Giorgio Pastore

Phase diagram of Janus particles.

We thoroughly investigate a simple model representative of the recently synthesized Janus particles, i.e., colloidal spherical particles whose surface is divided into two areas of different chemical composition. When the two surfaces are solvophilic and solvophobic, these particles constitute the simplest example of surfactants. The phase diagram includes a colloidal-poor (gas), colloidal-rich (liquid) demixing region, which is progressively suppressed by the insurgence of micelles, providing the first model in which micellization and phase separation are simultaneously observed. The coexistence curve is found to be negatively sloped in the temperature-pressure plane, suggesting that Janus particles can provide a colloidal system with anomalous thermodynamic behavior.

Additive and non-additive hard sphere mixtures: Monte Carlo simulation and integral equation results.

Binary mixtures of additive (d 12 = (d 1 + d 2)/2) and non-additive (d 12 ≠ (d 1 + d 2)/2) hard spheres have been studied with new integral equations. Accurate Monte Carlo results for some non-additive cases have been generated and tables of radial distribution functions are explicitly given. Comparison of structural and thermodynamic results demonstrates that an approximate integral equation recently proposed by Martynov and Sarkisov for one-component hard spheres significantly improves on the Percus-Yevick approximation also in the mixture case, without introducing free parameters. A simple further generalization gives a very good parameterization of the computer experiment results. An analytical fit of the equation of state for symmetric non-additive systems is also given.

Effects of patch size and number within a simple model of patchy colloids.

We report on a computer simulation and integral equation study of a simple model of patchy spheres, each of whose surfaces is decorated with two opposite attractive caps, as a function of the fraction chi of covered attractive surface. The simple model explored--the two-patch Kern-Frenkel model--interpolates between a square-well and a hard-sphere potential on changing the coverage chi. We show that integral equation theory provides quantitative predictions in the entire explored region of temperatures and densities from the square-well limit chi=1.0 down to chi approximately 0.6. For smaller chi, good numerical convergence of the equations is achieved only at temperatures larger than the gas-liquid critical point, where integral equation theory provides a complete description of the angular dependence. These results are contrasted with those for the one-patch case. We investigate the remaining region of coverage via numerical simulation and show how the gas-liquid critical point moves to smaller densities and temperatures on decreasing chi. Below chi approximately 0.3, crystallization prevents the possibility of observing the evolution of the line of critical points, providing the angular analog of the disappearance of the liquid as an equilibrium phase on decreasing the range for spherical potentials. Finally, we show that the stable ordered phase evolves on decreasing chi from a three-dimensional crystal of interconnected planes to a two-dimensional independent-planes structure to a one-dimensional fluid of chains when the one-bond-per-patch limit is eventually reached.

Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures

The existence of systematic variations of isobaric fragility in different supercooled Lennard-Jones binary mixtures is revealed by molecular dynamics simulations. The connection between fragility and local structures in the bulk is analyzed by means of a Voronoi construction. It is found that clusters of particles belonging to locally preferred structures form slow, long-lived domains, whose spatial extension increases with decreasing temperature. As a general rule, a more rapid growth, upon supercooling, of such domains is associated with a more pronounced super-Arrhenius behavior, and hence to a larger fragility.

Phase diagram and structural properties of a simple model for one-patch particles

We study the thermodynamic and structural properties of a simple, one-patch fluid model using the reference hypernetted-chain (RHNC) integral equation and specialized Monte Carlo simulations. In this model, the interacting particles are hard spheres, each of which carries a single identical, arbitrarily oriented and attractive circular patch on its surface; two spheres attract via a simple square-well potential only if the two patches on the spheres face each other within a specific angular range dictated by the size of the patch. For a ratio of attractive to repulsive surface of 0.8, we construct the RHNC fluid-fluid separation curve and compare with that obtained by Gibbs ensemble and grand canonical Monte Carlo simulations. We find that RHNC provides a quick and highly reliable estimate for the position of the fluid-fluid critical line. In addition, it gives a detailed (though approximate) description of all structural properties and their dependence on patch size.

Dynamics and energy landscape in a tetrahedral network glass-former: direct comparison with models of fragile liquids

We report molecular dynamics simulations for a new model of tetrahedral network glass-former, based on short-range spherical potentials. Despite the simplicity of the forcefield employed, our model reproduces some essential physical properties of silica, an archetypal network-forming material. Structural and dynamical properties, including dynamic heterogeneities and the nature of local rearrangements, are investigated in detail and a direct comparison with models of close-packed, fragile glass-formers is performed. The outcome of this comparison is rationalized in terms of the properties of the potential energy surface, focusing on the unstable modes of the stationary points. Our results indicate that the weak degree of dynamic heterogeneity observed in network glass-formers may be attributed to an excess of localized unstable modes, associated with elementary dynamical events such as bond breaking and reformation. In contrast, the more fragile Lennard-Jones mixtures are characterized by a larger fraction of extended unstable modes, which lead to a more cooperative and heterogeneous dynamics.

First‐principles molecular‐dynamics simulation of liquid CsPb

Many alkali–post‐transition group IV alloy systems exhibit clearly defined equiatomic compounds together with a pronounced intermediate range ordering, indicated by a first sharp diffraction peak at ≊0.9 A−1. These phenomena have been explained assuming that tetrahedral group IV anions, ‘‘Zintl’’ ions, survive in the liquid state. As a prototype system we considered liquid CsPb, for which several experimental results are available, and studied it by means of first‐principles molecular‐dynamics. Agreement with experiment is satisfactory, provided the 5s and 5p electrons of cesium are explicitly taken into account in the computation of the electronic valence charge density. In particular, our calculations reproduce the structure factor prepeak reasonably well. The local liquid structure however is quite complex. This can be described as a disordered network, which still has many features in common with the ‘‘Zintl’’ ion model. For instance, the average Pb‐Pb coordination is close to 3, the value for perfect...

Atomic size effects on local coordination and medium-range order in molten trivalent metal chlorides

Structural correlations in molten trivalent metal chlorides are evaluated as functions of the metal ion size RM across the range from LaCl3 (RM approximately=1.4 AA) to AlCl3 (RM approximately=0.8 AA), using a charged soft-sphere model and the hypernetted chain approximation. Main attention is given to trends in the local liquid structure (partial radial distribution functions, coordination numbers and bond lengths) and in the intermediate range order (first sharp diffraction peak in the number-number and partial structure factors). The trend towards fourfold local coordination of the metal ions, the stabilization of their first-neighbour chlorine cage and the growth of medium-range order are found to proceed in parallel as the size of the metal ion is allowed to decrease at constant number density and temperature. A tendency to molecular-type local structure and liquid-vapour phase separation is found within the hypernetted chain scheme at small metal ion sizes corresponding to AlCl3 and is emphasized by decreasing the number density of the fluid.

Understanding fragility in supercooled Lennard-Jones mixtures. II. Potential energy surface

The connection between isobaric fragility and the properties of high-order stationary points of the potential energy surface in different supercooled Lennard-Jones mixtures was investigated. The increase of effective activation energies upon supercooling appears to be driven by the increase of average potential energy barriers measured by the energy dependence of the fraction of unstable modes. Such an increase is sharper, the more fragile the mixture. Correlations between fragility and other properties of high-order stationary points, including the vibrational density of states and the localization features of unstable modes, are also discussed.

The role of excluded volume effects on the structure and chemical short-range order of Ni33Y67 metallic glass

The possibility of modelling the partial structure factors of the Ni33Y67 amorphous alloy with simple purely repulsive potentials is investigated by extending and refining an approach previously proposed for the NixTi1-x alloys. Non-additive hard- and soft-sphere models are carefully studied by using integral equations originally derived in the theory of liquids. The quality of the approximations used and the consequences of softening the repulsion have been examined. The overall agreement between model and experimental data allows a meaningful discussion of the role of different regions of the interaction potential in the real system.