Synthesis of Mesoporous Silica using Mixed Surfactant Templates: Predictions from Ternary Liquid Crystal Phase Diagrams
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena - III (T2-6c)
Keywords: mesoporous, self-assembly, liquid crystals, ceramics
Dilute micellar solutions of mixed surfactants display interesting nonlinear phenomena including synergism in the properties of mutually soluble surfactants and demixing of surfactants that are mutually immiscible. Tuning the size, charge density, shape, and functionality of micelles by mixing surfactants should have many benefits for mesoporous materials synthesis as well. Here we discuss the use of ternary liquid crystal phase diagrams of mixed surfactants in water for predictive synthesis of mesoporous silica. Specifically, we examine the naïve hypothesis that the phase of a mesoporous material can be predicted by subsituting the volume of water needed at a particular point in the phase diagram with the equivalent volume of condensed silica. This hypothesis has some merit because both water and silica are protic, polar media and because both substances form tetrahedrally coordinated clusters. In practice, the hypothesis works quite well at predicting the structure of silica-surfactant mesophases formed from a mixture of a cationic surfactant and a surfactant with a single glucopyranoside headgroup. The phase diagram measured using polarized optical microscopy matches the mesostructures measured by XRD, and TEM of silica-surfactant composites formed by acid-catalyzed polycondensation in concentrated solutions. However, the mesophase of the material is not always well predicted for the same cationic surfactant mixed with a maltoside-terminated surfactant. Differences may be due to the specific hydrogen bonding envirnonment near the sugar headgroup. The success of this hypothesis in mixtures of cationic hydrogenated and cationic fluorinated surfactants will also be examined using a combination of polarized optical microscopy, TEM, XRD, and gas adsorption.
Presented Thursday 20, 11:20 to 11:40, in session Interfacial & Colloidal Phenomena - III (T2-6c).
