Biphasic synthesis of colloidal mesoporous silica nanoparticles using primary amine catalysts

Evanescent wave-cavity ring-down spectroscopy (EW-CRDS) is employed to characterize micellization of anionic surfactants and the related capability of removing cationic substance off the silica surface. Crystal violet (CV(+)) cationic dye is used as a molecular probe to effectively determine critical hemimicelle concentration (HMC) of surfactants on the surface. The HMC results are 1×10(-2), 4×10(-3), 8×10(-4), and 2.5×10(-4) mol/L for sodium sulfate salts with a carbon-chain length of C-10, C-12, C-14, and C-16, respectively. A stronger hydrophobic interaction results in a less concentration required to undergo micellization. The HMC values on the surface are about half of those in solution. When NaCl solution is added, the electrolyte helps reduce the electrostatic repulsion between the anionic sulfate heads to facilitate the surfactant aggregation, and thus, the subsequent HMC is reduced. Furthermore, the probable phase change for dye-surfactant interactions on the surface at the concentration below HMC is observed, and the desorption rates of CV(+) are measured as a function of concentration and carbon-chain length of surfactants above HMC. Given each surfactant concentration at its respective HMC, the corresponding desorption rates are along the order of C-12