Chemical and energetic control of phase stability in silica/surfactant composite materials

Self-organized silica/surfactant composite materials provide a unique method to produce an inorganic framework with tunable nanoscale architecture through inorganic/organic co-assembly. These composite materials show well-devolved nanoscale periodicity in the range of 10–200 Å and may be used to produce a porous inorganic with a narrow pore size distribution by removal of the organic component. This thesis addresses issues of structural stability in inorganic/organic composite materials. All silica/surfactant composites are kinetically trapped structures, thus to better understand metastability in these materials it is necessary to understand how structures can form and rearrange on the nanometer length scale. This is investigated by following the kinetics of a hexagonal-to-lamellar phase transformation that occurs under hydrothermal conditions.; To understand the kinetic data, we must explore how structural changes on the nanometer length scale are coupled to silica bonding changes that occur on the atomic length scale. Changes in the bonding of the inorganic framework (which can be simply controlled through pH) help reveal the molecular basis for kinetic barriers. We show that the most important factor in increasing nanoscale phase stability is the regularity of the inorganic framework. In addition, we discover that pre-transformation chemical and thermal treatments, which change the inorganic framework, can be used to inhibit a phase transformation. We also follow changes in the organic packing with temperature to better understand the driving force for structural rearrangement in these inorganic/organic composite materials. Heating surfactant results in an increase of surfactant tail volume, which also decreases organic domain curvature. However, a decrease in micelle curvature, rather than an increase in volume, is necessary to drive phase transformations in these composites. Lastly, hydrothermal differential scanning calorimetry is performed to learn about the energetics rather than the kinetics of rearrangement. We find that restructuring of the nanoscale periodicity is the dominant energetic process during a phase transition while atomic scale silica chemistry (hydrolysis and condensation) is dominant at all other times.