetics for Pharmaceutical Formulation and Processing
Discussion of how differences in dispersive energy and basicity may influence interactions with excipients, devices, and processing equipment, emphasizing the need to control both solid state and processing route.