The phase behavior of terminal (R-Si-(OEt)₃) and bridging ((EtO)₃-Si-R-Si-(OEt)₃)) organosilica precursors (OSPs) has been studied in order to understand how to control the chemical and structural properties of hybrid materials, that can yield periodic mesoporous materials, useful in targeted industrial applications. Experimental evidence indicates that bridging OSPs (b-OSPs) form ordered structures more easily than terminal OSPs (t-OSPs)¹. In this work, we have analyzed how the hydrophobic/hydrophilic nature and solubility of the OSPs affect the self-assembling of these systems. MC simulations in the NVT ensemble have been used to model self-assembling ternary systems composed of amphiphilic molecules, silica or OSPs, and solvent. These systems are able to form periodic complex structures, such as hexagonal-ordered cylinders, depending on the precursors used and the surfactant concentration². In the region where two phases coexist, it has been possible to identify a “concentrated phase”, rich in surfactant and OSPs, and a “dilute phase” composed mainly of solvent. Our simulations show that it is easier to form ordered structures in systems with b-OSPs because the surfactant concentration in the concentrated phase is higher than in systems with t-OSPs. Nevertheless, the chemical nature of the OSPs is key to determine whether or not ordered structures are formed because OSPs with functional hydrophobic groups are unable to form ordered structures, regardless of the surfactant concentration, even when a microphase separation has been observed in b-OSPs. In general, when using partially soluble hydrophilic OSPs, the concentrated phase has a higher surfactant concentration than those observed with completely soluble OSPs. Therefore, in the former case ordered phases are observed over a wider range of conditions. We have detected from the cluster size distribution that in all ternary systems studied the size of the aggregates increases and their separation is reduced when the surfactant concentration increases. This behavior has not been observed in binary systems with the same symmetric surfactant, where the aggregate size is practically insensitive to changes in surfactant concentration. At high concentrations, adjacent spherical aggregates touch each other, and as surfactant concentration increases, hexagonal phases appear. The radial distribution functions in systems presenting hexagonal phases have shown that increasing the surfactant concentration reduces the distance between the cylindrical rods and the surface area per unit volume for the resulting porous material becomes higher.

References

(1) Hatton, B.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Accounts of Chemical Research 2005, 38, 305-312.

(2) Patti, A.; Mackie, A. D.; Siperstein, F. R. COPS VII, Studies in Surface Science and Catalysis 2006, 495.