arXiv subjects

Silvina M. Gatica

Publications and source records attributed to Silvina M. Gatica.

17 recordsLinked to original sources

arXiv2020

Selective adsorption of Carbon Dioxide from Mixed Vapors by Blockage of Methane in Graphene Nanoribbons

We study numerically the adsorption of a mixture of CO$_2$ and CH$_4$ on a graphite substrate covered by graphene nanoribbons (NRs). The NRs are flat and parallel to the graphite surface, at a variable distance ranging from 6 Å to 14 Å. We show that the NRs-graphite substrate acts as an effective filter for CO$_2$. Our study is based on Molecular Dynamics (MD) simulations. Methane is considered a spherical molecule, and carbon dioxide is represented as a linear rigid body. Graphite is modeled as a continuous material, while the NRs are approached atomistically. We observe that when the NRs are placed 6 Å above the graphite surface, methane is blocked out, while CO$_2$ molecules can diffuse and be collected in between the NRs and the graphite surface. Consequently, the selectivity of CO$_2$ is extremely high. We also observe that the initial rate of adsorption of CO$_2$ is much higher than CH$_4$. Overall we show that the filter can be optimized by controlling the gap between NRs and the NRs-graphite separation.

cond-mat.mtrl-sci↗

arXiv2016

Monolayer Adsorption of Noble Gases on Graphene

We report our results on the adsorption of noble gases such as argon, krypton and xenon on a graphene sheet, using Grand Canonical Monte Carlo (GCMC) simulations. We calculated the two-dimensional gas-liquid critical temperature for each adsorbate, resulting in fair agreement with theoretical predictions and experimental values of gases on graphite. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton.

cond-mat.mtrl-sci↗

arXiv2010

Solid phase of Krypton on the exterior of individual single-walled carbon nanotubes

We have computed the adsorption of Krypton in a closed single-walled carbon nanotube using the method of Grand Canonical Monte Carlo. Our results indicate evidence of an incommensurate solid formed at high pressure and low temperature (T<85K), before the formation of a second layer. The solid melts above that temperature. Our simulations are in good agreement with novel experimental results for adsorption in individual carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2009

Quasi-one dimensional fluids that exhibit higher dimensional behavior

Fluids confined within narrow channels exhibit a variety of phases and phase transitions associated with their reduced dimensionality. In this review paper, we illustrate the crossover from quasi-one dimensional to higher effective dimensionality behavior of fluids adsorbed within different carbon nanotubes geometries. In the single nanotube geometry, no phase transitions can occur at finite temperature. Instead, we identify a crossover from a quasi-one dimensional to a two dimensional behavior of the adsorbate. In bundles of nanotubes, phase transitions at finite temperature arise from the transverse coupling of interactions between channels.

cond-mat.stat-mech↗

arXiv2009

To wet or not to wet: that is the question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H2 molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, water, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential's well-depth D is smaller than, or comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At the wetting temperature, Tw, the transition to wetting occurs, for entropic reasons, when the liquid's surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid- surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the "simple model", which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.

cond-mat.mtrl-sci↗

arXiv2008

Effects of substrate relaxation on adsorption in pores

Fluids in porous media are commonly studied with analytical or simulation methods, usually assuming that the host medium is rigid. By evaluating the substrate's response (relaxation) to the presence of the fluid we assess the error inherent in that assumption. One application is a determination of the ground state of 3He in slit and cylindrical pores. With the relaxation, there results a much stronger cohesion than would be found for a rigid host. Similar increased binding effects of relaxation are found for classical fluids confined within slit pores or nanotube bundles.

cond-mat.mtrl-sci↗

arXiv2006

Thermodynamic properties and correlation functions of Ar films on the surface of a bundle of nanotubes

We employ canonical Monte Carlo simulations to explore the properties of an Ar film adsorbed on the external surface of a bundle of carbon nanotubes. The study is concerned primarily with three properties: specific heat $c(T)$, differential heat of adsorption $q_d$, and Ar-Ar correlation functions $g(\mathbf{r})$. These measurable functions exhibit information about the dependence of film structure on coverage and temperature.

cond-mat.soft↗

arXiv2005

Capillary condensation in cylindrical nanopores

Using grand canonical Monte Carlo simulations, we have explored the phenomenon of capillary condensation (CC) of Ar at the triple temperature inside infinitely long, cylindrical pores. Pores of radius R= 1 nm, 1.7 nm and 2.5 nm have been investigated, using a gas-surface interaction potential parameterized by the well-depth D of the gas on a planar surface made of the same material as that comprising the porous host. For strongly attractive situations, i.e., large D, one or more (depending on R) Ar layers adsorb successively before liquid fills the pore. For very small values of D, in contrast, negligible adsorption occurs at any pressure P below saturated vapor pressure P0; above saturation, there eventually occurs a threshold value of P at which the coverage jumps from empty to full, nearly discontinuously. Hysteresis is found to occur in the simulation data whenever abrupt CC occurs, i.e. for R>= 1.7 nm, and for small D when R=1nm. Then, the pore-emptying branch of the adsorption isotherm exhibits larger N than the pore-filling branch, as is known from many experiments and simulation studies. The relation between CC and wetting on planar surfaces is discussed in terms of a threshold value of D, which is about one-half of the value found for the wetting threshold on a planar surface. This finding is consistent with a simple thermodynamic model of the wetting transition developed previously.

cond-mat.mtrl-sci↗

arXiv2003

Intriguing examples of inhomogeneous broadening

Three problems are considered in which inhomogeneous broadening can yield unusual consequences. One problem involves the energy levels of atoms moving within nanopores of nearly cylindrical cross section. A second involves atomic or molecular motion in a quasi-one dimensional interstitial channel within a bundle of carbon nanotubes. The third problem involves motion within a groove between two nanotubes at the surface of such a bundle. In each case, the density of states at low energy is qualitatively different from that occurring in the perfectly homogeneous case.

cond-mat.mtrl-sci↗

arXiv2003

Universal anisotropic condensation transition of gases in nanotube bundles

Gases adsorbed within bundles of carbon nanotubes (inside of the nanotubes or in the interstitial channels between the tubes) exhibit a variety of phase transitions with the help of interactions between molecules in neighboring channels or tubes. Because the channels/tubes are widely separated, these transverse interactions are weaker than the (longitudinal) interactions within the same channel. The transition temperatures that result are therefore lower than those of typical two- or three-dimensional transitions of the same species of molecules. We discuss here the condensation transition of such a gas to form a liquid, expressing the transition behavior in universal form, where the reduced critical temperature T_c* is a universal function of the reduced transverse interaction.

cond-mat.soft↗

arXiv2003

Bose-Einstein Condensation of Molecular Hydrogen in Nanotube Bundles

We evaluate the effects of heterogeneity on the density of states of H$_2$ molecules inside interstitial channels within bundles of carbon nanotubes. As temperature (T) falls, the density increases within those tubes having the greatest binding energy. At T ~ 10 mK, the molecules undergo Bose-Einstein condensation, exhibiting a singular heat capacity.

cond-mat.soft↗

arXiv2003

Three-body interactions involving clusters and films

The three body (triple dipole) interaction of Axilrod, Teller and Muto (ATM) contributes 5 to 10 % of the total energy of condensed phases of inert elements. It is shown in this paper for clusters and films that a much larger or smaller ATM contribution can arise for other geometries or other atomic species. The ratio R of the three body interaction energy to the two body energy is evaluated for a wide variety of configurations. This ratio varies considerably with the geometry. For highly polarizable atoms in certain geometries, the magnitude of the three body energy is comparable to that of the two body energy and can be either attractive or repulsive. Systematic trends are established and explained.

cond-mat.soft↗

arXiv2003

Peas in a pod: quasi-one-dimensional C60 molecules in a nanotube

We evaluate the equation of state of the quasi-one-dimensional (1D) phase of C60 molecules in small carbon nanotubes, nicknamed ``peas in a pod''. The chemical potential and 1D pressure are evaluated as functions of the temperature and density, initially with the approximation of nearest neighbor interactions and classical statistical mechanics. Quantum corrections and long-range interaction corrections are discussed, as are the effects of interactions with neighboring peapods. Transition phenomena involving the 3D coupling are evaluated.

cond-mat.soft↗

arXiv2002

Ground state and thermal properties of a lattice gas on a cylindrical surface

Adsorbed gases within, or outside of, carbon nanotubes may be analyzed with an approximate model of adsorption on lattice sites situated on a cylindrical surface. Using this model, the ground state energies of alternative lattice structures are calculated, assuming Lennard-Jones pair interactions between the particles. The resulting energy and equilibrium structure are nonanalytic functions of radius (R) because of commensuration effects associated with the cylindrical geometry. Specifically, as R varies, structural transitions occur between configurations differing in the "ring number", defined as the number of atoms located at a common value of the longitudinal coordinate (z). The thermodynamic behavior of this system is evaluated at finite temperatures, using a Hamiltonian with nearest-neighbor interactions. The resulting specific heat bears a qualitative resemblance to that of the one-dimensional Ising model.

cond-mat.soft↗

arXiv2001

Condensed phases of gases inside nanotube bundles

An overview is presented of the various phases predicted to occur when gases are absorbed within a bundle of carbon nanotubes. The behavior may be characterized by an effective dimensionality, which depends on the species and the temperature. Small molecules are strongly attracted to the interstitial channels between tubes. There, they undergo transitions between ordered and disordered quasi-one dimensional (1D) phases. Both small and large molecules display 1D and /or 2D phase behavior when adsorbed within the nanotubes, depending on the species and thermodynamic conditions. Finally, molecules adsorbed on the external surface of the bundle exhibit 1D behavior (striped phases), which crosses over to 2D behavior (monolayer film) and eventually 3D behavior (thick film) as the coverage is increased. The various phases exhibit a wide variety of thermal and other properties that we discuss here.

cond-mat.soft↗

arXiv1999

Helium mixtures in nanotube bundles

An analogue to Raoult's law is determined for the case of a 3He-4He mixture adsorbed in the interstitial channels of a bundle of carbon nanotubes. Unlike the case of He mixtures in other environments, the ratio of the partial pressures of the coexisting vapor is found to be a simple function of the ratio of concentrations within the nanotube bundle.

cond-mat.stat-mech↗

arXiv1999

Atoms in nanotubes: small dimensions and variable dimensionality

Newly discovered carbon nanotubes provide an environment in which small atoms move relatively freely. An assembly of such atoms provides a realization of a quasi-one dimensional system which is an ideal testing ground for concepts and mathematics of statistical physics.

cond-mat.stat-mech↗