arXiv subjects

Murilo S. Marques

Publications and source records attributed to Murilo S. Marques.

7 recordsLinked to original sources

arXiv2026

Amorphous Radial Frustration and Water-Like Anomalies in a Ramp-Shoulder Fluid

We investigate the thermodynamic, structural, and dynamic behavior of a three-dimensional coarse-grained ramp-shoulder fluid derived from effective interactions between polymer-grafted nanoparticles. The interaction combines a softened repulsive ramp with a shallow attractive shoulder, stabilizing competing local organizations over a broad pressure interval. Molecular dynamics simulations reveal density, diffusion, and structural anomalies together with crystalline, amorphous, and fluid regions in the phase diagram. Unlike conventional isotropic core-softened fluids, the anomalous hierarchy becomes partially decoupled: the density anomaly extends beyond the structural anomaly, while the diffusion anomaly becomes closely connected to amorphization and shell migration processes. Analysis of radial distribution functions, excess entropy, translational and orientational order, and coordination-shell organization shows that the anomalies are not controlled solely by shell competition. Instead, they emerge from cooperative radial restructuring in a regime where radial correlations increase without the development of crystalline orientational order. The results indicate that the detailed shape of the softened interaction region strongly influences the structural pathways explored under compression, leading to a regime of amorphous radial frustration associated with anomalous diffusion and frustrated shell reorganization.

cond-mat.soft↗

arXiv2021

Phase classification using neural networks: application to supercooled, polymorphic core-softened mixtures

Characterization of phases of soft matter systems is a challenge faced in many physicochemical problems. For polymorphic fluids it is an even greater challenge. Specifically, glass forming fluids, as water, can have, besides solid polymorphism, more than one liquid and glassy phases, and even a liquid-liquid critical point. In this sense, we apply a neural network (NN) algorithm to analyze the phase behavior of a core-softened mixture of core-softened CSW fluids that have liquid polymorphism and liquid-liquid critical points, similar to water. We also apply the NN to mixtures of CSW fluids and core-softened alcohols models. We combine and expand two methods based on bond-orientational order parameters to study mixtures, applied to mixtures of hardcore fluids by Boattini and co-authors [Molecular Physics 116, 3066-3075 (2018)] and to supercooled water by Martelli and co-authors [The Journal of Chemical Physics 153, 104503 (2020)], to include longer range coordination shells. With this, the trained neural network (NN) was able to properly predict the crystalline solid phases, the fluid phases and the amorphous phase for the pure CSW and CSW-alcohols mixtures with high efficiency. More than this, information about the phase populations, obtained from the NN approach, can help verify if the phase transition is continuous or discontinuous, and also to interpret how the metastable amorphous region spreads along the stable high density fluid phase. These findings help to understand the behavior of supercooled polymorphic fluids and extend the comprehension of how amphiphilic solutes affect the phases behavior.

cond-mat.soft↗

arXiv2021

Core-softened water-alcohol mixtures: the solute-size effects

In a recent work [\textit{J. Mol. Liq}, 2020, \textbf{320}, 114420], using molecular dynamics simulations and a core-softened potential approach, we have shown that adding a simple solute as methanol can "kill" the density anomalous behavior as the LLCP is suppressed by the spontaneous crystallization in a hexagonal closed packing (HCP) crystal near the LLPT. Now, we extend this work in order to realize how longer-chain alcohols will affect the complex behavior of water-alcohol mixtures in the supercooled regime. Besides core-softened (CS) methanol, ethanol and 1-propanol were added to a system of identical particles that interact through the continuous shouldered well (CSW) potential. We observed that the density anomaly gradually decreases its extension in phase diagrams until disappearing with the growth of the non-polar chain and the alcohol concentration, differently from the liquid-liquid phase transition (and the LLCP), which remained present in all analyzed mixtures, in according to \textit{Nature}, 2001, \textbf{409}, 692. For our model, the longer non-polar chains and higher concentrations gradually impact the competition between the scales in the CS potential, leading to a gradual disappearing of the anomalies until the TMD total disappearance is observed when the first coordination shell structure is also affected: the short-range ordering is favored, leading to less competition between short- and long-range ordering and, consequently, to the extinction of anomalies. Also, the non-polar chain size and concentration have an effect on the solid phases, favoring the hexagonal closed packed (HCP) solid and the amorphous solid phase over the body-centered cubic (BCC) crystal.

cond-mat.soft↗

arXiv2021

Hard core-soft shell particles near repulsive interfaces: interplay between adsorption, aggregation and diffusion

The behavior of colloidal particles with a hard core and a soft shell has attracted the attention for researchers in the physical-chemistry interface not only due the large number of applications, but due the unique properties of these systems in bulk and at interfaces. The adsorption at the boundary of two phases can provide information about the molecular arrangement. In this way, we perform Langevin Dynamics simulations of polymer-grafted nanoparticles. We employed a recently obtained core-softened potential to analyze the relation between adsorption, structure and dynamic properties of the nanoparticles near a solid repulsive surface. Two cases were considered: flat or structured walls. At low temperatures, a maxima is observed in the adsorption. It is related to a fluid to clusters transition and with a minima in the contact layer diffusion - and is explained by the competition between the scales in the core-softened interaction. Due the long range repulsion, the particles stay at the distance correspondent to this length scale at low densities, and overcome the repulsive barrier as the packing increases, However, increasing the temperature, the gain in kinetic energy allows the colloids to overcome the long range repulsion barrier even at low densities. As consequence, there is no competition and no maxima was observed in the adsorption.

cond-mat.soft↗

arXiv2020

Competing interactions near the liquid-liquid phase transition of core-softened water/methanol mixtures

Water is an unique material with a long list of thermodynamic, dynamic and structural anomalies, which are usually attributed to the competition between two characteristic length scales in the intermolecular interaction. It has been argued that a potential liquid-liquid phase transition (LLPT) ending at a liquid-liquid critical point (LLCP) lies at the core of the anomalous behavior of water. This transition which has been evidenced in multiple simulation studies seems to be preempted experimentally by spontaneous crystallization. Here, in order to expose the connection between the spontaneous crystallization observed in the supercooled regime in the vicinity of the LLPT, and the density anomaly, we perform extensive Molecular Dynamics simulations of a model mixture of core-softened water and methanol. The pure water-like fluid exhibits a LLPT and a density anomaly. In contrast, our pure methanol-like model does have a LLPT but lacks the density anomaly. Our results illustrate the relation between the vanishing of the density anomaly and an increase in the temperature of the spontaneous crystallization: once this temperature surpasses the LLCP critical temperature, no density anomaly is observed. This peculiar feature illustrates how fine tuning the competitive interactions determine the anomalous behavior of water/alcohol mixtures.

cond-mat.stat-mech↗

arXiv2020

Modeling the temperature of maximum density of aqueous tert-butanol solutions

Short-chain alcohols at high dilution are among the very few solutes that enhance the anomalous behavior of water, in particular the value of the temperature of maximum density. This peculiar feature, first discovered experimentally in the early sixties, has remained elusive to a full explanation in terms of atomistic models. In this paper, we first introduce a two-site model of tert-butanol in which the interactions involving hydrogen bonding are represented by a Stillinger-Weber potential, following the ideas first introduced by Molinero and Moore, [J. Phys. Chem. B, 113, 4008, (2009)]. Our model parameters are fit so as to semi-quantitatively reproduce the experimental densities and vaporization enthalpies of previously proposed united atom and all atom OPLS models. Water is represented using the aforementioned potential model introduced by Molinero and Moore, with cross interaction parameters between water and tert-butanol optimized to yield a reasonable description of the experimental excess enthalpies and volumes over the whole composition range of the mixture. We will see that our simple model is able to reproduce the presence of a maximum in the change of the temperature of maximum density for very low alcohol mole fractions, followed by a considerable decrease until the density anomaly itself disappears. We have correlated this behavior with changes in the local structure of water and compared it with the results of all-atom simulations of water/tert-butanol mixtures.

cond-mat.soft↗

arXiv2019

Waterlike anomalies in hard core-soft shell nanoparticles using a effective potential approach: pinned vs adsorbed polymers

In this work, a two dimensional system of polymer grafted nanoparticles is analyzed using large-scale Langevin Dymanics simulations. Effective core-softened potentials were obtained for two cases: one where the polymers are free to rotate around the nanoparticle core and a second where the polymers are fixed, with a $45^\circ$ angle between them. The use of effective core-softened potentials allow us to explore the complete system phase space. In this way, the $PT$, $Tρ$ and $Pρ$ phase diagrams for each potential were obtained, with all fluid and solid phases. The phase boundaries were defined analyzing the specific heat at constant pressure, the system mean square displacement, the radial distribution function and the discontinuities in the density-pressure phase diagram. Also, due the competition in the system we have observed the presence of waterlike anomalies, such as the temperature of maximum density - in addition with a tendency of the TMD to move to lower temperatures (negative slope)- and the diffusion anomaly. It was observed different morphologies (stripes, honeycomb, amorphous) for each nanoparticle. We observed that for the fixed polymers case the waterlike anomalies are originated by the competition between the potential characteristic length scales, while for the free to rotate case the anomalies arises due a smaller region of stability in the phase diagram and no competition between the scales was observed.

cond-mat.soft↗