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The current project explores our recently proposed approach to the judicious selection of micelle expanders (swelling agents) in the surfactant-templated synthesis of ultra-large-pore ordered mesoporous materials (OMMs). The swelling agent candidates are selected on the basis of a hypothesis that substances that solubilize in micelles of a particular surfactant to a moderate extent promise to be micelle expanders leading to well-ordered OMMs with significantly enlarged mesopores, perhaps of size beyond that hitherto achieved. On the other hand, substances that solubilize too strongly are likely to afford non-uniform very-large-pore products (perhaps foams), while substances solubilizing to a small extent may lead to no appreciable pore size enlargement. The suggested way to identify swelling agents is to examine data on the extent of solubilization of organic compounds in surfactant solutions, with a particular attention to families of compounds for which the extent of solubilization varies in a wide range. For instance, in the case of alkyl-substituted benzenes, the extent of solubilization in Pluronic triblock copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO), decreases as the number and/or size of alkyl substituants increases. If there are experimental data available on the micelle expander performance of one or more of the members of a particular swelling agent family in conjunction with a particular surfactant (or surfactant family), one can consider whether the experimentally observed swelling action was excessive or too weak. On this basis, one can attempt to identify a member of the swelling agent family that would solubilize in the considered surfactant in an optimal way. In particular, in the synthesis of ordered mesoporous silicas (known as SBA-15) with cylindrical mesopores using EO₂₀PO₇₀EO₂₀ surfactant Pluronic P123 (with relatively large fraction of the hydrophobic block), 1,3,5-triisopropylbenzene (TIPB) was identified earlier as a promising swelling agent. TIPB indeed allowed us to synthesize large-pore and ultra-large-pore 2-D hexagonal SBA-15 silica with cylindrical mesopores of diameter from 10 nm up to 26 nm, some of which had never been documented before. On the other hand, it was noticed that EO₁₀₆PO₇₀EO₁₀₆ triblock copolymer Pluronic F127 with a low fraction of the hydrophobic block was swollen to a limited extent by 1,3,5-trimethylbenzene (TMB; the most commonly used swelling agent) in the synthesis of LP-FDU-12 silica with face-centered cubic, fcc, structure (Fm3m symmetry) of spherical mesopores. Therefore, more strongly solubilized compounds (xylene and toluene) were selected as swelling agents, indeed leading to significant unit-cell size and pore-diameter increases.

Over the last year, the work on the project was focused on: (i) better understanding of the selection principles for swelling agents, (ii) elaboration of new procedures for the synthesis of FDU-12; (iii) optimization of the synthesis of large-pore SBA-15; (iv) application of the micelle expander selection principles to periodic mesoporous organosilicas (PMOs), and (v) application of the micelle expander selection principles to hollow single-micelle-templated silica and organosilica nanoparticles.

The literature data on solubilization of alkylbenzenes in micelles of Pluronics surfactants suggested that xylene and ethylbenzene, which have the same number (two) of carbon atoms in alkyl substituents solubilize to a similar extent. We investigated whether this result implies that the suitability of alkylbenzenes as swelling agents in conjunction with Pluronics block copolymers can be predicted on the basis of the total number of carbon atoms in alkyl substituents, without considering the number, position and size of the alkyl substituents. In particular, FDU-12 silica was synthesized under the same conditions using Pluronic F127 and either individual xylene isomers or ethylbenzene as swelling agents. The results were compared with those for xylenes (xylene isomer mixture possibly containing some ethylbenzene), which had been obtained earlier. It was found that all these swelling agents perform quite similarly in ultra-large-pore FDU-12 synthesis, although xylenes (isomer mixture) and ethylbenzene allowed us to obtain unit-cell parameters up to 55-56 nm and pore diameters up to 36 nm, while individual xylene isomers afforded slightly lower unit-cell parameters. The results for all the considered swelling agents were significantly different from those for TMB and toluene with three and one carbon atom in alkyl substituents, respectively. These results suggest a simplified procedure for the swelling agent identification in families of compounds in the absence of experimental data on solubilization of particular swelling agent candidates in surfactant solutions. Moreover, these results prompted us to further investigate the use of toluene as a swelling agent in FDU-12 synthesis. Other implications of the above insight into the selection of the swelling agents are also under investigation.

The synthesis (in the absence of NH₄F used in our original synthesis) of highly ordered large-pore SBA-15 with (100) interplanar spacing of ~17 nm and large lateral size of ordered domains using TIPB as a swelling agent was explored, including its extension over larger unit-cell sizes and pore diameters.

In addition, highly ordered organosilicas with organic bridging groups integrated into silica-based frameworks and with face-centered cubic (Fm3m) structures were successfully obtained with unit-cell parameters up to ~50 nm using EO₁₀₆PO₇₀EO₁₀₆ and xylene or toluene as swelling agents. Several of these large-unit-cell PMOs do not have counterparts reported in the scientific literature. For these cubic PMOs, the pore diameter was controllable by the adjustment of the inorganic salt concentration in the synthesis mixture and other parameters.

Pluronic block copolymers and judiciously selected swelling agents were also employed in the synthesis of hollow silica and organosilica nanoparticles (typically nanospheres) templated by single surfactant micelles.

The PRF award significantly facilitated PI’s advancement as an assistant professor and contributed to a favorable decision regarding the promotion to an associate professor position. It provided funds to support one Ph.D. student (US citizen), as well as two undergraduate students (both being minority students). The PRF award also provided much needed funding for chemicals and supplies used by one postdoctoral fellow, two graduate students, and three undergraduate students working on research projects within the scope of the PRF award.