A spin probe study of mesoporous silica formation via a neutral templating route

The structure of dodecylamine (DDA) aggregates acting as templates in the synthesis of mesoporous silica (in the DDA/water/ethanol/tetraethoxysilane (TEOS) system) has been studied via the EPR spectra of a variety of spin probes and of a spin-labeled silica precursor. Two recipes have been examined: one, starting from a water-rich DDA solution, A(0), reported to produce a silica with framework mesopores and textural pores comparable in volume, and another, starting from an ethanol-rich solution, B-0 which yields a solid having practically only framework pores. For both compositions the relative positions of the spin probes in the aggregates have been determined, the local polarities expressed in terms of the Kosower scale and their motions examined. The aggregates formed have been characterized as emulsion droplets for A(0) and micelles with radii <25 A for B-0. After reaction of A(0) and B-0 with TECS, all probes showed significantly hindered motion in the wet precipitates. In the solid B, precipitated from the ethanol-rich solvent, the spin probes 5- and 7-doxyl stearic acid (5- and 7-DSA) are strongly immobilized, while other probes maintain quasi-isotropic motion but with very large rotational correlation times. This pattern is characteristic of micelles with hindered tumbling and/or lateral diffusion and is assigned to those aggregates (in interaction with the silica component) on which the framework mesopores are formed. In solid A, precipitated from A(0), 5- and 7-DSA spin probes maintain the spectra with anisotropic features, characterized by an order degree, found in the starting emulsion, but with evidence of tighter packing, while other probes have their rotation retarded. These aggregates have a low curvature interface, characteristic of emulsion droplets and are assumed to be at the origin of the large textural pores, which characterize this solid. Adsorption of probes after synthesis rather than before precipitation supports these assignments. We have also found that the surfactant-silica interaction is critically dependent on solvent. The strong immobilization of the probes in the interface region, observed during drying at low temperatures, is a reversible process, since under these conditions the silica network cross-linking is only incipient; Si-29 NMR data show that cross-linking becomes important only above 90degreesC.