Preparation of Highly Crystalline Magnesium Oxide and Polystyrene/Magnesium Hydroxide Composite Particles by Sol–Gel Processes in an Ionic Liquid

Abstract

Highly crystalline magnesium hydroxide (Mg(OH)2) was obtained by sol–gel process using magnesium nitrate (Mg(NO3)2) with ammonium hydroxide (NH4OH) in a hydrophilic ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) as a medium. Crystallite size of Mg(OH)2 obtained in [Bmim]Cl was larger than that obtained in 2-propanol (used as a medium for comparison), which indicates that [Bmim]Cl worked as a template. The obtained Mg(OH)2 was transformed to magnesium oxide (MgO) by heat treatment under atmospheric pressure. After heat treatment, crystallite size of the MgO obtained in [Bmim]Cl was also larger than that obtained in 2-propanol, reflecting the crystallinity of Mg(OH)2 before heat treatment. Polystyrene (PS)/Mg(OH)2 composite particles were successfully prepared by seeded dispersion sol–gel process in [Bmim]Cl in the presence of PS seed particles. The composite particles had smooth surfaces and core–shell morphology consisting of PS core and Mg(OH)2 shell having high crystallinity. The Mg(OH)2 content of the composite particles calculated from the weight loss measured by thermogravimetry (17 wt %) agreed well with the theoretical value (19 wt %). Using 2-propanol as a medium, secondary nucleation of Mg(OH)2 was observed, and composite particles were not obtained. It was clarified that the core–shell structure was consistent with the predicted morphology from the viewpoint of the spreading coefficients calculated from the interfacial tensions.