Hygroscopic properties modeling and thermodynamic analysis of a seamless popping capsule

This study investigated the thermal stability of seamless "popping" capsules at different storage temperatures to address the challenges of adhesion and texture changes affecting shelf life. Storage experiments at different temperatures identified optimal conditions for maintaining product integrity. Moisture adsorption behavior was analyzed using isothermal adsorption curves at 25 °C, 32 °C, and 37 °C, along with sorption kinetics at 68% relative humidity (RH). Capsules lost commercial viability at 37 °C and higher, and the moisture sorption isotherms followed Type II patterns. The Smith model best fit data at 25 °C and 37 °C (R² > 0.98), while the GAB model was more suitable at 32 °C (R² > 0.98). The storage humidity should not exceed 61% RH. The sorption process reached a state of saturation within two hours and followed first-order kinetics (R² > 0.95). Thermodynamic analysis showed that sorption was enthalpy-driven and non-spontaneous, with an isokinetic temperature of 324.76 K and Gibbs free energy of 45.57 J/(mol·K). Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) identified moisture-binding sites, and Scanning Electron Microscopy (SEM) revealed structural changes due to moisture exposure. These findings improve understanding of moisture sensitivity in seamless capsules and guide optimal storage conditions to preserve product quality.