Structure and phase behavior of colloidal dumbbells with tunable attractive interactions

We investigate thermodynamic and structural properties of colloidal dumbbells in the framework provided by the Reference Interaction Site Model (RISM) theory of molecular fluids and Monte Carlo simulations. We consider two different models: in the first one we set identical square-well attractions on the two tangent spheres composing the molecule (SW-SW model); in the second scheme, one of square-well interactions is switched off (HS-SW model). Appreciable differences emerge between the physical properties of the two models. Specifically, the $k \to 0$ behavior of SW-SW structure factors $S(k)$ points to the presence of a gas-liquid coexistence, as confirmed by subsequent fluid phase equilibria calculations. Conversely, the HS-SW $S(k)$ develops a low-$k$ peak, signaling the presence of aggregates; such a process destabilizes the gas-liquid phase separation, promoting at low temperatures the formation of a cluster phase, whose structure depends on the system density. We further investigate such differences by studying the phase behavior of a series of intermediate models, obtained from the original SW-SW by progressively reducing the depth of one square-well interaction. RISM structural predictions positively reproduce the simulation data, including the rise of $S(k \to 0$) in the SW-SW model and the low-$k$ peak in the HS-SW structure factor. As for the phase behavior, RISM agrees with Monte Carlo simulations in predicting a gas-liquid coexistence for the SW-SW model (though the critical parameters appears overestimated by the theory) and its progressive disappearance moving toward the HS-SW model.Comment: 12 pages, 13 figures, 1 table, 78 reference

Generation of well relaxed all atom models of stereoregular polymers: a validation of hybrid particle-field molecular dynamics for polypropylene melts of different tacticities

The tacticity of vinyl polymer chains strongly affects the physical properties of polymeric materials, as example the chain conformations and stiffness. In the present work we tested how the hybrid Particle-Field Molecular Dynamics (PF-MD) technique is capable to describe conformational differences of polymer chains as function of the tacticity. In particular, we focus on tacticity effect of atactic, isotactic, and syndiotactic polypropylene (PP) homopolymer melts. We found that PF-MD simulations exhibit dependence of Flory's Characteristic Ratio from the fraction of racemo diads along the PP chains in qualitative agreement with Small Angle Neutron Scattering (SANS) experiments and theoretical previsions. Finally, we calculated and compared the packing length parameter on very high stereoregular syndiotactic PP systems with rheological measurements. A qualitative agreement between the calculated and experimental packing length is found.Solid state NMR/Biophysical Organic Chemistr

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. Munao 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

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. (C) 2011 American Institute of Physics. [doi:10.1063/1.3582904

Effect of the ligand's bulkiness on the shape of functionalized gold nanoparticles in aqueous solutions: A molecular dynamics study

The functionalized gold nanoparticles (FGNPs) behaviour in biological environments is completely altered by the presence of protein corona. Avoid its formation is required to design NP surface to specific targets. In the present work we investigated, by using atomistic Molecular Dynamics (MD) simulations, the interfacial structural properties of a FGNP in physiological solutions. The behaviour of three zwitterionic sulfobetaine, proved to avoid protein corona formation, is compared with pegylated one. MD simulations describe a change in shape, named “chain blossom” as function of the bulkiness and hydrophobicity of sulfobetaine headgroup and its effect on the FNGP surface properties is discussed

Aortic endograft and bridging stent-graft remodeling after branched endovascular aortic repair

Objectives: The results of branched endovascular repair of thoracoabdominal aneurysms are mainly dependent on durability of the graft used. The purpose of this study was to evaluate postoperative aortic main body and bridging stent-graft remodeling, and their impact on bridging stent-graft instability at one year. Methods: Computed tomoangiographies of 43 patients (43 aortic main body mated with 171 bridging stent-grafts) were analyzed before and after branched endovascular repair as well as after a follow-up of 12 months. Primary endpoint was aortic main body remodeling (migration >5 mm, shortening >5 mm, scoliosis >5° or lordosis >5°). Shortening was defined as a reduced length in the long axis, scoliosis as left-right curvature, and lordosis as antero-posterior curvature. Aortic main body remodeling, aneurysm sac changes, and bridging stent-graft tortuosity were evaluated to study their correlations and the impact on the bridging stent-graft instability. Results: At 12 months, aortic main body remodeling was observed in 72% of the cases, migration in 39.5% (mean 5.21 mm), shortening in 41.9% (mean 5.79 mm), scoliosis in 58.1%, (mean 10.10°), lordosis in 44.2% (mean 5.78°). Migration, shortening, and scoliosis were more frequent in patients with larger aneurysms (p =.005), while scoliosis was significantly more frequent in type II thoracoabdominal aneurysm (p =.019). Aortic main body remodeling was significantly associated to bridging stent-graft remodeling (r: 0.3–0.48). The bridging stent-graft instability rate was 9.3%. Despite a trend toward significance (p =.07), none of the evaluated aortic main body and bridging stent-graft changes were associated with bridging stent-graft instability at 12 months. Conclusions: Aortic main body remodeling is frequent especially in large and extended thoracoabdominal aneurysm aneurysms. Aortic main body and bridging stent-graft remodeling was significantly correlated. While these geometric changes had no significant impact on bridging stent-graft instability at one year, a close long-term follow-up after branched endovascular repair could predict bridging stent-graft failures

Aortic endograft and bridging stent-graft remodeling after branched endovascular aortic repair

objectives: the results of branched endovascular repair of thoracoabdominal aneurysms are mainly dependent on durability of the graft used. the purpose of this study was to evaluate postoperative aortic main body and bridging stent-graft remodeling, and their impact on bridging stent-graft instability at one year. methods: computed tomoangiographies of 43 patients (43 aortic main body mated with 171 bridging stent-grafts) were analyzed before and after branched endovascular repair as well as after a follow-up of 12 months. primary endpoint was aortic main body remodeling (migration >5 mm, shortening >5 mm, scoliosis >5° or lordosis >5°). shortening was defined as a reduced length in the long axis, scoliosis as left-right curvature, and lordosis as antero-posterior curvature. aortic main body remodeling, aneurysm sac changes, and bridging stent-graft tortuosity were evaluated to study their correlations and the impact on the bridging stent-graft instability. results: At 12 months, aortic main body remodeling was observed in 72% of the cases, migration in 39.5% (mean 5.21 mm), shortening in 41.9% (mean 5.79 mm), scoliosis in 58.1%, (mean 10.10°), lordosis in 44.2% (mean 5.78°). migration, shortening, and scoliosis were more frequent in patients with larger aneurysms (p = .005), while scoliosis was significantly more frequent in type II thoracoabdominal aneurysm (p = .019). aortic main body remodeling was significantly associated to bridging stent-graft remodeling (r: 0.3-0.48). the bridging stent-graft instability rate was 9.3%. despite a trend toward significance (p = .07), none of the evaluated aortic main body and bridging stent-graft changes were associated with bridging stent-graft instability at 12 months. conclusions: aortic main body remodeling is frequent especially in large and extended thoracoabdominal aneurysm aneurysms. aortic main body and bridging stent-graft remodeling was significantly correlated. While these geometric changes had no significant impact on bridging stent-graft instability at one year, a close long-term follow-up after branched endovascular repair could predict bridging stent-graft failures

Messina. Dal “degrado pianificato” delle periferie alla “periferizzazione” della città

Il saggio riassume il processo di costruzione e il progressivo degrado delle periferie messinesi, sottolineando come il disagio sociale ed il degrado urbanistico ed edilizio piuttosto che essere un effetto imprevisto e perverso della costruzione della città, sia un esito (prevedibile e scontato) di errate politiche urbane, scarsa rilevanza attribuita agli strumenti urbanistici ed una ferma volontà politica di non governare i processi territoriali

Molecular structure and multi-body potential of mean force in silica-polystyrene nanocomposites

We perform a systematic application of the hybrid particle-field molecular dynamics technique [Milano, et al., J. Chem. Phys., 2009, 130, 214106] to study interfacial properties and potential of mean force (PMF) for separating nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or grafted with Polystyrene chains, aiming to shed light on the interactions among free and grafted chains affecting the dispersion of NPs in the nanocomposite. The proposed hybrid models show good performances in catching the local structure of the chains, and in particular their density profiles, documenting the existence of the "wet-brush-to-dry-brush" transition. By using these models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene matrix are calculated. Moreover, we estimate the three-particle contribution to the total PMF and its role in regulating the phase separation on the nanometer scale. In particular, the multi-particle contribution to the PMF is able to give an explanation of the complex experimental morphologies observed at low grafting densities. More in general, we propose this approach and the models utilized here for a molecular understanding of specific systems and the impact of the chemical nature of the systems on the composite final properties