[show abstract][hide abstract] ABSTRACT: We report a computational study on the spontaneous self-assembly of spherical particles into two-dimensional crystals. The experimental observation of such structures stabilized by spherical objects appeared paradoxical so far. We implement patchy interactions with the patches point-symmetrically (icosahedral and cubic) arranged on the surface of the particle. In these conditions, preference for self-assembly into sheet-like structures is observed. We explain our findings in terms of the inherent symmetry of the patches and the competition between binding energy and vibrational entropy. The simulation results explain why hollow spherical shells observed in some Keplerate-type polyoxometalates (POM) appear. Our results also provide an explanation for the experimentally observed layer-by-layer growth of apoferritin--a quasi-spherical protein.
The Journal of chemical physics 04/2012; 136(14):144706. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: The equilibrium size of two largely different kinds of hollow objects behave qualitatively differently with respect to certain experimental conditions. Yet, we show that they can be described within the same theoretical framework. The objects we consider are 'minivesicles' of ionic and nonionic surfactant mixtures, and shells of Keplerate-type polyoxometalates. The finite-size of the objects in both systems is manifested by electrostatic interactions. We emphasize the importance of constant charge and constant potential boundary conditions. Taking these conditions into account, indeed, leads to the experimentally observed qualitatively different behavior of the equilibrium size of the objects.
[show abstract][hide abstract] ABSTRACT: We demonstrate a mechanism to intrinsically stabilize a hollow shell composed of individual nanoparticles. Using Monte Carlo simulations, we show that if nanoparticles that interact via short-range attraction and long-range repulsion are assembled on a template, the resulting shell can be stabilized upon the removal of the template. The interplay of attractive and repulsive interactions provides energy barriers that dynamically arrest the particles and stabilize the shell. We present a well-defined stability region in the interaction parameters space. We find a transition from single layered to multilayered stable shell by increasing the range of attraction, and show that the mechanism is not limited to spherical shells but can also be extended to stabilize nonspherical shells such as torus shells. This study can potentially be useful in understanding and engineering the assembly of nanoparticles into hollow objects of various shapes.
[show abstract][hide abstract] ABSTRACT: The Keplerate-type polyoxometalates (POMs) are known to self-assemble as single-layered hollow shells. It has been regarded that only when POMs carry moderate amount of charges, shells are observed in experiments. Using a coarse-grained molecular model for POMs and invoking patchy hydrogen bonding attractions, we show from Simulated Annealing simulations that patchy attraction alone is sufficient to stabilize sheet-like structure, which is the precursor for the formation of shells. The electrostatic interactions may play a role only in the folding of the sheet-like structure into a spherical shell. Simulation results suggest that shells can be formed even at low charge density. We report a theoretical model to predict the radius of the shell (R∗) formed by weakly charged POMs in the limit of constant charge density. The model predicts that R∗ scales with (i) dielectric constant of the solution (ε) as R∗∝ε1/3 and (ii) with charge density (σ) as R∗∝σ-2/3. This behavior is qualitatively different from the highly charged limit that is often encountered in these systems. Using the model, we find that the radius of the shell (20–120nm) generally observed in experiments corresponds to charge densities in the range of 10−3–10−2nm−2, and the number of charges per POM in the range of 0.02–0.2.