Attachment, Coalescence, and Spreading of Carbon Dioxide Nanobubbles at Pyrite Surfaces.

Recently, it was reported that using CO2 as a flotation gas increases the flotation of auriferous pyrite from high carbonate gold ores of the Carlin Trend. In this regard, the influence of CO2 on bubble attachment at fresh pyrite surfaces was measured in the absence of collector using an induction timer, and it was found that nitrogen bubble attachment time was significantly reduced from 30 ms to less than 10 ms in CO2 saturated solutions. Details of CO2 bubble attachment at a fresh pyrite surface have been examined by atomic force microscopy (AFM) measurements and molecular dynamics (MD) simulations, and the results used to describe the subsequent attachment of a N2 bubble. As found from MD simulations, unlike the attached N2 bubble, which is stable and has a contact angle of about 90°, the CO2 bubble attaches, and spreads, wetting the fresh pyrite surface and forming a multilayer of CO2 molecules, corresponding to a contact angle of almost 180°. These MDS results are complemented by in situ AFM images, which show that, after attachment, CO2 nano-/microbubbles spread to form pancake bubbles at the fresh pyrite surface. In summary, it seems that CO2 bubbles have a propensity to spread, and whether CO2 exists as layers of CO2 molecules (gas pancakes) or as nano-/microbubbles, their presence at the fresh pyrite surface subsequently facilitates film rupture and attachment of millimeter N2 bubbles and, in this way, improves the flotation of pyrite.

Rate theory of ion pairing at the water liquid-vapor interface: A case of sodium iodide.

Studies on ion pairing at interfaces have been intensified recently because of their importance in many chemical reactive phenomena, such as ion-ion interactions that are affected by interfaces and their influence on kinetic processes. In this study, we performed simulations to examine the thermodynamics and kinetics of small polarizable sodium iodide ions in the bulk and near the water liquid-vapor interface. Using classical transition state theory, we calculated the dissociation rates and corrected them with transmission coefficients obtained from the reactive flux formalism and Grote-Hynes theory. Our results show that in addition to affecting the free energy of ions in solution, the interfacial environments significantly influence the kinetics of ion pairing. The results on the relaxation time obtained using the reactive flux formalism and Grote-Hynes theory present an unequivocal picture that the interface suppresses ion dissociation. The effects of the use of molecular models on the ion interactions as well as the ion-pair configurations at the interface are also quantified and discussed.

Li+ solvation and kinetics of Li+-BF4-/PF6- ion pairs in ethylene carbonate. A molecular dynamics study with classical rate theories.

Using our polarizable force-field models and employing classical rate theories of chemical reactions, we examine the ethylene carbonate (EC) exchange process between the first and second solvation shells around Li+ and the dissociation kinetics of ion pairs Li+-[BF4] and Li+-[PF6] in this solvent. We calculate the exchange rates using transition state theory and correct them with transmission coefficients computed by the reactive flux, Impey, Madden, and McDonald approaches, and Grote-Hynes theory. We found that the residence times of EC around Li+ ions varied from 60 to 450 ps, depending on the correction method used. We found that the relaxation times changed significantly from Li+-[BF4] to Li+-[PF6] ion pairs in EC. Our results also show that, in addition to affecting the free energy of dissociation in EC, the anion type also significantly influences the dissociation kinetics of ion pairing.

Rate theory of solvent exchange and kinetics of Li(+) - BF4 (-)/PF6 (-) ion pairs in acetonitrile.

In this paper, we describe our efforts to apply rate theories in studies of solvent exchange around Li(+) and the kinetics of ion pairings in lithium-ion batteries (LIBs). We report one of the first computer simulations of the exchange dynamics around solvated Li(+) in acetonitrile (ACN), which is a common solvent used in LIBs. We also provide details of the ion-pairing kinetics of Li(+)-[BF4] and Li(+)-[PF6] in ACN. Using our polarizable force-field models and employing classical rate theories of chemical reactions, we examine the ACN exchange process between the first and second solvation shells around Li(+). We calculate exchange rates using transition state theory and weighted them with the transmission coefficients determined by the reactive flux, Impey, Madden, and McDonald approaches, and Grote-Hynes theory. We found the relaxation times changed from 180 ps to 4600 ps and from 30 ps to 280 ps for Li(+)-[BF4] and Li(+)-[PF6] ion pairs, respectively. These results confirm that the solvent response to the kinetics of ion pairing is significant. Our results also show that, in addition to affecting the free energy of solvation into ACN, the anion type also should significantly influence the kinetics of ion pairing. These results will increase our understanding of the thermodynamic and kinetic properties of LIB systems.

Nuclear quantum effects in water exchange around lithium and fluoride ions.

We employ classical and ring polymer molecular dynamics simulations to study the effect of nuclear quantum fluctuations on the structure and the water exchange dynamics of aqueous solutions of lithium and fluoride ions. While we obtain reasonably good agreement with experimental data for solutions of lithium by augmenting the Coulombic interactions between the ion and the water molecules with a standard Lennard-Jones ion-oxygen potential, the same is not true for solutions of fluoride, for which we find that a potential with a softer repulsive wall gives much better agreement. A small degree of destabilization of the first hydration shell is found in quantum simulations of both ions when compared with classical simulations, with the shell becoming less sharply defined and the mean residence time of the water molecules in the shell decreasing. In line with these modest differences, we find that the mechanisms of the exchange processes are unaffected by quantization, so a classical description of these reactions gives qualitatively correct and quantitatively reasonable results. We also find that the quantum effects in solutions of lithium are larger than in solutions of fluoride. This is partly due to the stronger interaction of lithium with water molecules, partly due to the lighter mass of lithium and partly due to competing quantum effects in the hydration of fluoride, which are absent in the hydration of lithium.

Computational studies of [bmim][PF6]/n-alcohol interfaces with many-body potentials.

In this paper, we present the results from molecular dynamics simulations of the equilibrium properties of liquid/liquid interfaces of room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and simple alcohols (i.e., methanol, 1-butanol, and 1-hexanol) at room temperature. Polarizable potential models are employed to describe the interactions among species. Results from our simulations show stable interfaces between the ionic liquid and n-alcohols, and we found that the interfacial widths decrease from methanol to 1-butanol systems and then increase for 1-hexanol interfaces. Angular distribution analysis reveals that the interface induces a strong orientational order of [bmim] and n-alcohol molecules near the interface, with [bmim] extending its butyl group into the alcohol phase, whereas the alcohol has the OH group pointing into the ionic liquid region, which is consistent with the recent sum-frequency-generation experiments. We found the interface to have a significant influence on the dynamics of ionic liquids and n-alcohols. The orientational autocorrelation functions illustrate that [bmim] rotates more freely near the interface than in the bulk, whereas the rotation of n-alcohol is hindered at the interface. Additionally, the time scale associated with the diffusion along the interfacial direction is found to be faster for [bmim] but slowed down for n-alcohols approaching the interface. We also calculate the dipole moment of n-alcohols as a function of the distance normal to the interface. We found that, even though methanol and 1-butanol have different dipole moments in bulk phase, they reach a similar value at the interface.

Computational studies of water exchange around aqueous Li+ with polarizable potential models.

To enhance our understanding of the mechanism of water-exchange around aqueous Li(+), we carried out a systematic study on this system using molecular dynamics simulations with polarizable potential models. The mechanistic properties associated with the water-exchange process, such as potentials of mean force, time dependent transmission coefficients, and the corresponding rate constants, were examined using transition rate theory, the reactive flux method, and Grote-Hynes treatments of the dynamic response of the solvent. We compared the computed rate theory results with results from previous corresponding studies in which classical non-polarizable force fields were used. Our computed barrier heights for water exchange are significantly larger than those obtained using classical non-polarizable force fields. We also studied the effect of pressure on water-exchange rates and the corresponding activation volume. Our computed rate results for water exchange increase with pressure; therefore, a small negative activation volume is observed.

Molecular mechanism of the adsorption process of an iodide anion into liquid-vapor interfaces of water-methanol mixtures.

To enhance our understanding of the molecular mechanism of ion adsorption to the interface of mixtures, we systematically carried out a free energy calculations study involving the transport of an iodide anion across the interface of a water-methanol mixture. Many body affects are taken into account to describe the interactions among the species. The surface propensities of I(-) at interfaces of pure water and methanol are well understood. In contrast, detailed knowledge of the molecular level adsorption process of I(-) at aqueous mixture interfaces has not been reported. In this paper, we explore how this phenomenon will be affected for mixed solvents with varying compositions of water and methanol. Our potential of mean force study as function of varying compositions indicated that I(-) adsorption free energies decrease from pure water to pure methanol but not linearly with the concentration of methanol. We analyze the computed density profiles and hydration numbers as a function of concentrations and ion positions with respect to the interface to further explain the observed phenomenon.

Computational study of ion distributions at the air/liquid methanol interface.

Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl(2) at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems, in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions, which is part of the reason they are driven to air/aqueous interfaces, was compensated by the surfactant nature of methanol itself. These halide anions had on average an induced dipole of moderate magnitude in bulk methanol. As a consequence, methanol hydroxy groups donated hydrogen bonds to anions where the negatively charged side of the anion induced dipole pointed, and methyl groups interacted with anions where the positively charged side of the anion-induced dipole pointed. Furthermore, salts were found to disrupt the surface structure of methanol. For the neat air/liquid methanol interface, there is relative enhancement of methyl groups at the outer edge of the air/liquid methanol interface in comparison with hydroxy groups, but with the addition of NaX this enhancement was reduced somewhat. Finally, with the additional of salts to methanol, the computed surface potentials decreased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential increases with the addition of salts. Both of these trends have been indirectly observed with experiments. The surface potential trends were found to be due to the greater propensity of anions for the air/water interface that is not present at the air/liquid methanol interface.

Structure, dynamics and vibrational spectrum of supercritical CO2/H2O mixtures from ab initio molecular dynamics as a function of water cluster formation.

In this study, we investigate the effect of water-cluster formation in the supercritical (SC) systems CO(2)/(H(2)O)(n) as a function of water content using DFT-based molecular dynamics simulations. The dependence of the intermolecular and intramolecular structure and dynamic properties upon water concentration in the supercritical CO(2)/H(2)O phase at a density of 0.74 g cm(-3) and temperature of 318.15 K is investigated in detail and compared to previous studies of the pure sc-CO(2) system, single D(2)O in sc-CO(2), and Monte-Carlo simulations of a single water molecule in sc-CO(2) phase. Analysis of radial and orientational distribution functions of the intermolecular interactions shows that the presence of water molecules does not disturb the previously established distorted T-shaped orientation of CO(2) molecules, though there is evidence of perturbation of the second shell structure which enhances the preference for the slipped parallel orientation in this region. There is also evidence of short-lived hydrogen bonds between CO(2) and water molecules. For higher water concentrations, water clustering is observed, consistent with the low solubility of water in CO(2) under these conditions of temperature and pressure. Finally, the water-water and water-CO(2) interactions are discussed and analyzed in terms of the water self-association and thermodynamic quantities derived from the molecular dynamics simulations.

Structure and dynamics of N,N-diethyl-N-methylammonium triflate ionic liquid, neat and with water, from molecular dynamics simulations.

We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methylammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema](+) are found to diffuse slightly faster than the triflate anions [Tfl](-), and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs. We also examined the effects of water through characterization of properties of dema:Tfl-water mixtures. Water molecules replace counterions in the coordination shell of both ions, thus weakening their association. As water concentration increases, water molecules start to connect with each other and then form a large network that percolates through the system. Water influences ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motions of [dema](+) and [Tfl](-) are significantly enhanced. As a result, higher vehicular ionic conductivity is observed with increased hydration level.

The behavior of NaOH at the air-water interface: a computational study.

Molecular dynamics simulations with a polarizable multistate empirical valence-bond model were carried out to investigate NaOH dissociation and pairing in water bulk and at the air-water interface. It was found that NaOH readily dissociates in the bulk and the effect of the air-water interface on NaOH dissociation is fairly minor. Also, NaOH complexes were found to be strongly repelled from the air-water interface, which is consistent with surface tension measurements. At the same time, a very strong preference for the hydroxide anion to be oriented toward the air was found that persisted a few angstroms toward the liquid from the Gibbs dividing surface of the air-water interface. This was due to a preference for the hydroxide anion to have its hydrogen pointing toward the air and the fact that the sodium ion was more likely to be found near the hydroxide oxygen than hydrogen. As a consequence, the simulation results show that surfaces of NaOH solutions should be negatively charged, in agreement with experimental observations, but also that the hydroxide has little surface affinity. This provides the possibility that the surface of water can be devoid of hydroxide anions, but still have a strong negative charge.

Computational investigation of the influence of organic-aqueous interfaces on NaCl dissociation dynamics.

NaCl pairing and dissociation was investigated at the CCl(4)-water and 1,2-dichloroethane (DCE)-water interfaces, and compared with dissociation results in the bulk and at the air-water interface utilizing polarizable potentials. The transition path sampling methodology was used to calculate the rate constant for dissociation, while umbrella sampling was used to map out a free energy profile for NaCl dissociation. The results found that ion pairing was weakest at the organic-water interfaces, even weaker than in the water bulk. This is in contrast to what has been observed previously for the air-water interface, in which NaCl ion paring is stronger than in the bulk [C. D. Wick, J. Phys. Chem. C 113, 6356 (2009)]. A consequence of the weaker binding at the organic-water interfaces was that ion dissociation was faster than in the other systems studied. Interactions of the organic phase with the ions influenced the magnitude of the Cl(-) induced dipole moment, and at the organic-water interfaces, the average Cl(-) induced dipole was found to be lower than at the air-water interface, weakening interactions with Na(+). These weaker interactions were found to be responsible for the weaker ion pairing found at the organic-water interfaces.

Solvation of dimethyl succinate in a sodium hydroxide aqueous solution. A computational study.

Molecular dynamics simulations were carried out to study dimethyl succinate/water/NaOH solutions. The potential of mean force method was used to determine the transport mechanism of a dimethyl succinate (a diester) molecule across the aqueous/vapor interface. The computed number density profiles show a strong propensity for the diester molecules to congregate at the interface, with the solubility of the diester increasing with increasing NaOH concentration. It is observed that the major contribution to the interfacial solvation free-energy minimum is from electrostatic interactions. Even at higher NaOH concentrations, the increasing electrostatic interaction between the diester and ions is not large enough to favor the solvation of diester in bulk solutions. The calculated solvation free energies are found to be -2.6 to -3.5 kcal/mol in variant concentrations of NaOH aqueous solutions. These values are in qualitative agreement with the corresponding experimental measurements. The computed surface potential indicates that the contribution of diester molecules to the total surface potential is about 25%, with the major contribution from interfacial water molecules.

Computational studies of structures and dynamics of 1,3-dimethylimidazolim salt liquids and their interfaces using polarizable potential models.

The structures, thermodynamics, and dynamical properties of bulk and air/liquid interfaces of three ionic liquids, 1,3-dimethylimidazolium [dmim](+) with Cl(-), Br(-), and I(-) were studied using molecular dynamics techniques and polarizable potential models. In bulk melts, the radial distribution functions reveal a significant long-range structural correlation in these ionic liquids. The single-ion dynamics are studied via mean-square-displacements, velocity and orientational correlation functions. We observe that anion size plays an important role in the dynamics of ionic liquids, with larger anions inducing faster cation and anion motion. The computed density profiles of the ionic liquid/vapor interface exhibit oscillatory behavior, indicative of surface layering at the interface. The computed surface tensions indicate small differences between these ionic liquids and decrease with the increasing anion size. The magnitudes of the computed potential drops of these ionic liquids are found to be small and negative and increase with the decreasing anion size. These results could imply that the cation dipoles on average orient more in the interfacial plane than perpendicular to it. Our results showed that anion type plays a major role in determining IL interfacial behavior.

Investigating hydroxide anion interfacial activity by classical and multistate empirical valence bond molecular dynamics simulations.

Molecular dynamics simulations were carried out to understand the propensity of the hydroxide anion for the air-water interface. Two classes of molecular models were used, a classical polarizable model and a polarizable multistate empirical valence bond (MS-EVB) potential. The latter model was parametrized to reproduce the structures of small hydroxide-water clusters based on proton reaction coordinates. Furthermore, nuclear quantum effects were introduced into the MS-EVB model implicitly by refitting its potential energy function to account for them. The final MS-EVB model showed reasonable agreement with experiment and ab initio molecular dynamics simulations for dynamical and structural properties. The free-energy profiles for both the classical and MS-EVB models were mapped out across the air-water interface, and the classical model gave a higher free energy at the interface with respect to bulk. However, the MS-EVB model gave little free-energy difference between when the hydroxide anion was in the bulk and when it was present at the air-water interface with its oxygen fully solvated and its hydrogen pointing toward the vapor. When the hydroxide oxygen started to desolvate, the free energy increased dramatically, suggesting that the hydroxide anion can be found in the interfacial region.

Computational studies of load-dependent guest dynamics and free energies of inclusion for CO2 in low-density p-tert-butylcalix[4]arene at loadings up to 2:1.

The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) added beyond 1:1 loading shows a significant drop in absorption energy to -1.9 and +1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO(2) at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO(2) above 1:1 is less favorable.

Computational studies of aqueous interfaces of RbBr salt solutions.

We computed the structure factor and the corresponding x-ray reflectivity of an aqueous interface of RbBr salt solution and used molecular dynamics techniques to compare polarizable and nonpolarizable potential models for molecular interaction. Our computed electron and number density profiles clearly demonstrate that the polarizable Br(-) anions are enhanced at the water/vapor surface while the nonpolarizable Br(-) anions are depleted from it. The observation of Br(-) ions at the interface contradicts a recent interpretation that was based on experimentally measured x-ray reflectivity data.

Interfacial Water Features at Air-Water Interfaces as Influenced by Charged Surfactants.

The features of interfacial water at air-water interfaces of anionic sodium dodecyl sulfate (SDS) and cationic dodecyl amine hydrochloride (DDA) solutions were examined by combining sum frequency generation (SFG) vibrational spectroscopy measurements and molecular dynamics simulations (MDS). The SFG spectra revealed that interfacial water molecules for SDS solutions were highly ordered compared with those for DDA solutions. To elucidate this observation, in addition to agreement with the literature in regards to the interfacial electric field at the interfaces, we investigated the features of interfacial water molecules with respect to their network and their interaction with surfactant head groups. Our simulation analysis results revealed a higher number density, more strongly connected hydrogen bonding, and more orderly oriented interfacial water molecules at the interface of the SDS solutions as compared to the DDA solutions. The goal of this research is  to identify significant features of interfacial water for our improved understanding of such interfacial phenomena.

Microscopic Behaviors of Tri- n-Butyl Phosphate, n-Dodecane, and Their Mixtures at Air/Liquid and Liquid/Liquid Interfaces: An AMBER Polarizable Force Field Study.

In solvent extraction processes for recovering metal ions from used nuclear fuel, as well as other industrial applications, a better understanding of the metal complex phase transfer phenomenon would greatly aid ligand design and process optimization. We have approached this challenge by utilizing the classical molecular dynamics simulations technique to gain visual appreciation of the vapor/liquid and liquid/liquid interface between tri- n-butyl phosphate (TBP) and n-dodecane with air and water. In this study, we successfully reparameterized polarizable force fields for TBP and n-dodecane that accurately reproduced several of their thermophysical properties such as density, heat of vaporization, and dipole moment. Our models were able to predict the surface and interfacial tension of different systems when compared to experimental results that were also performed by us. Through this study, we gained atomistic understanding of the behaviors of TBP and n-dodecane at the interface against air and water, useful in further computational studies of such systems. Finally, our studies indicate that the initial configuration of a simulation may have a large effect on the final result.