In this study, the rapid expansion of the supercritical solutions (RESS) process was used to produce microparticles of a commonly used anti-inflammatory drug, ethenzamide. The effects of process parameters in RESS including the extraction temperature, pre-expansion temperature, and post-expansion temperature were investigated using the Box–Behnken design. According to the results of the analysis of variance (ANOVA), the effect of pre-expansion temperature is the most significant parameter on the mean size of RESS-produced ethenzamide. A higher pre-expansion temperature benefits the production of smaller crystals. In addition, a quadratic effect of the post-expansion temperature was also identified. Through RESS, ethenzamide microparticles with a mean size of 1.6 μm were successfully produced. The solid-state properties including the crystal habit, crystal form, thermal behavior, and spectrometric property were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectrometer (FTIR), differential scanning calorimeter (DSC), and powder X-ray diffraction (PXRD). These analytical results show that the rod-like crystals were generated through RESS, and the crystal form, thermal behavior, and spectrometric property of RESS-produced crystals are consistent with the unprocessed ethenzamide.
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