Phase transitions in mesostructured silica/surfactant composites: Surfactant packing and the role of charge density matching

Time-resolved X-ray diffraction is utilized to follow phase transitions in nanostructured silica/surfactant composites in real time under hydrothermal conditions. The data allow us both to obtain kinetic parameters and to obs erve intermediate phases. In all cases, changes in the packing of the organ ic component of these composites drives the transformation, indicating that surfactant packing is a dominant factor in determining the overall structu re of these materials. For materials heated in pure water, however, high ac tivation energies for transformation were measured, suggesting that large k inetic barriers can stabilize structures against surfactant-driven rearrang ements. Matching between the interfacial charge density of the inorganic si lica framework and the charge density of the surfactant headgroups is also found to affect the kinetics of transformation. Lamellar-to-hexagonal trans itions, which complement condensation-induced changes in charge density, ar e observed to be continuous, while hexagonal-to-lamellar transitions, which proceed contrary to these charge density changes, are discontinuous. For m aterials heated in their high-pH synthesis solutions, more complex phase be haviors are observed. Hexagonal (p6mm) structures transform either to a bic ontinuous cubic phase (Ia3d) or to a lamellar structure. Lamellar phases ar e observed at either long or short polymerization times, while cubic phases dominate at intermediate polymerization times. The production of these dif ferent phases can be understood by considering the interplay between organi c packing, charge density matching, and changing activation energies. At sh ort times, high charge on the inorganic framework favors transformation to the low-curvature lamellar structures. At very long times, silica condensat ion both reduces this charge density and cross-links the framework. This cr osslinking raises kinetic barriers for transformation and again favors the topologically simpler hexagonal-to-lamellar transition. Transformations to the cubic phase are only observed at intermediate times, when these effects are balanced.