The Quality Assessment of Solid Oral Dosage Forms Using Parameters of Thermal Emissivity

Emissivity is a parameter allowing the assessment of thermal/optical properties of active pharmaceutical ingredients (APIs). ε reflects radiative properties, changes with product aging, and correlates with surface characteristics. This study analyzed the thermal emissivity of commercial tablets—extended-release tablets with metformin hydrochloride (from two manufacturers: XR I and XR II), coated (Co) tablets with ibuprofen, and chewable (Ch) tablets with sodium aluminum dihydroxycarbonate—and compared unexpired vs. expired products. We used the ET 100 emissometer (Surface Optics Corporation, USA; IR range 1.5–21 µm) to measure directional–hemispherical reflectance (DHR) at 300 K, and on the basis of these values, directional thermal emissivity at 20° (DTE20) and 60° (DTE60) and hemispherical thermal emissivity (HTE) were calculated. Then, emissivity parameters were evaluated at 500 K, 800 K, and 1200 K. The DHR values at a 60° angle differed between unexpired and expired XR II tablets across all spectral bands and for XR I tablets, except in the 3.0–4.0 micron range. In turn, for DHR at 20°, high effect sizes were demonstrated between unexpired and expired Ch tablets for 1.5–2.0, 2.0–3.5, 4.0–5.0, and 5.0–10.5 microns. For the DHR at 60°, the high effect size between unexpired and expired Ch tablets was found at 1.5–2.0, 2.0–3.5, and 4.0–5.0 microns. At 300 K, XR I and XR II tablets showed comparable DTE20, DTE60, and HTE. The Ch tablets had higher DTE20 than XR I and XR II (0.968 vs. 0.954 and 0.958, respectively; p < 0.001) and Co tablets (0.968 vs. 0.930; p < 0.001). The Co tablets had the highest DTE60 mean values (0.941 vs. 0.926 for Ch, p < 0.001; 0.926 for XR I, p < 0.001; 0.932 for XR II, p = 0.001). The HTE value was the highest for Ch tablets (p < 0.001 vs. others). During thermal modeling of the emissivity parameters, all DTE20, DTE60, and HTE values decreased with temperature, reaching their lowest values at 1200 K. The largest relative decrease in HTE values (over 15%) between the standard measurement temperature of 300 K and the modeled temperature of 1200 K was found for Ch tablets. Tablets with different release profiles show distinct DTE20, DTE60, and HTE values, suggesting that emissivity may serve as a rapid, non-destructive screening tool that could support further pharmaceutical evaluation during storage. However, emissivity alone does not establish pharmaceutical quality, and the present findings should be interpreted as proof-of-concept rather than as validation of a stand-alone quality-control method.