Navegando por Autor "Barbosa, Márcia Cristina Bernardes"

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Core-softened fluids, water-like anomalies, and the liquid-liquid critical points.
(2011) Salcedo, Evy; Oliveira, Alan Barros de; Barraz Júnior, Ney Marçal; Chakravarty, Charusita; Barbosa, Márcia Cristina Bernardes
Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.
Density anomaly in a competing interactions lattice gas model.
(2005) Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
Water and other tetrahedral liquids are characterized by a density anomaly whose origin is not well understood. A very simple model of a short-range attraction followed by an outer shell repulsion is proposed as a test potential for the density anomaly. We show that these competing interactions when applied to a two-dimensional lattice gas leads to the formation of two liquid phases and to the appearance of a density anomaly. The coexistence line between the two liquid phases meets a critical line between the fluid and the low-density liquid phase at a tricritical point. The line ofmaximum density emerges in the vicinity of the tricritical point, close to the demixing transition.
Dewetting in associating lattice gas model confined by hydrophobic walls.
(2019) Fonseca, Tássylla Oliveira; Szortyka, Marcia Martins; Dallagnollo, Patricia Ternes; Gavazzoni, Cristina; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
The phase behavior of a two dimensional fluid confined within hydrophobic walls is obtained by Monte Carlo simulations. The fluid is described by the associating lattice gas model which reproduces the density and diffusion anomalous behavior of water. The confined fluid exhibits a liquid-liquid critical temperature which decreases with the decrease of the distance between the confining walls. In contact with the wall a dewetting is observed. The thickness of this interfacial layer is independent of the distance between the two walls. Even for very small distances between the two walls no total depletion is observed and consequently no drying transition is present.
Diffusion behaviour of water confined in deformed carbon nanotubes.
(2019) Mendonça, Bruno Henrique da Silva e; Freitas, Débora Nazaré de; Kohler, Mateus Henrique; Batista, Ronaldo Junio Campos; Barbosa, Márcia Cristina Bernardes; Oliveira, Alan Barros de
We use molecular dynamics simulations to study the diffusion of water inside deformed carbon nanotubes with different degrees of eccentricity at 300 K. We found a water structural transition between tubular-like to single-file for (7,7) nanotubes associated with change from a high to low mobility regimes. Water is frozen when confined in a perfect (9,9) nanotube and it becomes liquid if such a nanotube is deformed above a certain threshold. Water diffusion enhancement (suppression) is related to a reduction (increase) in the number of hydrogen bonds. This suggests that the shape of the nanotube is an important ingredient when considering the dynamical and structural properties of confined water.
Diffusion enhancement in core-softened fluid confined in nanotubes.
(2012) Bordin, José Rafael; Oliveira, Alan Barros de; Diehl, Alexandre; Barbosa, Márcia Cristina Bernardes
We study the effect of confinement in the dynamical behavior of a core-softened fluid. The fluid is modeled as a two length scales potential. This potential in the bulk reproduces the anomalous behavior observed in the density and in the diffusion of liquid water. A series of NpT molecular dynamics simulations for this two length scales fluid confined in a nanotube were performed. We obtain that the diffusion coefficient increases with the increase of the nanotube radius for wide channels as expected for normal fluids. However, for narrow channels, the confinement shows an enhancement in the diffusion coefficient when the nanotube radius decreases. This behavior, observed for water, is explained in the framework of the two length scales potential.
Effects of the attractive interactions in the thermodynamic, dynamic, and structural anomalies of a two length scale potential.
(2010) Silva, Jonathas Nunes da; Salcedo, Evy; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
Using molecular dynamic simulations, we study a system of particles interacting through a continuous core-softened potentials consisting of a hard core, a shoulder at closest distances, and an attractive well at further distance. We obtain the pressure–temperature phase diagram of this system for various depths of the tunable attractive well. Since this is a two length scale potential, density, diffusion, and structural anomalies are expected. We show that the effect of increasing the attractive interaction between the molecules is to shrink the region in pressure in which the density and the diffusion anomalies are present. If the attractive forces are too strong, particle will be predominantly in one of the two length scales and no density of diffusion anomaly is observed. The structural anomalous region is present for all the cases.
Entropy, diffusivity and the energy landscape of a waterlike fluid.
(2010) Oliveira, Alan Barros de; Salcedo, Evy; Chakravarty, Charusita; Barbosa, Márcia Cristina Bernardes
Molecular dynamics simulations and instantaneous normal mode _INM_ analysis of a fluid with core-softened pair interactions and waterlike liquid-state anomalies are performed to obtain an understanding of the relationship between thermodynamics, transport properties, and the potential energy landscape. Rosenfeld scaling of diffusivities with the thermodynamic excess and pair correlation entropy is demonstrated for this model. The INM spectra are shown to carry information about the dynamical consequences of the interplay between length scales characteristic of anomalous fluids, such as bimodality of the real and imaginary branches of the frequency distribution. The INM spectral information is used to partition the liquid entropy into two contributions associated with the real and imaginary frequency modes; only the entropy contribution from the imaginary branch captures the nonmonotonic behavior of the excess entropy and diffusivity in the anomalous regime of the fluid.
Interplay between structure and density anomaly for an isotropic core-softened ramp-like potential.
(2007) Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes; Netz, Paulo Augusto
Using molecular dynamics simulations and integral equations wee investigate the structure, the thermodynamics, and the dynamics of a system of particles interacting through a continuous core-softened ramp-like interparticle potential. We found density, dynamic and structural anomalies similar to that found in water. Analysis of the radial distribution function for several temperatures at fixed densities shows a pattern that may be related to the origin of density anomaly.
Modelos efetivos para separação solvente-soluto através de nanoestruturas : teoria e simulações computacionais.
(2016) Vasconcelos, Cláudia Karina Barbosa de; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes; Barboza, Ana Paula Moreira; Chacham, Helio; Matos, Matheus Josué de Souza
A escassez de água potável nos mais diversos países tem se mostrado um problema cada vez mais relevante. Estima-se que, atualmente, aproximadamente 748 milhões de pessoas no planeta não têm acesso à fontes de água potável. Sob esse aspecto, a dessalinização de água do mar tem se mostrado uma alternativa promissora, não apenas por 97% da água do planeta estar concentrada nos oceanos e mares mas também pelo fato do percentual de água obtido desta maneira ainda ser muito pequeno. Em especial, o processo de dessalinização da água através de nanoporos tem recebido grandes esforços científicos e tecnológicos. Apesar dos resultados animadores, um dos problemas encontrados é que o processo de dessalinização de água é tipicamente macro. Mesmo com o poder computacional disponível atualmente, é impossível simular este problema em escala-macro. Daí a importância de se procurar modelos mais simples e computacionalmente mais baratos. Nesse sentido, uma alternativa eficaz para a simulação computacional de fluidos complexos é a utilização dos potenciais efetivos do tipo caroço atenuado (core-softened potentials). Estes potenciais tem sido utilizados para investigar fluidos anômalos dando bons resultados na descrição de propriedades dinâmicas, termodinâmicas e estruturais desses fluidos. Devido à sua simplicidade, as simulações se tornam mais rápidas e tratamentos matemáticos se tornam possíveis. Baseado nesses potenciais, este trabalho propõe um modelo para a separação soluto solvente a partir de osmose reversa. O comportamento das partículas filtradas de solvente e as propriedades, tais como rejeição de soluto e fluxo volumétrico de solvente, foram comparadas com resultados prévios apresentados na literatura para modelos moleculares clássicos. O objetivo deste trabalho é o de dar os primeiros passos para o desenvolvimento de um modelo coarse-grained de dessalinização de água, onde a abordagem de problemas em escalas maiores, tanto em tempo quanto em tamanho, possam ser viáveis computacionalmente.
Simulação computacional da difusão da água confinada em Nanotubos de Carbono Deformados.
(2017) Mendonça, Bruno Henrique da Silva e; Oliveira, Alan Barros de; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes; Batista, Ronaldo Junio Campos
Water diffusion in carbon nanotubes : interplay between confinement, surface deformation, and temperature.
(2020) Mendonça, Bruno Henrique da Silva e; Ternes, Patricia; Salcedo, Evy; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
In this article, we investigate, through molecular dynamics simulations, the diffusion behavior of the TIP4P/2005 water confined in pristine and deformed carbon nanotubes (armchair and zigzag). To analyze different diffusive mechanisms, the water temperature was varied as 210 ≤ T ≤ 380 K. The results of our simulations reveal that water presents a non-Arrhenius to Arrhenius diffusion crossover. The confinement shifts the diffusion transition to higher temperatures when compared with the bulk system. In addition, for narrower nanotubes, water diffuses in a single line, which leads to its mobility independent of the activation energy.
Water diffusion in carbon nanotubes for rigid and flexible models.
(2023) Mendonça, Bruno Henrique da Silva e; Moraes, Elizane Efigenia de; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes; Chacham, Helio
We compared the diffusion of water confined in armchair and zigzag carbon nanotubes for rigid and flexible water models. Using one rigid model, TIP4P/2005, and two flexible models, SPC/Fw and SPC/FH, we found that the number of the hydrogen bonds that water forms depends on the structure of the nanotube, directly affecting the diffusion of water. The simulation results reveal that, due to the hydrophobic nature of carbon nanotubes and the degrees of freedom imposed by the water force fields, water molecules tend to avoid the surface of the carbon nanotube. This junction of variables plays a central role in the diffusion of water, mainly in narrow and/or deformed nanotubes, governing the mobility of confined water in a nontrivial way, where the greater the degree of freedom of the water force field, the smaller mobility it will have in confinement as we limit the competition between area and volume and it no longer plays the unique role in changing water diffusivity.
Water diffusion in carbon nanotubes under directional electric frields : coupling between mobility and hydrogen bonding.
(2020) Freitas, Débora Nazaré de; Mendonça, Bruno Henrique da Silva e; Kohler, Mateus Henrique; Barbosa, Márcia Cristina Bernardes; Matos, Matheus Josué de Souza; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de
Molecular Dynamics simulations of water confined in carbon nanotubes subjected to external electric fields show that water mobility strongly depends on the confining geometry, the intensity and directionality of the electric field. While fields forming angles of 0° and 45° slow down the water dynamics by increasing organization, perpendicular fields can enhance water diffusion by decreasing hydrogen bond formation. For 1.2 diameter long nanotubes, the parallel field destroys the ice-like water structure increasing mobility. These results indicate that the structure and dynamics of confined water are extremely sensitive to external fields and can be used to facilitate filtration processes.
Water diffusion in rough carbon nanotubes.
(2020) Mendonça, Bruno Henrique da Silva e; Ternes, Patricia; Salcedo, Evy; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
We use molecular dynamics simulations to study the diffusion of water inside deformed carbon nanotubes with different degrees of deformation at 300 K. We found that the number of hydrogen bonds that water forms depends on nanotube topology, leading to enhancement or suppression of water diffusion. The simulation results reveal that more realistic nanotubes should be considered to understand the confined water diffusion behavior, at least for the narrowest nanotubes, when the interaction between water molecules and carbon atoms is relevant.