Ariel A. Chialvo

Water in carbon nanotubes: Adsorption isotherms and thermodynamic properties from molecular simulation

Grand canonical Monte Carlo simulations are performed to study the adsorption of water in single-walled (6:6), (8:8), (10:10), (12:12), and (20:20) carbon nanotubes in the 248-548 K temperature range. At room temperature the resulting adsorption isotherms in (10:10) and wider single-walled carbon nanotubes (SWCNs) are characterized by negligible water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption/desorption hysteresis loops. The width of the hysteresis loops decreases as pore diameter narrows and it becomes negligible for water adsorption in (8:8) and (6:6) SWCNs. Results for the isosteric heat of adsorption, density profiles along the pore axis and across the pore radii, order parameter across the pore radii, and x-ray diffraction patterns are presented. Layered structures are observed when the internal diameter of the nanotubes is commensurate to the establishment of a hydrogen-bonded network. The structure of water in (8:8) and (10:10) SWCNs is ordered when the temperature is 298 and 248 K, respectively. By simulating adsorption isotherms at various temperatures, the hysteresis critical temperature, e.g., the lowest temperature at which no hysteresis can be detected, is determined for water adsorbed in (20:20), (12:12), and (10:10) SWCNs. The hysteresis critical temperature is lower than the vapor-liquid critical temperature for bulk Simple Point Charge-Extended (SPCE) water model.

From dimer to condensed phases at extreme conditions: Accurate predictions of the properties of water by a Gaussian charge polarizable model

Water exhibits many unusual properties that are essential for the existence of life. Water completely changes its character from ambient to supercritical conditions in a way that makes it possible to sustain life at extreme conditions, leading to conjectures that life may have originated in deep-sea vents. Molecular simulation can be very useful in exploring biological and chemical systems, particularly at extreme conditions for which experiments are either difficult or impossible; however this scenario entails an accurate molecular model for water applicable over a wide range of state conditions. Here, we present a Gaussian charge polarizable model (GCPM) based on the model developed earlier by Chialvo and Cummings [Fluid Phase Equilib. 150, 73 (1998)] which is, to our knowledge, the first that satisfies the water monomer and dimer properties, and simultaneously yields very accurate predictions of dielectric, structural, vapor-liquid equilibria, and transport properties, over the entire fluid range. This model would be appropriate for simulating biological and chemical systems at both ambient and extreme conditions. The particularity of the GCPM model is the use of Gaussian distributions instead of points to represent the partial charges on the water molecules. These charge distributions combined with a dipole polarizability and a Buckingham exp-6 potential are found to play a crucial role for the successful and simultaneous predictions of a variety of water properties. This work not only aims at presenting an accurate model for water, but also at proposing strategies to develop classical accurate models for the predictions of structural, dynamic, and thermodynamic properties.

Engineering a simple polarizable model for the molecular simulation of water applicable over wide ranges of state conditions

We perform a systematic analysis of the relationship between the molecular geometry, the force‐field parameters, the magnitude of the induced dipoles, and the resulting site–site microstructure of a model for water consisting of simple point charges plus a self‐consistent point dipole polarizability. We constrain the model to represent the experimental values of the pressure and the configurational internal energy of water at ambient conditions, while keeping a permanent dipole moment of 1.85 D. The resulting force fields are then used to perform additional simulations at high temperature to determine the effect of polarizabilities on the site–site structure, and to make contact with neutron scattering experiments as well as ab initio simulation results. We show that the parameterization of the model is possible for 0≤ROM≤0.25 A, where ROM is the oxygen‐to‐negative charge distance along the bisectrix of the H–O–H angle, resulting in total dipole moments from 2.88 to 3.03 D, with polarization energies acco...

Na+–Cl− ion pair association in supercritical water

Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent‐separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean‐force potential from simulation and the impact of the assumptions involved.

Hydrogen bonding in supercritical water

We study the hydrogen bonding structure of water models at supercritical conditions by molecular dynamics to directly compare with recent microstructural data obtained by neutron diffraction with isotopic substitution (NDIS) experiments. We also study the angular dependence of the hydrogen–oxygen pair distribution function to gain insight into the hydrogen bonding mechanism in the molecular models for water. The simulation results suggest that the angle‐averaged radial distribution function gOH(r) measured by NDIS experiments may not provide a complete picture of the degree of hydrogen bonding.

Supercritical fluid behavior at nanoscale interfaces: Implications for CO2 sequestration in geologic formations

Injection of CO2 into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO2. A key aspect of this process is the prevention of leakage from the host formation by an effective cap or seal rock which has low porosity and permeability characteristics. Shales comprise the majority of cap rocks encountered in subsurface injection sites with pore sizes typically less than 100 nm and whose surface chemistries are dominated by quartz (SiO2) and clays. We report the behavior of pure CO2 interacting with simple substrates, i.e. SiO2 and muscovite, that act as proxies for more complex mineralogical systems. Modeling of small-angle neutron scattering (SANS) data taken from CO2–silica aerogel (95% porosity; ∼7 nm pores) interactions indicates the presence of fluid depletion for conditions above the critical density. A theoretical framework, i.e. integral equation approximation (IEA), is presented that describes the fundamental behavior of near-critical adsorption onto a non-confining substrate that is consistent with SANS experimental results. Structural and dynamic behavior for supercritical CO2 interaction with muscovite (KAl2Si3AlO10(OH)2) was assessed by classical molecular dynamics (CMD). These results indicate the development of distinct layers of CO2 within slit pores, reduced mobility by one to two orders of magnitude compared to bulk CO2 depending on pore size and formation of bonds between CO2 oxygens and H from muscovite hydroxyls. Analysis of simple, well-characterized fluid-substrate systems can provide details on the thermodynamic, structural and dynamic properties of CO2 at conditions relevant to sequestration.

Simulated water adsorption isotherms in carbon nanopores

Water adsorption isotherms are calculated by grand canonical Monte Carlo simulations for the SPC/E water model in carbon nanopores at 298 K. The pores are of slit or cylindrical morphology. Carbon-slit pores are of widths 0.8, 1.0 and 1.6 nm. The simulated single-walled carbon nanotubes are of 1.4 and 2.7 nm diameter ((10:10) and (20:20) respectively). In all cases considered, the adsorption isotherms are characterized by negligible adsorption at low pressures, pore filling by a capillary-condensation-like mechanism and adsorption–desorption hysteresis loops. For both pore morphologies considered, the relative pressures at which pore filling occurs, and the width of the adsorption–desorption hysteresis loop decrease with decreasing pore size. Adsorption isotherms simulated for water in carbon nanotubes show pore filling at lower relative pressures and narrower adsorption–desorption hysteresis loops when compared to adsorption isotherms simulated in carbon-slit pores of similar sizes. By using representative simulation snapshots, the mechanisms of pore filling and pore emptying are discussed. Pore filling happens by growth of hydrogen-bonded clusters of adsorbed water molecules, without the formation of monolayers as observed in the adsorption of simple fluids. Pore emptying occurs by the formation of bubbles, often in contact with the hydrophobic surface, followed by the coalescence and growth of these bubbles.

The structure of CaCl2 aqueous solutions over a wide range of concentration. Interpretation of diffraction experiments via molecular simulation

A detailed analysis of the water structure around the ionic species in aqueous CaCl2 solutions is performed by molecular dynamics over a wide range of concentration (0<m⩽9.26) at ambient conditions. The goals behind the study are to address a long-standing controversy regarding the actual coordination number of aqueous Ca+2, to characterize the hydration structure of Ca+2, and consequently to interpret the origin of the disparity in the reported experimental data, to describe the additional information available from simulation to augment the first-order difference method in the determination of the species coordination numbers from neutron diffraction with isotopic substitution (NDIS) experiments, and to attain additional insights into the distribution of the “tilting” and “wagging” ion–water angles and their relation with the conventional configurational definitions.

Simulated water adsorption in chemically heterogeneous carbon nanotubes

Grand canonical Monte Carlo simulations are used to study the adsorption of water in single-walled (10:10), (12:12), and (20:20) carbon nanotubes at 298 K. Water is represented by the extended simple point charge model and the carbon atoms as Lennard-Jones spheres. The nanotubes are decorated with different amounts of oxygenated sites, represented as carbonyl groups. In the absence of carbonyl groups the simulated isotherms are characterized by negligible amounts of water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. In the presence of a few carbonyl groups the simulated adsorption isotherms are characterized by pore filling at lower pressures and by narrower adsorption-desorption hysteresis loops compared to the results obtained in the absence of carbonyl groups. Our results show that the distribution of the carbonyl groups has a strong effect on the adsorption isotherms. For carbonyl groups localized in a narrow section the adsorption of water may be gradual because a cluster of adsorbed water forms at low pressures and grows as the pressure increases. For carbonyl groups distributed along the nanotube the adsorption isotherm is of type V.

Understanding controls on interfacial wetting at epitaxial graphene: Experiment and Theory

The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution x-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle {Theta}{sub c} = 73{sup o}) is substantially smaller than that of multilayer graphene ({Theta}{sub c} = 93{sup o}). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.