| Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol-gel derived silica nanoparticles, resulting in a conformal "cross-linked" coating that renders the interparticle neck zone wider. Upon drying, these cross-linked networks yield aerogels which are up to ∼3x more dense than native aerogels based on the underlying silica framework, but also ∼10x less hygroscopic and ∼300x stronger. Furthermore, it has been found that cross-linking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Cross-linked aerogels can be also dried under ambient pressure without supercritical fluid (SCF) extractions. The best solvent identified for that purpose is pentane, and the resulting aerogels are both microscopically and macroscopically identical to their SCF-CO2 dried counterparts. Being able to dry monolithic aerogels without SCF extraction, opens the way to broader industrial application of this class of materials.; In a different line of research, we discovered that the 4-benzoyl- N-methylpyridinium cation undergoes two successive, well-separated 1-e reductions in analogy to other main stream redox-active classes of compounds like viologens (4,4'-bipyridinium dications). Unlike viologens, however, 4-benzoyl-N-methylpyridinium cations are unsymmetric redox systems. In order to elucidate the site of the 1 st- and 2nd-e reduction, we synthesized several 4-( p-substituted benzoyl)-N-methylpyridinium cations and we studied their redox properties from a Hammett Linear Free Energy Relationships perspective. Linearity was associated with both 1-e reductions of the 4-benzoyl- N-methylpyridinium system. The slopes indicate that 1st-e reduction takes place at the pyridinium ring, while the 2nd-e reduction takes place at the carbonyl. Along the way it was also discovered that the 1-e reduced form of the nitro group is an extremely strong electron donor with sp-NO˙-2 = -097. Furthermore, in an effort to investigate the effect of substitution in fine-tuning the two redox potentials of the 4-benzoylpyridinium system, we also synthesized a series of 4-(p-substituted benzoyl)-N-(p-substituted benzyl)pyridinium cations and we discovered, through the nonlinear behavior in the Hammett Free Energy Relationships, that multiple susbtitution effects are not additive. |
