Ken'ichi Aoki

Solid phase adsorption of crystal violet lactone on silica nanoparticles to probe mechanochemical surface modification.

The solid phase adsorption of crystal violet lactone (CVL) on five types of Stober silica nanopowders with BET specific surface areas in the range of 50-800 m2/g under dry milling conditions was described for the first time. The hydrogen bonding between surface silanol and the carboxylate of the ring-opened triphenylmethane dye (CVL+) led to the formation of monolayers of CVL+ in a flat-laid configuration. The lambda max of CVL+ in diffusive reflection visible spectra was influenced by the particle size of silica powders, suggesting that the microenvironmental polarity of adsorbed CVL+ is considerably reduced along with the decrease of the particle size. The solid phase adsorption of CVL obeyed Langmuir adsorption isotherms to give a saturated amount of CVL+ for every silica nanoparticle. The surface concentration of CVL+ on nanoparticles at the saturation was estimated to be 0.31 mg/m2 on average, disclosing that about 52% of the surface can be covered by CVL+ under the assumption that the BET-specific surface areas are equivalent to the real surfaces active for the CVL adsorption. The generation of the blue color of CVL provided a convenient means to estimate qualitative and quantitative analysis of the surface coverage with surface-active reagents, which conceal surface silanols. Subsequently, silica nanoparticles were milled with a surface modifier, followed by milling with CVL to observe the intensity of the blue color in order to disclose that the surface coverage with oligo- and polyethylene glycols as well as with nonionic surfactants by dry milling was specifically determined by the number of repeating oxyethylene units. Although the surface-active reagents were easily desorbed in water, the desorption was notably suppressed by milling with CVL, suggesting that the surface-modified particles with the surface-active reagents are covered with ultrathin films of CVL.

Properties of core–shell structured nanopowders of molecular crystals fabricated by dry grinding

A generic way to prepare nano-sized molecular crystals in large scale is described. The dry mechanical grinding of a mixture of molecular crystals with surface-modified Stober silica nanoparticles gave voluminous hybridised powders with core–shell nanostructures, which were revealed by TEM and EFTEM measurements. Fluorescence spectra of the hybrids of emissive crystals suggested that crystal structures of the bulk are preserved even after the milling. Calorimetric measurements disclosed that melting points of crystals are lowered as a result of the nanohybridisation and that the depression of melting point and heat of fusion are reversibly proportional to shell thickness.

Dry grinding of polymer solids with silica nanoparticles to give powdery nanocomposites: mechanochemical effect on thermotropic behaviour and solid-state dissolution of molecular crystals

The dry beads grinding of polystyrene with surface-modified silica nanoparticles resulted in the formation of powdery nanocomposites. Molecular weights of the polymer were scarcely altered even after the grinding, indicating that the polymer exhibits resistance toward mechanochemical fission of main chains under the milling conditions. The procedure was applied to various types of polymers to give powdery nanohybrids, which are thought to be comprised of silica nanoparticles and polymer coils adhered on them. The nanopowders thus prepared gave slurries in water even in the absence of a surfactant, though the original polymeric powders were not dispersed in water at all. Particular events brought about by the milling caused the marked modification of thermotropic properties of polymer solids, implying the mechanochemically induced conformational alteration of polymer coils. As a result, pyrene crystals were able to be dissolved in polymer solids molecularly in a mechanochemical way under dry conditions, as evidenced by the generation of monomer emission.