[Show abstract][Hide abstract] ABSTRACT: Adsorption of supercritical CO 2 in nanoporous silica aerogel was investigated by a combination of experiments and molecular-level computer modeling. High-pressure gravimetric and vibrating tube densimetry techniques were used to measure the mean pore fluid density and excess sorption at 35 and 50 °C and pressures of 0−200 bar. Densification of the pore fluid was observed at bulk fluid densities below 0.7 g/cm 3 . Far above the bulk critical density, near-zero sorption or weak depletion effects were measured, while broad excess sorption maxima form in the vicinity of the bulk critical density. The CO 2 sorption properties are very similar for two aerogels with bulk densities of 0.1 and 0.2 g/cm 3 , respectively. The spatial distribution of the confined supercritical fluid was analyzed in terms of two nanodispersed phases with sorption-and bulk-phase densities and their volumes by means of the adsorbed phase model (APM), which used data from gravimetric sorption and small-angle neutron scattering experiments. To gain more detailed insight into supercritical fluid sorption, large-scale lattice gas GCMC simulations were utilized and tuned to resemble the experimental excess sorption data. The computed three-dimensional pore fluid density distributions show that the observed maximum of the excess sorption near the critical density originates from large density fluctuations pinned to the pore walls. At this maximum, the size of these fluctuations is comparable to the prevailing pore sizes.
The Journal of Physical Chemistry C 06/2014; 118:15525. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Since the single-ion thermodynamic properties of bulk solutions are not directly accessible from experiments, extrapolations have been devised to estimate them from experimental measurements on small-clusters. Extrapolations based on the cluster-pair-based approximation (CPA) technique (Tissandier et al, J. Phys. Chem A 1998, 102, 7787-7794) and its variants are currently considered as one of the most reliable source of single-ion hydration thermodynamic data, and have been used as a benchmark for the development of molecular and continuum solvation models. Despite its importance, the CPA has not been thoroughly tested, while recent studies have indicated inconsistencies with molecular simulations. The present work challenges the key CPA assumptions that the hydration properties of single cations and anions in growing clusters rapidly converge to each other following a monotonous trend. Using a combination of simulation techniques to study the transition between alkali halide ions in small clusters and bulk solution, we show that this convergence is rather slow and involves a surprising change in trends, which can result in significant errors from the original estimated single-ion properties. When these cluster-size-dependent effects are taken into account, the inconsistencies between molecular models and experimental predictions disappear, and the value of the proton hydration enthalpy based on the CPA aligns with estimates based on other principles.
The Journal of Physical Chemistry A 10/2013; · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We carried out a systematic molecular simulation study of the behavior of a pair of finite-size graphene plates immersed in water at isobaric-isothermal conditions to provide insights into the nature of the water-graphene (corrugated) surface interactions. The goal was to address the link between the corrugation-driven hydration free energy changes in the association process involving graphene plates and the resulting water-graphene interfacial tension, to interrogate the effect of the surface corrugation and confinement on the thermodynamic response functions and the dynamics of confined water and to put the observed behavior in the context of Wenzel's modification of Young's equation. We found that graphene confinement induces a significant increase in the isothermal compressibility and isobaric thermal expansivity as well as a pronounced slowdown of the dynamics of water over that of the corresponding bulk counterpart, whose magnitudes depend on the type of surface corrugation involved. Our simulation results for different types of corrugated graphene plates involving identical surface areas do not support the meaning of the "r"-factor underlying Wenzel's equation for corrugated nanoscale surfaces.
Journal of Physical Chemistry C. 01/2013; 117(45):23875-23886.
[Show abstract][Hide abstract] ABSTRACT: In this communication we illustrate the occurrence of a recently reported new
phenomenon of surface-charge amplification, SCA, (originally dubbed
overcharging, OC), [Jimenez-Angeles F. and Lozada-Cassou M., J. Phys. Chem. B,
2004, 108, 7286] by means of molecular dynamics simulation of aqueous
electrolytes solutions involving multivalent cations in contact with charged
graphene walls and the presence of short-chain lithium polystyrene sulfonates
where the solvent water is described explicitly with a realistic molecular
model. We show that the occurrence of SCA in these systems, in contrast to that
observed in primitive models, involves neither contact co-adsorption of the
negatively charged macroions nor divalent cations with a large size and charge
asymmetry as required in the case of implicit solvents. In fact the SCA
phenomenon hinges around the preferential adsorption of water (over the
hydrated ions) with an average dipolar orientation such that the charges of the
water's hydrogen and oxygen sites induce magnification rather than screening of
the positive-charged graphene surface, within a limited range of surface-charge
[Show abstract][Hide abstract] ABSTRACT: The interactions of electrolyte fluids with carbon-based electrodes
control many complex interfacial processes encountered in
electrochemical energy storage systems. However, our knowledge of the
atomic/nanoscale reactivity at interfaces of electrolytes with
electrodes remain scares due to the incomplete understanding of
interfacial structures and processes in-situ and real-time encountered
in real operation conditions. In this talk, we will present our efforts
to obtain a molecular-scale perspective of the interactions of
electrolytes with carbon surfaces near ``real world'' conditions.
Structures of various electrolytes including slat aqueous and ionic
liquids on atomically flat graphene (epitaxially grown on a SiC
substrate), an ideal model fluid-solid interface system, were
investigated by coupling high-resolution interface X-ray scattering
techniques with molecular modeling-simulation approaches. These results
provide a base-line for understanding relevant electrolyte/carbon
interactions and will lead to fundamentally new insights and provide
unique tests of atomistic fluid-solid interface models for energy
[Show abstract][Hide abstract] ABSTRACT: 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 θc = 73°) is substantially smaller than that of multilayer graphene (θc = 93°). 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.
Physical Review B 01/2012; 85(3). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a detailed molecular-based characterization via isobaric isothermal-molecular dynamics simulation of the microstructure and dynamics of water-rich aqueous CO2 solutions at silica surfaces and under extreme confinement between finite silica plates at state conditions relevant to geologic capture and sequestration of carbon dioxide. The study comprises three types of slit-pore plates to represent two extreme cases of surface polarity and a mismatched pair of plates to interrogate the fluid behavior at and confined between heterogeneous surfaces. We found layer formation of H2O and CO2 whose strength depends on the nature of the plate surface, i.e., stronger H2O layering at hydrophilic than at hydrophobic plates with simultaneous weaker water-mediated CO2/hydrophilic-surface interactions. We observed the opposite behavior with the hydrophobic plates in which the weaker water layering results from the CO2-mediated H2O/hydrophobic-surface interactions. Moreover, we illustrate how the interplay between these types of interactions and extreme fluid confinement, i.e., strong overlapping of interfacial structures, can induce a drying out of the pore environment whose immediate consequence is a significant CO, concentration enhancement relative to that of the bulk environment. Finally, we assessed the effect of the nature of the plate surfaces on the translational diffusion coefficient of water. We found that this property changes monotonically at purely interfacial regions but nonmonotonically under confinement.
Journal of Physical Chemistry C. 01/2012; 116(26):13904-13916.
[Show abstract][Hide abstract] ABSTRACT: Injection of CO2 into geologic formations has been proposed as a key
element to reduce the impact of greenhouse gases emissions. Quantitative
understanding of CO2 adsorption in porous mineral environments at
thermodynamic conditions relevant to proposed sequestration sites is
thus a prerequisite for the assessment of their viability. In this study
we use a combination of neutron scattering, adsorption experiments, and
computer modeling to investigate the thermodynamics of near-critical
carbon dioxide in the pores of SiO2 aerogel, which serves as a model of
a high-porosity reservoir rock. Small angle neutron scattering (SANS)
experiments provide input for the optimization of the computer model of
the aerogel matrix, and also serve as a sensitive probe of local density
changes of confined CO2 as a function of external pressure. Additional
details of the aerogel basic building blocks and SiO2 surface are
derived from TEM images. An independent source of global adsorption data
is obtained from gravimetric experiments. The structural and
thermodynamic aspects of CO2 sorption are linked using computer
simulations, which include the application of the optimized diffusion
limited cluster-cluster aggregation algorithm (DLCA), classical density
functional theory (DFT) modeling of large-scale CO2 density profiles,
and molecular dynamics simulations of the details of interactions
between CO2 molecules and the amorphous silica surfaces. This integrated
approach allows us to span scales ranging from 1Å to 1μm, as
well as to infer the detailed structure of silica threads forming the
framework of the silica matrix.
[Show abstract][Hide abstract] ABSTRACT: The unlike-pair interaction parameters for the SPC/E-EPM2 models have been optimized to reproduce the mutual solubility of water and carbon dioxide at the conditions of liquid-supercritical fluid phase equilibria. An efficient global optimization of the parameters is achieved through an implementation of the coupling parameter approach, adapted to phase equilibria calculations in the Gibbs ensemble, that explicitly corrects for the overpolarization of the SPC/E water molecule in the nonpolar CO(2) environments. The resulting H(2)O-CO(2) force field accurately reproduces the available experimental solubilities at the two fluid phases in equilibria as well as the corresponding species tracer diffusion coefficients.
The Journal of Physical Chemistry B 06/2011; 115(27):8775-84. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We carry out a systematic microstructural characterization of the solid-fluid interface (SFI) of water and simple metal chloride aqueous solutions in contact with a free-standing plate or with two such plates separated by an interplate distance 0 ≤ h (Å) ≤ 30 at ambient conditions via isothermal-isobaric molecular dynamics. With this characterization, we target the interrogation of the system in search for answers to fundamental questions regarding the structure of the "external" and "internal" (confined) SFIs, the effect of the differential hydration behavior among species, and its link to species expulsion from confinement. For water at ambient conditions, we found that the structure of the "external" SFIs is independent of the interplate distance h in the range 0 ≤ h (Å) ≤ 30, that is, the absence of wall-mediated correlation effects between "external" and "internal" SFIs, and that for h < 9 Å the slit-pores dewet. Moreover, we observed a selective expulsion of ions caused by the differential hydration between the anion and the cations with a consequent charging of the slit-pore. All these observations were interpreted in terms of the axial profiles for precisely defined order parameters, including tetrahedral configuration, hydrogen bonding, and species coordination numbers.
The Journal of Physical Chemistry A 06/2011; 115(23):5918-27. · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The purpose of this communication is to provide an atomistic view, via molecular dynamic simulation, of the contrasting interfacial behavior between high temperature dry- and (10-40 vol%) wet-air in contact with stainless steels as represented by Fe20Cr. It was found that H2O preferentially adsorbs and displaces oxygen at the metal/fluid interface. Comparison of these findings with experimental studies reported in the literature is discussed. Keywords: Fe-Cr alloys, metal-fluid interfacial behavior, wet-air, molecular simulation
Scripta Materialia - SCRIPTA MATER. 01/2011; 64(11):1027-1030.
[Show abstract][Hide abstract] ABSTRACT: Injection of CO2 into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO2. Regardless of the formation type, the CO2 will encounter a complex heterogeneous porous matrix with widely varying pore size and pore distribution, interconnectivity, and surface composition. A small but non-trivial percentage of the pore space is comprised of voids that range from 100 nm down to a few nm in size. These nanoporous environments are more dominant in the cap or seal rocks, such as shale or clay-rich mudstones that act as confining barriers to leakage of CO2 out of the storage reservoir. A concern 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 investigated the behavior of pure CO2 and CO2-H2O mixtures interacting with simple substrates, e.g. SiO2 and muscovite, that act as proxies for more complex mineralogical systems. SANS results were described for sorption properties of supercritical CO2 inside mesoporous silica aerogel (95% porosity; 5-40 nm pores), a proxy for the quartz sub-system. The Adsorbed Phase Model (APM) allows, for the first time, a means to quantify the physical properties (e.g. excess, absolute and total adsorption) of the adsorbed phase formed by fluids inside porous media in terms of the mean density and volume of the sorption phase. The results show clear evidence for fluid depletion for conditions above the critical density. Classical molecular dynamics (CMD) modeling of CO2-silica aerogel interactions also indicates the presence of fluid depletion for conditions above the critical density consistent with SANS results. CMD was also used to assess the microscopic behavior of CO2-H2O mixture (2.3 mol % CO2) at a silica surfaces and within silica 1 nm slit pores at 45oC and 200 b. For the case of a silica plate we observed significant layering within 10 Å from the mineral surface, promotion of CO2 co-sorption with H2O on hydroxylated surfaces and stronger H2O adsorption at non-hydroxylated surfaces. In the 1 nm slit pore case, nano-confinement strongly influences the sorption behavior wherein for hydroxylated surfaces H2O is preferentially enriched in the pore but for non-hydroxylated surfaces, H2O dewetting occurs with enhanced capillary uptake of CO2. Structural and dynamic behavior for supercritical CO2 interaction with muscovite (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 magnitudes compared to bulk CO2 depending on pore size and formation of bonds between CO2oxygens 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 and CO2-H2O mixtures at conditions relevant to sequestration.
[Show abstract][Hide abstract] ABSTRACT: We study the polarization behavior of water under geologically relevant extreme aqueous environments along four equidistant supercritical isotherms, 773<or=T(K)<or=1373, and over a wide pressure range, 0<P(GPa)<or=30, by isobaric-isothermal molecular dynamics simulations of the Gaussian charge polarizable water model, to unravel and discuss the underlying link between two precisely defined orientational order parameters and the magnitude of the average induced dipole moment of water. The predicted behavior indicates an isothermal linear dependence (a) between the magnitude of the average induced dipole moment mu(ind) and the average system density rho, (b) between the magnitude of the average induced dipole mu(ind) and that of the total dipole mu(tot), resulting from (c), a compensating (inverse) dependence between the permanent-to-induced dipolar angle theta and the magnitude of the average induced dipole moment mu(ind). Moreover, we interpret this behavior in terms of the evolution of the state dependent tetrahedral order parameter q(T) and the corresponding bond-order parameter Q(6), supplemented by the microstructural analysis based on the three site-site radial distribution functions of water and the distance-ranked nearest-neighbor distributions. Finally, we show that while water exhibits a dramatic microstructural transformation from an open four-coordinated hydrogen-bonded network at normal conditions to a quasi-close-packed coordination, it still preserves a significant degree of hydrogen bonding.
The Journal of Chemical Physics 08/2010; 133(7):074504. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We analyzed the solvation behavior of aqueous lithium, nickel, and ytterbium sulfates at ambient conditions in terms of the relevant radial distributions functions and the corresponding first-order difference of the sulfur-site neutron-weighted distribution functions generated by isothermal-isobaric molecular dynamics simulation. We determined the partial contributions to the neutron-weighted distribution functions, to identify the main contributing peaks of the corresponding radial distribution functions, and the effect of the contact ion-pair configuration on the resulting water’s hydrogen coordination around the sulfate’s sulfur site. Finally, we assessed the extent of the ion-pair formation according to Poirier–DeLap formalism and highlighted the significant increase of the ion-pair association exhibited by these salts with cation charge.
Collection of Czechoslovak Chemical Communications 01/2010; · 1.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We discuss the interplay between theory, molecular simulation and electric conductance experiments as an important tool for the extraction of ion-pair interaction potentials to make possible the bridging of the density gap between the lowest experimentally attainable conductance measurement and the theoretically reachable zero-density limit of the ion-pair association constant. Then, we predict the density dependence of the Na+!Cl! pair association constant in ultra supercritical steam environments by constraint molecular dynamics simulation over state conditions relevant to the new generation of ultra-supercritical steam power plants. Finally, we draw attention to relevant modeling challenges associated to the behavior of these systems around the zero-density limit and discuss ways to overcome them.
Journal of Chemical & Engineering Data 01/2010; 55(5). · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: 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.
Philosophical Magazine A 01/2010; 90(17-18):2339-2363.
[Show abstract][Hide abstract] ABSTRACT: The liquid-vapor equilibrium isotopic fractionation of water is determined by molecular-based simulation, via Gibbs ensemble Monte Carlo and isothermal-isochoric molecular dynamics involving two radically different but realistic models, the extended simple point charge, and the Gaussian charge polarizable models. The predicted temperature dependence of the liquid-vapor equilibrium isotopic fractionation factors for H(2) (18)O/H(2) (16)O, H(2) (17)O/H(2) (16)O, and (2)H(1)H(16)O/(1)H(2) (16)O are compared against the most accurate experimental datasets to assess the ability of these intermolecular potential models to describe quantum effects according to the Kirkwood-Wigner free energy perturbation variant Planck's over h(2)-expansion. Predictions of the vapor pressure isotopic effect for the H(2) (18)O/H(2) (16)O and H(2) (17)O/H(2) (16)O pairs are also presented in comparison with experimental data and two recently proposed thermodynamic modeling approaches. Finally, the simulation results are used to discuss some approximations behind the microscopic interpretation of isotopic fractionation based on the underlying rototranslational coupling.
The Journal of Chemical Physics 04/2009; 130(9):094509. · 3.12 Impact Factor