Formation of hollow solid lipid microparticles from natural waxes using supercritical carbon dioxide

In this study, the potential of natural waxes, namely, candelilla wax (CLW) and carnauba wax (CW), was explored to form extra-hard shell hollow solid lipid particles using a particle formation process based on supercritical fluid technology. Melting behavior and volumetric expansion of CLW and CW in pressurized CO₂ were investigated as a function of pressure. The melting point of waxes decreased linearly with increasing pressures up to a certain level; then stayed constant regardless of further elevated pressures. The highest melting point depressions of 13.0% and 8.3% were observed for CLW and CW at 140 and 130 bar CO₂ pressure, respectively. A positive correlation between the melting point depression and volumetric expansion was observed for both waxes. The volumetric expansion of waxes increased with increasing pressures within the linear range of the melting curves. The highest volumetric expansion was achieved at 140 bar/68°C and 130 bar/85°C for CLW (7.1%) and CW (8.5%), respectively. Hollow solid lipid particles with smooth surfaces were obtained from both CLW and CW. There was no difference between the melting behavior of the original waxes and their particles. Increasing SC-CO₂ pressure decreased the particle size and inner volume of the hollow particles. D_[4,3] of CLW ranged between 9.3 and 25.4 μm, whereas it ranged between 8.7 and 34.2 μm for CW. CLW and CW were found to be suitable high-melting lipids to form hollow solid lipid particles via SC-CO₂-assisted particle formation process to develop hollow solid lipid particles with an extra-hard protective shell.