Role of Hydrophobic Attraction and Hydrophobic Nanoconfined Channels in the Spray Drying Process for Synthesizing Hollow Microspheres from Silica Sol

The impact of particle hydrophobicity on the morphology and structure of hollow microspheres prepared by spray drying of silica sol was investigated through modification with γ-methylacryloxypropyltrimethoxysilane (γ-MPS). Hydrophobic attraction among the hydrophobic silica particles in the aqueous phase drives particles to gather inward the droplet at the early stages of dehydration, which delays the shell formation of the droplets. During the later stages of dehydration, hydrophobic particles form hydrophobic nanoconfined channels among themselves. Afterward, water undergoes rapid evaporation via the formation of molecular clusters, facilitating the formation of a thicker and stronger shell, which mitigates the formation of the tail vortex resulting from the adverse effects of airflow. After continued dehydration, regular hollow microspheres are formed. Cationic surfactant cetyltrimethylammonium bromide (CTAB) was absorbed onto the negatively charged particle surface of silica sol, thereby inducing hydrophobicity via electrostatic attraction. Subsequently, the CTAB-adsorbed silica sol is spray dried in a mixed solvent phase of ethanol and water, resulting in the preparation of regular, morphologically uniform hollow microspheres. The robust synthesis and the underlying mechanistic study provide a scalable and sustainable route to prepare hollow microspheres for potential application in cosmetics.