A new minimal surface and the structure of mesoporous silicas.

The formation of complex inorganic superstructures in nature relies on the interaction between organic and inorganic species to direct the inorganic form away from its “usual” morphology. During synthesis the superstructures are soft and dynamic which makes a study of the nature of the ephemeral interface difficult. However, the inorganic skeletons formed are stable and consequently amenable to detailed examination. In 1992 researchers at Mobil and in Japan made the remarkable discovery that the subtle forms of mesoscopic organization of surfactant molecules could be imprinted on oxide structures. Herein we report our studies of the structure of the surfactant-templated, cubic, mesoporous silica superstructure, SBA-1 and provide a formulation in terms of curvature that has important repercussions for both surfactant structures and the mechanism of formation of inorganic replicas. We establish that the crucial interface that determines the inorganic structure is between the silica and water adsorbed at the micelle surface, not between silica and surfactant, thus challenging the present synthesis mechanisms. We adopt a general protocol for understanding the surface curvature and energy which could be applied widely to the growth of inorganic structures in biology, including nonperiodic and disordered structures. Currently there are a number of ways to address the mesophase structure of these mesoporous silicas. Terasaki and co-workers adopted the direct structure elucidation approach by using electron crystallography. With this method they were able to reconstruct an electron density map directly with electron micrographs recorded along several zone axes. This has the distinct advantage that no predefined knowledge is required and that the diffraction intensities are optimized through the observation of individual particles. However, its disadvantage is that the electron density map does not easily yield clues about the synthesis mechanism. The technique is also technically very demanding and is only suitable for highly organized 3D structures that are stable in the electron microscope. It is not a routine technique and cannot be used to monitor many sample preparations, or to look at preparations in situ. Another route, first adopted by Anderson and Alfredsson, and more recently by Solovyov and co-workers, and Sch th and co-workers, 9] involves the determination of a structural model from some known data followed by its refinement according to certain parameters, for instance, against X-ray diffraction patterns or electron micrographs. This method has the advantage that it is flexible and can be applied to a variety of preparations; the results also give information about synthesis mechanisms. The disadvantage is that the process does require input from other techniques and some prior knowledge. Also, the powder X-ray diffractogram is an average technique and as a consequence, it is important that the sample is relatively homogeneous. Herein we introduce a general mathematical approach to the modeling of mesoporous silicas, which allows easy generation of electron density maps and diffraction patterns for comparison with experimental data. SBA-1 was synthesized in HCl (4m) with hexadecyltriethylammonium bromide (HTEABr) as surfactant and tetraethylorthosilicate (TEOS) as a source of silica. The structure is known from electron crystallography to belong to the space group Pm3̄n and consists of cages connected through windows. The corresponding surfactant phase is known as the I1 (or Q 223 referring to the space group number, or Pm3n as an identifier for Pm3̄n ) phase which is a cubic, isotropic, micellar phase. Curiously, the I1 phase was not observed in the phase behavior of HTEABr, in which a hexagonal phase occurs at the micellar solution boundary. Consequently, we studied the phase behavior of this surfactant with a temperature-programmed penetration scan (Supporting Information). This shows that in the presence of impurities that are likely to remain from the surfactant synthesis, such as bromohexadecane, a cubic phase is induced below 32 8C. Significantly, this phase occurs between the hexagonal phase and isotropic solution, which suggests a higher curvature of the micellar phase. More importantly, when HTEABr is exchanged with chloride ions to make HTEACl the penetration scan reveals the presence of three cubic mesophases. HTEACl is probably the templating agent in SBA-1 preparations owing to the large excess of chloride anions in the preparation. [*] Prof. M. W. Anderson, Dr. C. C. Egger # Department of Chemistry, UMIST Manchester, M601QD (UK) Fax: (+44)161-200-4511 E-mail: [email protected] Prof. G. J. T. Tiddy Department of Chemical Engineering, UMIST Manchester, M601QD (UK)