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ABSTRACT: The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.
Physical Chemistry Chemical Physics 06/2012; 14(30):10799-808. · 3.57 Impact Factor
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ABSTRACT: A series of results from computer simulations of sodium chloride ionic solutions at both supercritical and ambient conditions
are presented. We considered infinite dilute and finite concentration solutions (m=1, 2, 4 molkg−1) at variable densities. Structure of water around ionic species is carefully analyzed. Special attention is devoted to the
effects of ion pairing and clustering. Running coordination numbers and residence times of water molecules are also reported
Theoretical Chemistry Accounts 04/2012; 115(2):161-169. · 2.16 Impact Factor
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ABSTRACT: Wetting of a single hexagonal boron nitride sheet by liquid water has been investigated by molecular dynamics simulations within a temperature range between 278 and 373 K. The wetting temperature was found to be ~310 K, while the onset of prewetting happens around the much higher temperature of 354 K. The static (hydrogen-bond populations, density profiles, energy per molecule) and dynamic (diffusion coefficients) properties of water in the stable phases in this temperature range were also studied and compared to those of water on graphene. The results indicate that hydrophobicity of boron nitride is milder than that of graphene.
Physical Review E 07/2011; 84(1 Pt 1):011602. · 2.26 Impact Factor
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ABSTRACT: In this paper, we summarize the main results obtained in our group about the behavior of water confined inside or close to different graphene surfaces by means of molecular dynamics simulations. These include the inside and outside of carbon nanotubes, and the confinement inside a slit pore or a single graphene sheet. We paid special attention to some thermodynamical (binding energies), structural (hydrogen-bond distributions) and dynamic (infrared spectra) properties, and their comparison to their bulk counterparts.
Journal of Physics Condensed Matter 07/2010; 22(28):284111. · 2.55 Impact Factor
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ABSTRACT: The study of NaCl solutions in water at finite concentration, explicitly including polarization in water molecules and ions, has been carried out by molecular dynamics simulations. A comparison of the RPOL polarizable model with the rigid SPC/E potential for water has been included. Structure obtained with the two models does not show significant differences, although some deviations in the NaNa radial distribution functions at all concentrations are observed. Dielectric properties such as total and molecular dipole moment correlation functions revealed decay times of the order of 10 ps, roughly independent of concentration. The analysis of electric conductivity by means of current-current correlation functions also included the calculation of cross terms corresponding to dipole moment-current correlations, which proved to be non-neglectable at short times and especially relevant at high concentrations (m=4 mol kg(-1)). Frequency dependent dielectric constants and conductivities have been computed and the role of cross correlations has been analyzed. In all cases both concentration and cross correlations have significant influence in the results.
The Journal of chemical physics 06/2010; 132(21):214505. · 3.09 Impact Factor
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ABSTRACT: We studied the effects that deviations from a perfectly flat structure have in the adsorption of water on a single graphene sheet at room temperature. To do so we performed molecular dynamics simulations of water on top of a single sheet of carbon atoms whose positions were changed periodically or randomly and the results compared with those obtained for an ideal flat surface. Different textures were considered, and in all cases, the corresponding geometries of the single graphene layers were kept frozen through the simulations. Our results indicate that the effect of the roughness in the water molecules depends basically on the average amplitude of the distortions in the Z direction and not of their particular type (random or periodic). Binding energies and water structure are scarcely affected by corrugation, with an average number of 3.5-3 hydrogen bonds per molecule through the distance perpendicular to the surface. Analysis of water translational diffusion and spectroscopical densities of states indicates changes when water is close to the vacuum interface and far from the graphene surface.
The Journal of Physical Chemistry B 03/2010; 114(13):4583-9. · 3.70 Impact Factor
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ABSTRACT: We report the results of a series of molecular dynamics simulations of water inside a carbon-slit pore at supercritical conditions. A range of densities corresponding from liquid (0.66gcm;{-3}) to gas environments (0.08gcm;{-3}) at the supercritical temperature of 673K were considered. Our findings are compared with previous studies of liquid water confined in graphene nanochannels at ambient and high temperatures, and indicate that the microscopic structure of water evolves from hydrogen bond networks characteristic of hot dense liquids to looser arrangements where the dominant units are water monomers and dimers. Water permittivity was found to be very small at low densities, with a tendency to grow with density and to reach typical values of unconfined supercritical water at 0.66gcm;{-3}) . In supercritical conditions, the residence time of water at interfaces is roughly similar to that of water in the central regions of the slabs, if the size of the considered region is taken into account. That time span is long enough to compute dynamical properties such as diffusion or spectral densities. Water diffusion in supercritical states is much faster at low densities, and it is produced in such a way that, at interfaces, translational diffusion is mainly produced along planes parallel to the carbon walls. Spectral frequency shifts depend on several factors, being temperature and density effects the most relevant. However, we can observe corrections due to confinement, important both at the graphene interface and in the central region of the water slab.
Physical Review E 04/2009; 79(3 Pt 1):031606. · 2.26 Impact Factor
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ABSTRACT: We present a study of the microscopic dynamics of water trapped in reverse non-ionic micelles by means of a series of molecular dynamics simulations. The analysis of the effects of micellar confinement on spectroscopical properties of an excess proton has also been considered. Our micelles were microemulsions made with the neutral surfactant diethylene glycol monodecyl ether [CH(3)(CH(2))(11)(OC(2)H(4))(2)OH]. Simulation experiments including the proton species were performed using a multistate empirical valence bond Hamiltonian model. Diffusion of water in the micelle is markedly slower than that in the bulk liquid, in the same fashion as happens with reorientational dynamics. Spectral densities of hydrogens revealed a blue-shift of the OH-stretching vibration together with a split of the main band into two components. Absorption lineshapes of the solvated proton in the vicinity of the internal surface of the micelle indicate the coexistence of Eigen-like and Zundel-like structures and a tendency to red-shifting (compared to the aqueous unconstrained excess proton case) of the two relevant spectral bands (around 2000 and 2500 wavenumbers) mainly due to the slower dynamics of proton vibrations in water near interfaces.
Physical Chemistry Chemical Physics 04/2009; 11(10):1484-90. · 3.57 Impact Factor
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ABSTRACT: Structure, hydrogen bonding, electrostatics, dielectric, and dynamical properties of liquid water confined in flat graphene nanochannels are investigated by molecular dynamics simulations. A wide range of temperatures (between 20 and 360 degrees C) have been considered. Molecular structure suffers substantial changes when the system is heated, with a significant loss of structure and hydrogen bonding. In such case, the interface between adsorbed and bulk-like water has a marked tendency to disappear, and the two preferential orientations of water nearby the graphite layers at room temperature are essentially merging above the boiling point. The general trend for the static dielectric constant is its reduction at high temperature states, as compared to ambient conditions. Similarly, residence times of water molecules in adsorbed and bulk-like regions are significantly influenced by temperature, as well. Finally, we observed relevant changes in water diffusion and spectroscopy along the range of temperatures analyzed.
The Journal of Physical Chemistry B 12/2007; 111(43):12524-30. · 3.70 Impact Factor
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ABSTRACT: Electric and dielectric properties and microscopic dynamics of liquid water confined between graphite slabs are analyzed by means of molecular dynamics simulations for several graphite-graphite separations at ambient conditions. The electric potential across the interface shows oscillations due to water layering, and the overall potential drop is about -0.28 V. The total dielectric constant is larger than the corresponding value for the bulklike internal region of the system. This is mainly due to the preferential orientations of water nearest the graphite walls. Estimation of the capacitance of the system is reported, indicating large variations for the different adsorption layers. The main trend observed concerning water diffusion is 2-fold: on one hand, the overall diffusion of water is markedly smaller for the closest graphite-graphite separations, and on the other hand, water molecules diffuse in interfaces slightly slower than those in the bulklike internal areas. Molecular reorientational times are generally larger than those corresponding to those of unconstrained bulk water. The analysis of spectral densities revealed significant spectral shifts, compared to the bands in unconstrained water, in different frequency regions, and associated to confinement effects. These findings are important because of the scarce information available from experimental, theoretical, and computer simulation research into the dielectric and dynamical properties of confined water.
The Journal of Physical Chemistry B 12/2006; 110(47):23987-94. · 3.70 Impact Factor
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ABSTRACT: We report the results of a series of molecular-dynamics simulations of liquid water confined between two graphite plates with separations ranging from 7 to 15 A. Energies and free energies are provided, indicating also the corresponding stability density span of confined water phases. The structure of the different liquid layers is also discussed for all the considered systems. In particular, we studied atomic density profiles, two-dimensional radial distribution functions, hydrogen bonding, and angular orientations near the carbon plates.
The Journal of Chemical Physics 09/2005; 123(5):054707. · 3.33 Impact Factor
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ABSTRACT: The influence of temperature and density changes on the reorientational motions and dielectric properties of supercritical water is investigated by molecular dynamics simulations. A rigid simple point charged potential has been used to model water-water interactions. This model has revealed to be very satisfactory to reproduce thermodynamic, diffusive, and static dielectric properties of supercritical water. Reorientation times of water molecules have been computed along specific molecular directions and compared with available experimental data. The imaginary part of the frequency dependent dielectric constant, spectral densities of states, and residence times of water molecules in their first hydration shells are also evaluated.
Physical Review E 02/2004; 69(1 Pt 1):011502. · 2.26 Impact Factor
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ABSTRACT: Molecular dynamics simulations of liquid water near the external surface of a carbon nanotube bundle are presented. Flexible simple point charge and Lennard-Jones potentials have been chosen to model water–water and water–carbon forces, respectively. It has been recently shown that such a system suffers a first-order phase transition between low- and high-density phases. Hydrogen bonding, diffusive behavior, rotational and vibrational motions are analyzed in the low- and high-density regimes. © 2003 American Institute of Physics.
The Journal of Chemical Physics 12/2003; 119(23):12540-12546. · 3.33 Impact Factor
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ABSTRACT: Molecular-dynamics simulations of water on the outer surface of a bundle of carbon nanotubes are reported. We observed a first-order phase transition from a low-density regime in which the water molecules are confined in the grooves between tubes and a high-density one, characterized by adsorption of water on all the surface exposed. Both phases could be experimentally detected by the differences in the locations of the main peaks of their infrared spectra.
Phys. Rev. B. 05/2003; 67(20).
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ABSTRACT: Static and dynamic properties of a layer of water molecules on top of a graphite surface are studied by means of molecular dynamics simulations. The water molecules are described by a simple point charge flexible model, and the graphite is taken to be a set of featureless parallel sheets separated 3.4 Å in the z direction. Our results indicate that, even at the lower temperatures considered, the water layer is not flat, with some hydrogen atoms pointing perpendicularly to the surface plane. The O–H stretching frequencies are also different than those of bulk water, appearing a new peak in the simulated spectra at a frequency approximately 200 wave numbers higher than the main peak. This peak is associated with the presence of non-H-bonded molecules. © 2002 American Institute of Physics.
The Journal of Chemical Physics 08/2002; 117(7):3425-3430. · 3.33 Impact Factor
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ABSTRACT: We report a molecular dynamics simulation study of the behavior of liquid water adsorbed in carbon nanotubes under different thermodynamic conditions. A flexible simple point charged potential has been employed to model internal and intermolecular water interactions. Water-carbon forces are modeled with a Lennard-Jones-type potential. We have studied three types of tubes with effective radii ranging from 4.1 to 6.8 A and three temperatures, from 298 to 500 K for a fixed density of 1 g/cm(3). Structure of each thermodynamic state is analyzed through the characterization of the hydrogen-bond network. Time-dependent properties such as the diffusive behavior and molecular vibrational spectra are also considered. We observe the gradual destruction of the hydrogen-bond network together with faster diffusive regimes as temperature increases. A vibrational mode absent in bulk unconstrained water appears in the power spectra obtained from hydrogen velocity autocorrelation functions for all thermodynamic states. That frequency mode should be attributed to confinement effects.
Physical Review E 09/2001; 64(2 Pt 1):021504. · 2.26 Impact Factor
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ABSTRACT: Dynamics of liquid water and its isotopes when adsorbed inside carbon nanotubes of different radii is studied by means of molecular dynamics simulations. Water molecules have been modeled with a flexible simple point charged (SPC) potential while carbon forces were accounted for with Lennard–Jones-type potentials. Diffusive behavior and the librational, rotational, intra- and intermolecular motions of the constrained molecules have been investigated by means of the spectral densities computed from atomic velocity autocorrelation functions. The results show in all cases significant new vibrational bands and frequency shifts absent in bulk water. © 2001 American Institute of Physics.
The Journal of Chemical Physics. 06/2001; 114(23):10486-10492.
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J. Martí
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ABSTRACT: We report the results of a calculation of the vibrational and rotational spectra of water confined in carbon nanotubes. Water has been modeled with a flexible version of the simple point charge model of Berendsen et al., while the carbon nanotubes are taken to be soft potential walls interacting with water through a Lennard-Jones-type potential. The comparison of the computed absorption line shapes with the corresponding ones of liquid bulk water shows significant shifts in the positions of the spectral bands directly related to the tube radii.
Phys. Rev. B. 04/2001; 63(16).
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Molecular Simulation 10/1999; 23(1):55-62. · 1.33 Impact Factor
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ABSTRACT: Structure and dynamic properties of liquid water at temperatures between 298 and 523 K and densities between 0.75 and 1.20 g/cm3 have been investigated by molecular dynamics simulation. A flexible simple point charge potential has been asssumed for interactions. The hydrogen bonding structure in the different simulated states as well as the influence of the hydrogen bonds on the dynamic properties (self‐diffusion coefficients, vibrational spectra) is discussed. Special attention is paid to the intermolecular vibrational spectrum (10–400 cm−1). It has been corroborated that the band around 200 cm−1 can be attributed to intermolecular O–O stretching vibrations of pairs of H‐bonded bounded molecules. On the contrary, molecular dynamics results indicate that the band close to 50 cm−1 is independent of the existence of hydrogen bonds but depends on the density and temperature of the system. It is suggested that it is simply associated with vibrations of molecules in the cage formed by their neighbors. Shifts of librational and stretching bands as a function of the thermodynamic state are highly correlated with changes in the percentage of hydrogen bonded molecules. © 1996 American Institute of Physics.
The Journal of Chemical Physics 07/1996; 105(2):639-649. · 3.33 Impact Factor
