Gianmarco Munaò

Phase diagram of one-patch colloids forming tubes and lamellae

We numerically calculate the equilibrium phase diagram of one-patch particles with 30% patch coverage. It has been previously shown that in the fluid phase these particles organize into extremely long tubelike aggregates (G. Munaò et al. Soft Matter 2013, 9, 2652). Here, we demonstrate by means of free-energy calculations that such a disordered tube phase, despite forming spontaneously from the fluid phase below a density-dependent temperature, is always metastable against a lamellar crystal. We also show that a crystal of infinitely long packed tubes is thermodynamically stable, but only at high pressure. The full phase diagram of the model, beside the fluid phase, displays four different stable crystals. A gas-liquid critical point, and hence a liquid phase, is not detected.

Cluster formation in one-patch colloids: low coverage results

We perform Monte Carlo simulations of a simple one-patch colloidal model to investigate the cluster formation and the phase behavior of the system on changing the width of the patch. We investigate the parameter region where the coverage (defined as the ratio between attractive and total surface) varies from 50% (the Janus case) to zero (hard-sphere). Simulation results indicate that on decreasing the coverage, particles self-assemble into clusters of different shapes, from micelles close to the Janus case, to one and two dimensional aggregates (wires and lamellae) for smaller coverage. Close to the hard-sphere limit, small micelles and dimers dominate the scene. We never find evidence of a gas–liquid (colloidal-rich/colloidal-poor) phase separation: it confirms that self-assembly into clusters which expose to their neighbors mostly repulsive surfaces suppresses phase separation and stabilizes cluster phases.

Simulation and theory of a model for tetrahedral colloidal particles.

We study the thermodynamic and structural properties of a five-site tetrahedral molecular model by means of different Monte Carlo simulation techniques, and the reference interaction site model (RISM) theory of molecular fluids. Simulations and theory signal the onset, at sufficiently low temperatures, of two different tetrahedral molecular arrangements, with a more open topology progressively giving place to a fully bonded one, as the temperature decreases. The RISM theory reproduces the splitting of the static structure factor at low temperatures, a feature intimately related to the onset of the tetrahedral ordering. Less accurate predictions are obtained for the liquid-vapor coexistence and the short-range correlations.

Cooperative polymerization of one-patch colloids

We numerically investigate cooperative polymerization in an off-lattice model based on a pairwise additive potential using particles with a single attractive patch that covers 30% of the colloid surface. Upon cooling, these particles self-assemble into small clusters which, below a density-dependent temperature, spontaneously reorganize into long straight tubes. We evaluate the partition functions of clusters of all sizes to provide an accurate description of the chemical reaction constants governing this process. Our calculations show that, for intermediate sizes, the partition functions retain contributions from two different structures, differing in both energy and entropy. We illustrate the microscopic mechanism behind the complex polymerization process in this system and provide a detailed evaluation of its thermodynamics.

Reference interaction site model and molecular dynamics study of structure and thermodynamics of methanol

Thermodynamic and structural properties of various models of liquid methanol are investigated in the framework provided by the reference interaction site model (RISM) theory of molecular fluids. The theoretical predictions are systematically compared with molecular dynamics simulations both at ambient conditions and along a few supercritical isotherms. RISM results for the liquid-vapor phase separation are also obtained and assessed against available Gibbs ensemble Monte Carlo data. At ambient conditions, the theoretical correlations weakly depend on the specific details of the molecular models and reproduce the simulation results with different degrees of accuracy, depending on the pair of interaction sites considered. The position and the strength of the hydrogen bond are quite satisfactorily predicted. RISM results for the internal energy are almost quantitative whereas the pressure is generally overestimated. As for the liquid-vapor phase coexistence, RISM predictions for the vapor branch and for the critical temperature are quite accurate; on the other side, the liquid branch densities, and consequently the critical density, are underestimated. We discuss our results in terms of intrinsic limitations, and suitable improvements, of the RISM approach in describing the physical properties of polar fluids, and in the perspective of a more general investigation of mixtures of methanol with nonpolar fluids of specific interest in the physics of associating fluids.

Simulation and reference interaction site model theory of methanol and carbon tetrachloride mixtures.

We report molecular dynamics and reference interaction site model (RISM) theory of methanol and carbon tetrachloride mixtures. Our study encompasses the whole concentration range, by including the pure component limits. We majorly focus on an analysis of partial, total, and concentration-concentration structure factors, and examine in detail the k-->0 limits of these functions. Simulation results confirm the tendency of methanol to self-associate with the formation of ring structures in the high dilution regime of this species, in agreement with experimental studies and with previous simulations by other authors. This behavior emerges as strongly related to the high nonideality of the mixture, a quantitative estimate of which is provided in terms of concentration fluctuation correlations, through the structure factors examined. The interaggregate correlation distance is also thereby estimated. Finally, the compressibility of the mixture is found in good agreement with experimental data. The RISM predictions are throughout assessed against simulation; the theory describes better the apolar solvent than the alcohol properties. Self-association of methanol is qualitatively reproduced, though this trend is much less marked in comparison with simulation results.

Structure and thermodynamics of core-softened models for alcohols

The phase behavior and the fluid structure of coarse-grain models for alcohols are studied by means of reference interaction site model (RISM) theory and Monte Carlo simulations. Specifically, we model ethanol and 1-propanol as linear rigid chains constituted by three (trimers) and four (tetramers) partially fused spheres, respectively. Thermodynamic properties of these models are examined in the RISM context, by employing closed formulæ for the calculation of free energy and pressure. Gas-liquid coexistence curves for trimers and tetramers are reported and compared with already existing data for a dimer model of methanol. Critical temperatures slightly increase with the number of CH2 groups in the chain, while critical pressures and densities decrease. Such a behavior qualitatively reproduces the trend observed in experiments on methanol, ethanol, and 1-propanol and suggests that our coarse-grain models, despite their simplicity, can reproduce the essential features of the phase behavior of such alcohols. The fluid structure of these models is investigated by computing radial distribution function gij(r) and static structure factor Sij(k); the latter shows the presence of a low-k peak at intermediate-high packing fractions and low temperatures, suggesting the presence of aggregates for both trimers and tetramers.

Reference interaction site model and optimized perturbation theories of colloidal dumbbells with increasing anisotropy.

We investigate thermodynamic properties of anisotropic colloidal dumbbells in the frameworks provided by the Reference Interaction Site Model (RISM) theory and an Optimized Perturbation Theory (OPT), this latter based on a fourth-order high-temperature perturbative expansion of the free energy, recently generalized to molecular fluids. Our model is constituted by two identical tangent hard spheres surrounded by square-well attractions with same widths and progressively different depths. Gas-liquid coexistence curves are obtained by predicting pressures, free energies, and chemical potentials. In comparison with previous simulation results, RISM and OPT agree in reproducing the progressive reduction of the gas-liquid phase separation as the anisotropy of the interaction potential becomes more pronounced; in particular, the RISM theory provides reasonable predictions for all coexistence curves, bar the strong anisotropy regime, whereas OPT performs generally less well. Both theories predict a linear dependence of the critical temperature on the interaction strength, reproducing in this way the mean-field behavior observed in simulations; the critical density—that drastically drops as the anisotropy increases—turns to be less accurate. Our results appear as a robust benchmark for further theoretical studies, in support to the simulation approach, of self-assembly in model colloidal systems.

Properties of a soft-core model of methanol: An integral equation theory and computer simulation study

Thermodynamic and structural properties of a coarse-grained model of methanol are examined by Monte Carlo simulations and reference interaction site model (RISM) integral equation theory. Methanol particles are described as dimers formed from an apolar Lennard-Jones sphere, mimicking the methyl group, and a sphere with a core-softened potential as the hydroxyl group. Different closure approximations of the RISM theory are compared and discussed. The liquid structure of methanol is investigated by calculating site-site radial distribution functions and static structure factors for a wide range of temperatures and densities. Results obtained show a good agreement between RISM and Monte Carlo simulations. The phase behavior of methanol is investigated by employing different thermodynamic routes for the calculation of the RISM free energy, drawing gas-liquid coexistence curves that match the simulation data. Preliminary indications for a putative second critical point between two different liquid phases of methanol are also discussed.

On the determination of phase boundaries via thermodynamic integration across coexistence regions

Specialized Monte Carlo methods are nowadays routinely employed, in combination with thermodynamic integration (TI), to locate phase boundaries of classical many-particle systems. This is especially useful for the fluid-solid transition, where a critical point does not exist and both phases may notoriously go deeply metastable. Using the Lennard-Jones model for demonstration, we hereby investigate on the alternate possibility of tracing reasonably accurate transition lines directly by integrating the pressure equation of state computed in a canonical-ensemble simulation with local moves. The recourse to this method would become a necessity when the stable crystal structure is not known. We show that, rather counterintuitively, metastability problems can be alleviated by reducing (rather than increasing) the size of the system. In particular, the location of liquid-vapor coexistence can exactly be predicted by just TI. On the contrary, TI badly fails in the solid-liquid region, where a better assessment (to within 10% accuracy) of the coexistence pressure can be made by following the expansion, until melting, of the defective solid which has previously emerged from the decay of the metastable liquid.Specialized Monte Carlo methods are nowadays routinely employed, in combination with thermodynamic integration (TI), to locate phase boundaries of classical many-particle systems. This is especially useful for the fluid-solid transition, where a critical point does not exist and both phases may notoriously go deeply metastable. Using the Lennard-Jones model for demonstration, we hereby investigate on the alternate possibility of tracing reasonably accurate transition lines directly by integrating the pressure equation of state computed in a canonical-ensemble simulation with local moves. The recourse to this method would become a necessity when the stable crystal structure is not known. We show that, rather counterintuitively, metastability problems can be alleviated by reducing (rather than increasing) the size of the system. In particular, the location of liquid-vapor coexistence can exactly be predicted by just TI. On the contrary, TI badly fails in the solid-liquid region, where a better assessment (to within 10% accuracy) of the coexistence pressure can be made by following the expansion, until melting, of the defective solid which has previously emerged from the decay of the metastable liquid.