Lamellar mesostructured silicas with chemically significant hierarchical morphologies

We describe the hierarchical structures of mesostructured silicas assembled from electrically neutral and unsymmetrical Gemini surfactants of the type CnH2n+1NH(CH2)(m)NH2 with n = 10, 12, 14 and m = 3, 4. As expected for Gemini surfactants with an all anti-chain configuration and a packing parameter near 1.0, lamellar framework structures are formed, regardless of the length of the alkyl chain (n) and the number of carbon atoms (m) linking the two amino group centers. However, different layer curvatures and levels of hierarchical structure are observed depending on the delicate balance between the hydrophilic interactions at the surfactant head group-silica interface and the hydrophobic interactions between the surfactant alkyl groups. For Gemini derivatives with n = 12 or 14 and m = 3 or 4, well-expressed hierarchical vesicles are formed that are analogous to those assembled previously from Gemini surfactants with m = 2. However, for n = 10, a new coiled slab structure (m = 3) and an onion-like core-shell structure (m = 4) are formed. In addition, a previously unobserved stripe-like silica structure is obtained from a C-12+2+0(0) Gemini surfactant in combination with an alpha,omega-diamine co-surfactant. The relative stability of these hierarchical structures depends on the delicate competition between the long-range elastic forces occurring in the hydrophobic region of the assembled surfactant and the short-range chemical forces in the hydrophilic moiety. Lamellar silicas with hierarchical vesicular structures, the new coiled slab, and stripe-like phases promise to be chemically significant morphologies, because they can minimize the framework pore length and provide optimal access to the framework walls under diffusion-limited conditions.