Experimental and theoretical studies of the self-propelled motional dynamics of a new genre of catalytic sphere dimer, which comprises a non-catalytic silica sphere connected to a catalytic platinum sphere, are reported for the first time. Using aqueous hydrogen peroxide as the fuel to effect catalytic propulsion of the sphere dimers, both quasi-linear and quasi-circular trajectories are observed in the solution phase and analyzed for different dimensions of the platinum component. In addition, well-defined rotational motion of these sphere dimers is observed at the solution-substrate interface. The nature of the interaction between the sphere dimer and the substrate in the aqueous hydrogen peroxide phase is discussed. In computer simulations of the sphere dimer in solution and the solution-substrate interface, sphere-dimer dynamics are simulated using molecular-dynamics methods and solvent dynamics are modeled by mesoscopic multiparticle collision methods taking hydrodynamic interactions into account. The rotational and translational dynamics of the sphere dimer are found to be in good accord with the predictions of computer simulations.

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Adhesion between chemically dissimilar solids is not often observed for fundamental reasons, especially the large solid-solid interfacial tensions involved that, in turn, derive from the fundamental characteristics of van der Waals and other intermolecular interactions. However, this difficulty can be overcome in many cases by mixing particulate solids within aqueous media and drying the resulting dispersion. In this work, transmission electron microscopy (TEM) and scanning probe microscopy (SPM) were used to obtain evidence for strong adhesion between the following pairs of organic and inorganic nanoparticles: Sto ¨ber silica and poly(styreneco-butyl acrylate-co-acrylic acid) (SA) latex, calcium montmorillonite and the same latex, and titanium dioxide and another SA latex. Adhesion was observed even though the particles in each pair are highly dissimilar and thus are expected to have a high interfacial tension. Bulk or aggregate particle nanohybrids were obtained by drying mixed aqueous dispersions at different particle concentrations and were examined using bright-field and energy-filtered imaging in TEM, as well as intermittent-contact and phase-contrast SPM. Association between silica, clay, or TiO 2 and the latex particles was observed under several conditions, and partial particle segregation was also observed. A general mechanism for the formation of hybrid or composite monoliths is proposed, based on the action of capillary forces during the drying process followed by electrostatic interactions within the dry solid, between negative particles and cationic domains formed by dry serum solutes. Using this model, calculated electrostatic adhesion energy between dissimilar particles can be used to explain previous literature data. This mechanism is suitable for making hybrid monoliths from nanosized particles.

Na-montmorillonite (Na-MMT) and its ion-exchanged derivatives (K-, Li-, Ca-MMT) were used to make nanocomposites by mixing with a low-T g styrene-acrylic latex, and these materials were used to verify the effects of counterions on nanocomposite morphology and mechanical properties. The monovalent cation clays form exfoliated/intercalated nanocomposites with a more than 10-fold increase in modulus, as compared to the pristine polymer, and approximately 200% increase in tensile strength. In the case of Li nanocomposites the mechanical properties are strongly dependent on the extent of drying, as expected considering that these ions are strongly hydrated. Calcium clay-polymer particle adhesion is very good, as evidenced in transmission electron micrographs, but the extent of exfoliation is less pronounced and the changes in the mechanical properties are accordingly lower than using Na, Li, or K counterions. Analytical electron micrographs show that the counterions are always accumulated in the domains containing both clay and polymer, showing that the compatibility of these two phases, which carry excess negative charges, is achieved thanks to cation bridges at the interfaces, following a model that was previously put forward to explain latex-clay nanocomposite formation and properties.

Structure-function relationships in supramolecular systems depend on the spatial distribution of molecules, ions, and particles within complex arrays. Imaging the spatial distribution of molecular components within nanostructured solids is the objective of many recent techniques, and a powerful tool is electron spectroscopy imaging in the transmission electron microscope (ESI-TEM) in the low-energy-loss range, 0-80 eV. This technique was applied to particulate and thin film samples of dielectric polymers and inorganic compounds, providing excellent distinction between areas occupied by various macromolecules and particles. Domains differentiated by small changes in molecular composition and minor differences in elemental contents are clearly shown. Slight changes in the molecules produce intensity variations in molecular spectra that are in turn expressed in sets of low-energy-loss images, using the standard energy-filtered transmission electron microscopy (EFTEM) procedures. The molecular map resolution is in the nanometer range and very close to the bright-field resolution achieved for the same sample, in the same instrument. Moreover, contrast is excellent, even though sample exposure to the electron beam is minimal.