(521c) Ternary Phase Diagram of Cationic and Maltoside Surfactants in Water, and Its Use for Predictive Synthesis of Ordered Mesoporous Silica

Self-assembled nonionic alkyl glucoside surfactants are of interest for creating functional adsorption and catalytic sites at the surface of mesoporous sol-gel-derived materials. Alkyl glucoside surfactants also are “green” alternatives to other types of surfactants. However, these surfactants typically impart poor long-range order when they are used as pore templates. Improved order and control over the functional site density can be achieved by mixing the alkyl polyglucoside surfactant with a cationic surfactant. As an example, we investigate the lyotropic liquid crystalline phase behavior of aqueous solutions of the functional nonionic surfactant n-dodecyl â-D-maltoside (C12G2/DM) and cetyltrimethylammonium bromide (C16TAB) by low-angle x-ray diffraction and polarized optical microscopy (POM). A complete ternary phase diagram of the C16TAB-DM-H2O system is developed at 50°C. Multiple hydrogen bonding interactions between the DM headgroup and the surface silanol groups of silica may form recognition sites for simple carbohydrates. By replacing the volume of water in the phase diagram with an equivalent volume of silica, ordered mesoporous materials are prepared by nanocasting with variable DM:C16TAB ratios. Highly ordered hexagonal and cubic silica mesoporous materials can be synthesized with this mixed surfactant templating route. A series of materials are synthesized keeping the total amount of surfactant constant but varying the relative amounts of two surfactants. It is found that the pore size of the materials can be controlled by changing the ratio of DM to C16TAB surfactants. For a constant silica content, more DM surfactant gives larger pores. Synthesized materials corresponding to over 70 wt% of total surfactant in the aqueous system are not thermally stable, however, because of very thin silica walls.

X-ray diffraction, transmission electron microscopy, and nitrogen sorption confirm the formation of the phases predicted from the ternary phase diagram in most cases. Phases of the materials synthesized with pure DM surfactant at higher silicate concentrations can be predicted precisely from the phase diagram. However, the materials are only weakly ordered at low silica content / higher DM surfactant concentration. Increasing disorder of the mesoporous structures is observed as the relative amount of DM surfactant increases with respect to C16TAB surfactant for a constant percentage of total surfactant present in the system. For example at compositions corresponding to a total surfactant content of 60 wt% in the ternary diagram, the structure of mesoporous silica changes from long-range ordered 2D-hexagonal to disordered hexagonal to very weakly ordered (phases collapses during calcination) at DM contents of 48 wt% and 57 wt%, respectively. The observed deviations could be because of different hydrogen bonding abilities with hydroxyl groups of DM surfactant of water molecules vs. silicates. Stronger cationic interactions of C16TAB with silicates are beneficial to the formation of well ordered mesoporous silica compare to weaker hydrogen bonding interactions between the maltoside headgroups and silicates. Differences of hydrogen bonding behavior are most significant when only a small amount of silicates are available for hydrogen bonding with polar headgroups, and result in poorly ordered or lamellar silica. Slow kinetics of co-assembly might also play a role. In the next phase of this ongoing work, attention is given on the predictive synthesis of ordered mesoporous silica-titania mixed oxide thin films by utilizing this mixed templating route and the adsorption behavior of simple carbohydrates on theses surfaces.