New approach for the control of size and surface characteristics of mesoporous silica particles by using mixed surfactants in W/O emulsion

Mesoporous silica micro-spheres have been prepared using an emulsion-gel method in W/O emulsion consisting of aqueous solution of SDS or Tween 20 and n-octanol of hydroxypropyl cellulose (HPC) and Span 80. The morphology of water droplets in W/O emulsion was controlled by the concentration of the water-soluble surfactants of SDS and Tween 20 and the oil-soluble surfactant of Span 80. Since water droplets serve as a supporting structure for particle growth and aggregation, their morphology influences the shape, size, and size distribution of particles. When 3 wt.% and 5 wt.% of Span 80 were employed in our system, the effect of surfactant on the particle size distribution was more prominent than when 7 wt.% was used. Anionic surfactants were hardly used in aqueous phase to make mesoporous silica particles and one or more water-soluble surfactants were only used in aqueous phase to control the shape of particles. Interestingly, the particles of mesophase structure were synthesized when the anionic surfactant, SDS, was added to the water phase and the nonionic surfactant, Span 80, was employed in the oil phase. As the concentration of SDS and Tween 20 increases, the pore size of samples is altered from 7.2 nm up to 44.4 nm. This change of surface morphology occurred due to the solubilization ability of water-soluble surfactants. Moreover, depending on whether the anionic surfactant or nonionic surfactant is used, the degree of the change in pore size distribution of silica particles is relatively different. When SDS is used, the maximum peaks of the pore size distribution are located on the right rather than those in silica particles prepared using Tween 20. The structure of these materials was characterized by optical microscope, field-emission scanning electron microscopy, and nitrogen adsorption and desorption (BET isotherms and BJH pore size distribution measurements).