Synthesis of mesoporous silica particles using SDS-Pluronic couples

Abstract

Controlling the cooperative self assembly and micellization of pluronics and SDS (sodium dodecyl sulfate)are pivotal for the synthesis of mesoporous silica particles. The pH and temperature of the synthesis media, SDS/Pluronic mole ratio, TMOS (tetramethyl orthosilicate)amount, alkali salt amount of the synthesis solution are the parameters, which play significant roles on the micellization and self assembly of surfactants. The synthesis of mesoporous silica particles with distinct morphologies is possible with the precise optimizations of these parameters. In this thesis we have investigated the synthesis of mesoporous silica particles with a well defined morphology and structure using SDS-Pluronic couple as the template. The pore size can be tuned by changing the aggregation number of the surfactant molecules in the micelles, also by changing the pluronic type. The morphological control is achieved mainly by changing the pH and temperature of the synthesis media. At different temperatures and pHs, rods, spheres, muffin and ‘s’ shaped particles have been obtained. The addition of inorganic salts, such as NaNO3, NaCl, and KCl, has also effects on the morphology and meso-structure. Addition of a small amount of NaNO3 changes spherical particles to amorphous silica however, addition of large amount of NaNO3 gives well defined muffin shaped and worm-like particles. The concentration of nitrate ion also affects the pore size and wall thickness of the synthesized particles. The KCl or NaCl salts also have similar effects on the morphology of the silica particles, the morphological transitions have been observed but the role of Cl- ion is minor on the control of pore size. The SDS concentration has important effects on the micellization of pluronics, changing the SDS/Pluronic mole ratio (between 0.05 and 5.0) in the reaction media changes the structure of the mesoporous silica particles. Particularly the SDS concentration has important effects on the surface area of the synthesized particles. The surface area of the samples changes between 100 m2 /g and 700 m2 /g and the pore size of the particles changes between 3.0 and 6.0 nm by changing the SDS/Pluronic mole ratio. This ratio is also effective on the micropore amount of the samples together with mesopores. The tunable particle size (between 0.2µ to 1000µ) and morphology (spheres, rods, muffin and ‘s’ shaped.) can be achieved by changing the SDS concentration. Furthermore, the low reaction temperature (below RT) is essential for the synthesis of mesoporous silica particles in SDS-Pluronic system. However, the low temperature is a problem for micellization. This problem was overcome by using P123, which has low critical micellization concentration (CMC) and critical micellization temperature (CMT) values or by using Hofmeister ions to decrease the pluronic surfactant solubility and the CMC and CMT of the pluronics used. Decreasing solubility of the pluronics causes effective micellization of the surfactants. The well defined micelles are the templates for the synthesis of mesoporous silica particles. Overall , the effects of SDS/Pluronic mole ratio, pH and temperature of the synthesis solution, TMOS concentration, and the additives (alkali salts) have been investigated by synthesis of more than 300 samples that were analyzed using PXRD, SEM, TEM, POM, and N2 sorption techniques.