Evaporation-induced self-assembly (EISA) of amphiphilic molecules within aerosol droplets is an attractive method for synthesis of mesoporous silica particles. The aim of this research was to demonstrate synthetic methodologies to develop novel particle architectures using this technique, and to understand the influence of the competing dynamics within an evaporating droplet undergoing EISA on the particle morphology and mesostructure. Experiments were conducted to control particle characteristics. Particle size and distribution was varied by varying the size and distribution of starting droplets. The compressed gas atomizer, TSI 3076, gave a roughly micron-sized droplets with a polydisperse population, whereas the vibrating orifice aerosol generator (VOAG), TSI 3450, gave a highly monodisperse droplet population when orifices of diameters 10 μm and 20 μm were used. The mesopore size and mesostructure ordering were varied by employing amphiphiles of different geometry and by the use of 1,2,3-trimethylbenzene, a pore-swelling agent. The extent of ordering was influenced by factors that govern the rates of reactions of the silica precursors relative to the rates of amphiphile self-assembly. These factors included acid concentration, the alkyl group in the tetraalkoxysilane precursor, the time for which the sol was aged before droplet generation, and CTAB/Si ratio in the starting sol. Experiments and simulation studies were carried out for particles made using CTAB as the templating agent and TMB as a pore-swelling agent. Analysis of these experiments was used to get insight into the three main dynamic processes occurring inside these droplets: evaporation of the volatile species, amphiphile self-assembly and phase transformation, and hydrolysis and condensation reactions of the silica precursor species. Pore swelling was observed for particles made using the VOAG. Particles made using the 10 μm orifice retained their hexagonal mesostructure upon addition of TMB in the range 0 ≤ TMB/CTAB < 12.5. Particles made using the 20 μm orifice gave hexagonal mesostructure for the above-mentioned TMB/CTAB mole ratio range, except for 6.25, where a cubic mesostructure was obtained. On the contrary, particles made using the TSI 3076 showed no signs of pore swelling. Certain parameters were varied in droplets experiments as well as experiments in the bulk phase to understand the effect of TMB. It was found that addition of TMB increases the rate of condensation reactions of the silica precursor. For 0 ≤ TMB/CTAB ≤ 4.18, very little pore swelling was observed. For 4.18 ≤ TMB/CTAB ≤ 8.33, substantial pore swelling was observed, indicating significant TMB insertion into the hydrophobic regions of the self-assembled amphiphilic molecules. For 8.33 ≤ TMB/CTAB ≤ 12.5, very little change in mesopore diameter was observed, with evidence of an increase in microporosity. Presence of hysteresis in the nitrogen physisorption isotherms and a broadening of the BJH mesopore size was evident at higher TMB content (TMB/CTAB ≥ 6.25). The decreasing specific surface area and pore volume in the 0 ≤ TMB/CTAB ≤ 4.18 range implied that TMB addition leads to an increasing amount of a nonporous (or low porosity) silica phase. Simulation results show that there is ample time available once the solvent evaporation stops from micron-sized droplets till they reach the high temperature zone, where the rates of silicate reactions increase tremendously. This enabled the TMB molecules in the hydrophobic domains to diffuse out of the self-assembled structures and evaporate before the silicate reactions freezes the structure, and hence no pore swelling is observed in particles made from these droplets. However, in case of the larger droplets made from VOAG, a substantial fraction of TMB remains in the droplets that are still far from fully evaporated when they enter the high temperature region. Hence, pore swelling is observed in particles made from these droplets.
Particles with hierarchical internal morphology were produced by addition of a hydrophilic polymer, poly(acrylic) acid to the starting sol. There was no noticeable change in the CTAB-templated mesostructure observed. By varying certain parameters such as polymer concentration, polymer molecular weight and dilution of the sol (specifically by adding ethanol), particles with core-shell structure or domains of PAA dispersed throughout the particles were obtained. The addition of PAA adds another dynamic process to the three dynamic processes already present. The final product depends on the time available for the onset of the phase segregation of the polymer and the coalescence of these polymeric domains relative to the time required for the condensation reactions of the silica precursor to freeze the structure of the droplets.
|Advisor:||Ward, Timothy L.|
|School:||The University of New Mexico|
|School Location:||United States -- New Mexico|
|Source:||DAI-B 68/07, Dissertation Abstracts International|
|Subjects:||Chemical engineering, Materials science|
|Keywords:||Droplets, Evaporation-induced self-assembly, Mesoporous silica, Self-assembly|
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