| Popis: |
SEM micrographs of mesoporous SBA-16 type silica spheroidal microparticles prepared with the triblock copolymer surfactant PEO 140 PPO 39 PEO 140 between 80 and 125 °C and under different acid concentrations (HCl 0.2, 0.4 and 0.8 M), indicate that they are formed of coalesced nanoparticles. The coalescence degree is shown to increase with the temperature and with the presence of the co-surfactant cetyltrimethylammonium bromide (CTMABr), whereas the reverse trend is observed at high acidity. The nanoparticles formation from the initial triblock micelles and the silica source (TEOS) was followed in solution by dynamic light scattering (DLS) before the flocculation step (phase separation corresponding to the formation of micron-sized liquid particles by aggregation and fusion of the silica coated micelles). The variation of the scattered intensity and of the hydrodynamic diameter could be related to the TEOS hydrolysis degree leading to the formation of polycondensed siliceous species in the hydrophilic corona of the nanoparticles (composite colloids). Depending on the synthesis conditions, four types of nanoparticles could be defined according to the “hardness” of the siliceous species, which is related to their polycondensation degree and their distribution inside the corona of the nanoparticles. The morphology and texture of the resulting materials will depend on the type of nanoparticles and on the temperature at the phase separation (flocculation). In the absence of CTMABr, three kinds of materials are obtained: gel-like materials (low siliceous species hardness and low temperature), microparticles of mesoporous silica (medium hardness and medium temperature), blocks of aggregated nanoparticles (high hardness and high temperature). In the presence of CTMABr which acts like a buffer for the siliceous species hardness, well coalesced microparticles of mesoporous silica are obtained independently on the temperature and acidity conditions. |
Full Text from ScienceDirect