Phase behavior and material properties of hollow nanoparticles

The role of polymer-solvent interactions in polyvinyl-alcohol dispersions of multi-wall carbon nanotubes: from coagulant to dispersant

Dispersion of carbon nanotubes in solutions of polyvinyl-alcohol is required for solution casting of composite materials with improved interfacial adhesion where chains adsorbed on the nanotubes serve in the dual role of dispersant and compatible "connector" to the polyvinyl-alcohol matrix. Yet polyvinyl-alcohol is known to induce coagulation of nanotubes in aqueous solutions and thus far, it has not been used for dispersing pristine nanotubes. Here, we report that non-fully hydrolyzed (80-90%) polyvinyl-alcohol can be used for the preparation of stable, surfactant-free, dispersions of multi-wall carbon nanotubes in ethanol-water mixtures (of at least 50 vol% ethanol). Cryo-TEM imaging and rheological measurements of stable, long-lived dispersions reveal the formation of random networks of suspended tubes, with an averaged mesh size of ∼500 nm, indicating that the individual tubes do not aggregate or coagulate. We hypothesize that the polyvinyl-acetate sequences found in non-fully hydrolyzed polymers swell in the presence of ethanol, leading to the formation of a long-ranged steric (entropic) repulsion among polymer-decorated nanotubes. The unexpected role of the polyvinyl-acetate sequences along with a detailed dispersion mechanism are described.

Effect of the range of repulsions on the existence of a stable liquid phase

Experimental and theoretical results have established that the range of the attraction plays a critical role in determining whether a particle system exhibits a stable liquid phase. Changes to the range of the repulsions can similarly affect the existence of a stable liquid phase; however, these effects have not been clearly elucidated. We demonstrate that an increase in the range of repulsions can either enhance or decrease the stability of the liquid phase, depending on the form of the interaction potential. For either case, the critical variable that controls the stability of the liquid phase is the ratio of the representative energies of the liquid and solid phases.

Experimental and theoretical studies of the colloidal stability of nanoparticles-a general interpretation based on stability maps

The current work addresses the understanding of the stabilization of nanoparticles in suspension. Specifically, we study ZnO in ethanol for which the influence of particle size and reactant ratio as well as surface coverage on colloidal stability in dependence of the purification progress was investigated. The results revealed that the well-known ζ-potential determines not only the colloidal stability but also the surface coverage of acetate groups bound to the particle surface. The acetate groups act as molecular spacers between the nanoparticles and prevent agglomeration. Next to DLVO calculations based on the theory of Derjaguin, Landau, Verwey and Overbeek using a core-shell model we find that the stability is better understood in terms of dimensionless numbers which represent attractive forces as well as electrostatic repulsion, steric effects, transport properties, and particle concentration. Evaluating the colloidal stability in dependence of time by means of UV-vis absorption measurements a stability map for ZnO is derived. From this map it becomes clear that the dimensionless steric contribution to colloidal stability scales with a stability parameter including dimensionless repulsion and attraction as well as particle concentration and diffusivity of the particles according to a power law with an exponent of -0.5. Finally, we show that our approach is valid for other stabilizing molecules like cationic dendrons and is generally applicable for a wide range of other material systems within the limitations of vanishing van der Waals forces in refractive index matched situations, vanishing ζ-potential and systems without a stabilizing shell around the particle surface.

Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect

Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, we show that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.

Atomistic versus two-body central potential models of C(60): a comparative molecular dynamics study

We report on an extensive molecular dynamics investigation of two models of C60. The first model is based on an effective pair, central potential obtained by integrating the interaction between two carbon atoms over the fullerene cages [L.A. Girifalco, J. Phys. Chem. 96, 858 (1992)]. The second model explicitly takes into account the discrete, "atomistic" structure of the C60 molecules; we study two different parametrizations of the carbon-carbon interaction, one identical to that employed in the Girifalco approach, the other borrowed from previous studies on graphite [A. Cheng and M.L. Klein, J. Phys. Chem. 95, 6750 (1991)]. We consider a temperature range spanning from 300 to 1900 K, and pressures up to 200 kbar. Results for the lattice spacing and several thermodynamic quantities, as well as for the radial distribution functions, are reported and compared among each other and with experimental data. The central pair model yields only semiquantitative predictions at typical ambient densities, whereas pressures are generally overestimated. Atomistic simulations reproduce to an overall quantitative level of accuracy the experimental C60 properties. A comparison is also made of the central versus the atomistic potential predictions, when using the same potential parameters in the carbon-carbon interaction. We discuss applications of the adopted modelizations to fullerene systems of current interest, as well as different strategies to optimize the values of the potential parameters.

Elastic properties of polyelectrolyte capsules studied by atomic-force microscopy and RICM

Mechanical properties of polyelectrolyte multilayer capsules were studied using a new method combining atomic-force microscopy and reflection interference contrast microscopy. By measuring the force vs. deformation for poly(styrene sulfonate)/poly(allylamine) capsules the existence of different deformation regimes depending on the applied deformation was shown. The present paper focuses on the small-deformation regime. The elastic response of the deformed capsule was studied as a function of the wall thickness and the capsule size, and showed the theoretically expected variations. The Young modulus obtained from the experiments ranges between 1.3 and 1.9 GPa.

Theoretical description of phase coexistence in model C60

We have investigated the phase diagram of a pair interaction model of C60 fullerene [L. A. Girifalco, J. Phys. Chem. 96, 858 (1992)], in the framework provided by two integral equation theories of the liquid state, namely, the modified hypernetted chain (MHNC) implemented under a global thermodynamic consistency constraint, and the self-consistent Ornstein-Zernike approximation (SCOZA), and by a perturbation theory (PT) with various degrees of refinement, for the free energy of the solid phase. We present an extended assessment of such theories as set against a recent Monte Carlo study of the same model [D. Costa, G. Pellicane, C. Caccamo, and M. C. Abramo, J. Chem. Phys. 118, 304 (2003)]. We have compared the theoretical predictions with the corresponding simulation results for several thermodynamic properties such as the free energy, the pressure, and the internal energy. Then we have determined the phase diagram of the model, by using either the SCOZA, the MHNC, or the PT predictions for one of the coexisting phases, and the simulation data for the other phase, in order to separately ascertain the accuracy of each theory. It turns out that the overall appearance of the phase portrait is reproduced fairly well by all theories, with remarkable accuracy as for the melting line and the solid-vapor equilibrium. All theories show a more or less pronounced discrepancy with the simulated fluid-solid coexistence pressure, above the triple point. The MHNC and SCOZA results for the liquid-vapor coexistence, as well as for the corresponding critical points, are quite accurate; the SCOZA tends to underestimate the density corresponding to the freezing line. All results are discussed in terms of the basic assumptions underlying each theory. We have then selected the MHNC for the fluid and the first-order PT for the solid phase, as the most accurate tools to investigate the phase behavior of the model in terms of purely theoretical approaches. It emerges that the use of different procedures to characterize the fluid and the solid phases provides a semiquantitative reproduction of the thermodynamic properties of the C60 model at issue. The overall results appear as a robust benchmark for further theoretical investigations on higher order C(n>60) fullerenes, as well as on other fullerene-related materials, whose description can be based on a modelization similar to that adopted in this work.