Multistate Empirical Valence Bond Research Articles

Examination of the performance of degraded Nafion membrane with graphene oxide

The degradation of Nafion membrane easily occurs after a long run of proton exchange membrane fuel cell. Graphene-oxide (GO) is regarded as an ideal material to form composite membrane. In this work, a multistate empirical valence bond (MS-EVB) method and molecular dynamics method are employed to investigate the effect of GO on transport and solvation characteristics of protons in the degraded hydration membrane. The hydrophilic cluster and its connectivity are analyzed. The interaction between GO, Nafion membrane and water molecules is discussed. The influence of GO on the thermal conduction of Nafion membrane is also evaluated. The results demonstrate that with the addition of GO, the water cluster connectivity of Nafion membrane becomes better. The distribution of hydronium ions and membranes in the water is more uniform, which increases the proton conductivity. The larger GO may not interact well with water molecules, but promotes the thermal conductivity of degraded Nafion membrane more obviously.

Acidic Conditions Impact Hydrophobe Transfer across the Oil-Water Interface in Unusual Ways.

Molecular dynamics simulation and enhanced free energy sampling are used to study hydrophobic solute transfer across the water-oil interface with explicit consideration of the effect of different electrolytes: hydronium cation (hydrated excess proton) and sodium cation, both with chloride counterions (i.e., dissociated acid and salt, HCl and NaCl). With the Multistate Empirical Valence Bond (MS-EVB) methodology, we find that, surprisingly, hydronium can to a certain degree stabilize the hydrophobic solute, neopentane, in the aqueous phase and including at the oil-water interface. At the same time, the sodium cation tends to "salt out" the hydrophobic solute in the expected fashion. When it comes to the solvation structure of the hydrophobic solute in the acidic conditions, hydronium shows an affinity to the hydrophobic solute, as suggested by the radial distribution functions (RDFs). Upon consideration of this interfacial effect, we find that the solvation structure of the hydrophobic solute varies at different distances from the oil-liquid interface due to a competition between the bulk oil phase and the hydrophobic solute phase. Together with an observed orientational preference of the hydroniums and the lifetime of water molecules in the first solvation shell of neopentane, we conclude that hydronium stabilizes to a certain degree the dispersal of neopentane in the aqueous phase and eliminates any salting out effect in the acid solution; i.e., the hydronium acts like a surfactant. The present molecular dynamics study provides new insight into the hydrophobic solute transfer across the water-oil interface process, including for acid and salt solutions.

Molecular Origins of the Barriers to Proton Transport in Acidic Aqueous Solutions.

The self-consistent iterative multistate empirical valence bond (SCI-MS-EVB) method is used to analyze the structure, thermodynamics, and dynamics of hydrochloric acid solutions. The reorientation time scales of irreversible proton transport are elucidated by simulating 0.43, 0.85, 1.68, and 3.26 M HCl solutions at 270, 285, 300, 315, and 330 K. The results indicate increased counterion pairing with increasing concentration, which manifests itself via a reduced hydronium oxygen-chloride (O*-Cl) structuring in the radial distribution functions. Increasing ionic concentration also reduces the diffusion of the hydrated excess protons, principally by reducing the contribution of the Grotthuss proton hopping (shuttling) mechanism to the overall diffusion process. In agreement with prior experimental findings, a decrease in the activation energy of reorientation time scales was also observed, which is explicitly explained by using activated rate theory and an energy-entropy decomposition of the state-averaged radial distribution functions. These results provide atomistic verification of suggestions from recent two-dimensional infrared spectroscopy experiments that chloride anions (as opposed to hydrated excess protons) create entropic barriers to proton transport.

Proton Transport in [BMIM+][BF4-]/Water Mixtures Near the Percolation Threshold.

The incorporation of ionic liquids into existing proton exchange membrane (PEM) materials has been shown to enhance thermal stability and improve conductivity at reduced water content. Because proton transport is dictated by an interplay between vehicular diffusion and the Grotthuss mechanism, it is expected that the nanoscale structure of the resulting ionic liquid/water networks will sensitively influence transport properties. In this work, we study proton transport in [BMIM+][BF4-]/water mixtures of systematically varying water volume fraction, focusing on concentrations near the percolation threshold in which water networks are connected over macroscopic length scales. We utilize reactive molecular dynamics within the multistate empirical valence bond (MS-EVB) framework to explicitly model Grotthuss hopping processes. Excellent agreement with experimental conductivity data is obtained within the Nernst-Einstein approximation, indicating that proton transport proceeds in a largely uncorrelated manner even at pH <0. We additionally study the changing topology of the hydrogen-bonded water network in these mixtures using percolation and graph theory analysis. We find that the proton diffusion coefficient and forward hop rate increase linearly with water content at concentrations ranging from dilute through the percolation threshold; surprisingly, we find no deviation in this trend at the percolation transition. The high concentration of BF4- anions inherently alters the fraction of Eigen and Zundel proton states, producing a net detrimental effect on proton transport rates relative to bulk water. This mechanistic insight is useful for selecting ideal ionic liquid candidates and determining the optimal ionic liquid concentration to incorporate into PEM materials.

What Coordinate Best Describes the Affinity of the Hydrated Excess Proton for the Air-Water Interface?

Molecular dynamics simulations and free energy sampling are employed in this work to investigate the surface affinity of the hydrated excess proton with two definitions of the interface: The Gibbs dividing interface (GDI) and the Willard-Chandler interface (WCI). Both the multistate empirical valence bond (MS-EVB) reactive molecular dynamics method and the density functional theory-based ab initio molecular dynamics (AIMD) were used to describe the hydrated excess proton species, including "vehicular" (standard diffusion) transport and (Grotthuss) proton hopping transport and associated structures of the hydrated excess proton net positive charge defect. The excess proton is found to exhibit a similar trend and quantitative free energy behavior in terms of its surface affinity as a function of the GDI or WCI. Importantly, the definitions of the two interfaces in terms of the excess proton charge defect are highly correlated and far from independent of one another, thus undermining the argument that one interface is superior to the other when describing the proton interface affinity. Moreover, the hydrated excess proton and its solvation shell significantly influence the location and local curvature of the WCI, making it difficult to disentangle the interfacial thermodynamics of the excess proton from the influence of that species on the instantaneous surface curvature.

Proton Propensity and Orientation of Imidazolium Cation at Liquid Imidazole-Vacuum Interface: A Molecular Dynamics Simulation.

Imidazole has gained attention as an alternative to anhydrous proton conductor in high-temperature proton exchange membrane fuel cells. A detailed investigation of proton propensity and the orientation of the imidazolium cation at the liquid-vacuum interface is important for understanding the interfacial properties of imidazole-based proton-conductive materials. Here, we perform all-atom molecular dynamics simulation on a slab model of the liquid imidazole-vacuum interface. Proton transportation process between the imidazolium cation and neutral imidazole molecules is described by the multistate empirical valence bond model of imidazole developed previously. The imidazolium cation shows a tendency to stay in the bulk region rather than at the outermost surface, and the NN vectors and norm vectors of both the imidazolium cation and imidazole molecules are more probable to be perpendicular to the surface normal vector at the interface than in the bulk. The orientation of the hydrogen bond cluster shows the same tendency as the NN vectors, which indicates that proton transportation along the direction of the surface normal vector is hindered. The instantaneous surface analyses show that the fluctuation is depressed when the imidazolium cation is near the outermost surface, which makes it less favorable for the cation appearing at the interface.

Hydrogen Bond Dynamics in the Solvation Shell on Proton Transfer in Aqueous Solution.

Molecular dynamics (MD) simulations based on the multistate empirical valence bond model have been performed to study the proton transfer (PT) process in aqueous solution. This study focuses on the details of the hydrogen bond (HB) dynamics in the solvation shells of an excess proton accompanied by PT events. The HB dynamics analyses show that the three water molecules in the first solvation shell of hydronium (H3O+) tend to break their accepted HB to maintain a distorted Eigen (H9O4+) configuration. The results from MD simulations show that the cleavage and formation of accepted HBs on the water ligands in the first solvation shell of the proton acceptor water molecule and donor water molecule are crucial to drive the PT. Moreover, the water-donated and -accepted HBs around the H3O+ solvation shells are inequivalent, induced by the excess proton. Coupled with the PT, the donated HBs are enhanced on the proton acceptor side, while, in contrast, the accepted HBs are weakened on the same side.

High-Level VSCF/VCI Calculations Decode the Vibrational Spectrum of the Aqueous Proton

The hydrated excess proton is a common species in aqueous chemistry, which complexes with water in a variety of structures. The infrared spectrum of the aqueous proton is particularly sensitive to this array of structures, which manifests as continuous IR absorption from 1000 to 3000 cm-1 known as the "proton continuum". Because of the extreme breadth of the continuum and strong anharmonicity of the involved vibrational modes, this spectrum has eluded straightforward interpretation and simulation. Using protonated water hexamer clusters from reactive molecular dynamics trajectories, and focusing on their central H+(H2O)2 structures' spectral contribution, we reproduce the linear IR spectrum of the aqueous proton with a high-level local monomer quantum method and highly accurate many-body potential energy surface. The accuracy of this approach is first verified in the vibrational spectra of the two isomers of the protonated water hexamer in the gas phase. We then apply this approach to 800 H+(H2O)6 clusters, also written as [H+(H2O)2](H2O)4, drawn from multistate empirical valence bond simulations of the bulk liquid to calculate the infrared spectrum of the aqueous proton complex. Incorporation of anharmonic effects to the vibrational potential and quantum mechanical treatment of the proton produces a better agreement to the infrared spectrum compared to that of the double-harmonic approximation. We assess the correlation of the proton stretching mode with different atomistic coordinates, finding the best correlation with ⟨ROH⟩, the expectation value of the proton-oxygen distance ROH. We also decompose the IR spectrum based on normal mode vibrations and ⟨ROH⟩ to provide insight on how different frequency regions in the continuum report on different configurations, vibrational modes, and mode couplings.

Molecular Dynamics of Combustion Reactions in Supercritical Carbon Dioxide. Part 5: Computational Study of Ethane Dissociation and Recombination Reactions C2H6 ⇌ CH3 + CH3.

Fossil fuel oxy-combustion is an emerging technology where the habitual nitrogen diluent is replaced by high-pressure supercritical CO2 (sCO2), which increases the efficiency of energy conversion. In this study, the chemical kinetics of the combustion reaction C2H6 ⇌ CH3 + CH3 in the sCO2 environment is predicted at 30-1000 atm and 1000-2000 K. We adopt a multiscale approach, where the reactive complex is treated quantum mechanically in rigid rotor/harmonic oscillator approximation, while environment effects at different densities are taken into account by the potential of mean force, produced with classical molecular dynamics (MD). Here, we used boxed MD, where enhanced sampling of infrequent events of barrier crossing is accomplished without application of the bias potential. The multistate empirical valence bond model is applied to describe free radical formation accurately at the cost of the classical force field. Predicted rates at low densities agree well with the literature data. Rate constants at 300 atm are 2.41 × 1014 T-0.20 exp(-77.03 kcal/mol/ RT) 1/s for ethane dissociation and 8.44 × 10-19 T1.42 exp(19.89 kcal/mol/ RT) cm3/molecule/s for methyl-methyl recombination.

Proton Diffusion through Bilayer Pores.

The transport of protons through channels in complex environments is important in biology and materials science. In this work, we use multistate empirical valence bond simulations to study proton transport within a well-defined bilayer pore in a lamellar Lβ phase lyotropic liquid crystal (LLC). The LLC is formed from the self-assembly of dicarboxylate gemini surfactants in water, and a bilayer-spanning pore of radius of approximately 3-5 Å results from the uneven partitioning of surfactants between the two leaflets of the lamella. Local proton diffusion within the pore is significantly faster than diffusion at the bilayer surface, which is due to the greater hydrophobicity of the surfactant/water interface within the pore. Proton diffusion proceeds by surface transport along exposed hydrophobic pockets at the surfactant/water interface and depends on the continuity of hydronium-water hydrogen bond networks. At the bilayer surface, there is a reduced fraction of the "Zundel" intermediates that are central to the Grotthuss transport mechanism, whereas the fraction of these species within the bilayer pore is similar to that in bulk water. Our results demonstrate that the chemical nature of the confining interface, in addition to confinement length scale, is an important determiner of local proton transport in nanoconfined aqueous environments.

Restrained Proton Indicator in Combined Quantum-Mechanics/Molecular-Mechanics Dynamics Simulations of Proton Transfer through a Carbon Nanotube.

Recently, a collective variable "proton indicator" was purposed for tracking an excess proton solvated in bulk water in molecular dynamics simulations. In this work, we demonstrate the feasibility of utilizing the position of this proton indicator as a reaction coordinate to model an excess proton migrating through a hydrophobic carbon nanotube in combined quantum-mechanics/molecular-mechanics simulations. Our results indicate that applying a harmonic restraint to the proton indicator in the bulk solvent near the nanotube pore entrance leads to the recruitment of water molecules into the pore. This is consistent with an earlier study that employed a multistate empirical valence bond potential and a different representation (center of excess charge) of the proton. We attribute this water recruitment to the delocalized nature of the solvated proton, which prefers to be in high-dielectric bulk solvent. While water recruitment into the pore is considered an artifact in the present simulations (because of the artificially imposed restraint on the proton), if the proton were naturally restrained, it could assist in building water wires prior to proton transfer through the pore. The potential of mean force for a proton translocation through the water-filled pore was computed by umbrella sampling, where the bias potentials were applied to the proton indicator. The free energy curve and barrier heights agree reasonably with those in the literature. The results suggest that the proton indicator can be used as a reaction coordinate in simulations of proton transport in confined environments.

A comparison of sodium and hydrogen halides at the air-water interface.

New molecular models, parameterized to ab initio calculations, were developed to describe HBr and HI at the air-water interface. These were used to compare how the air-water interface influenced dissociation of NaX and HX, with X being Cl, Br, or I, and also their propensity for the interface. The polarizable multistate empirical valence bond method, which explicitly describes proton sharing, was used to model HX. Results showed that the air-water interface suppressed HX dissociation from a contact ion pair to a solvent separated to a greater degree than NaX dissociation. Furthermore, HX had a greater propensity for the interface than NaX, which was a consequence of the hydronium ion having a greatest interfacial activity of all species studied. As a consequence of this, the average configuration of dissociated HX, while in both contact ion and solvent separated ion pairs near the air-water interface, is with the dissociated hydrogen oriented more towards the air than the X atom.

IR spectral assignments for the hydrated excess proton in liquid water

The local environmental sensitivity of infrared (IR) spectroscopy to a hydrogen-bonding structure makes it a powerful tool for investigating the structure and dynamics of excess protons in water. Although of significant interest, the line broadening that results from the ultrafast evolution of different solvated proton-water structures makes the assignment of liquid-phase IR spectra a challenging task. In this work, we apply a normal mode analysis using density functional theory of thousands of proton-water clusters taken from reactive molecular dynamics trajectories of the latest generation multistate empirical valence bond proton model (MS-EVB 3.2). These calculations are used to obtain a vibrational density of states and IR spectral density, which are decomposed on the basis of solvated proton structure and the frequency dependent mode character. Decompositions are presented on the basis of the proton sharing parameter δ, often used to distinguish Eigen and Zundel species, the stretch and bend character of the modes, the mode delocalization, and the vibrational mode symmetry. We find there is a wide distribution of vibrational frequencies spanning 1200-3000 cm-1 for every local proton configuration, with the region 2000-2600 cm-1 being mostly governed by the distorted Eigen-like configuration. We find a continuous red shift of the special-pair O⋯H+⋯O stretching frequency, and an increase in the flanking water bending intensity with decreasing δ. Also, we find that the flanking water stretch mode of the Zundel-like species is strongly mixed with the flanking water bend, and the special pair proton oscillation band is strongly coupled with the bend modes of the central H5O2+moiety.

Comparing hydroxide and hydronium at the instantaneous air-water interface using polarizable multi-state empirical valence bond models

A new molecular model for the hydroxide anion was developed and its propensity for the air-water interface was compared with that of the hydronium ion. The multi-state empirical valence bond approach with polarizable potentials was used for both of these, allowing them to share their charge with adjacent water molecules. The hydroxide anion appeared to have a lower propensity for the air-water interface than that of hydronium when comparing its potential of mean force with respect to the Gibbs dividing surface. However, upon further inspection it was found that the hydroxide and hydronium ions had similar free energies at the instantaneous air-water interface. The reason for this discrepancy is that the hydroxide anion has a greater free energy when it was partially solvated. Or in other words, when the hydroxide anion is one molecular layer from the instantaneous air-water interface, but potentially still near the Gibbs dividing surface, it is less stable than the hydronium in a similar configuration. This shows that using the instantaneous interface can provide additional important insights.

Molecular modeling and assignment of IR spectra of the hydrated excess proton in isotopically dilute water.

Infrared (IR) spectroscopy of the water O-H stretch has been widely used to probe both the local hydrogen-bonding structure and dynamics of aqueous systems. Although of significant interest, the IR spectroscopy of excess protons in water remains difficult to assign as a result of extensive and strong intermolecular interactions in hydrated proton complexes. As an alternate approach, we develop a mixed quantum-classical model for the vibrational spectroscopy of the excess proton in isotopically dilute water that draws on frozen proton-water clusters taken from reactive molecular dynamics trajectories of the latest generation multi-state empirical valence bond proton model (MS-EVB 3.2). A semi-empirical single oscillator spectroscopic map for the instantaneous transition frequency and transition dipole moment is constructed using potential energy surfaces for the O-H stretch coordinate of the excess proton using electronic structure calculations. Calculated spectra are compared with experimental spectra of dilute H+ in D2O obtained from double-difference FTIR to demonstrate the validity of the map. The model is also used to decompose IR spectra into contributions from different aqueous proton configurations. We find that the O-H transition frequency continuously decreases as the oxygen-oxygen length for a special pair proton decreases, shifting from Eigen- to Zundel-like configurations. The same shift is accompanied by a shift of the flanking water stretches of the Zundel complex to higher frequency than the hydronium O-H vibrations.

Potentials of mean force in acidic proton transfer reactions in constrained geometries

Free energy barriers associated with the transfer of an excess proton in water and related to the potentials of mean force in proton transfer episodes have been computed in a wide range of thermodynamic states, from low-density amorphous ices to high-temperature liquids under the critical point for unconstrained and constrained systems. The latter were represented by set-ups placed inside hydrophobic graphene slabs at the nanometric scale allocating a few water layers, namely one or two in the narrowest case. Water–proton and carbon–proton forces were modelled with a Multi-State Empirical Valence Bond method. As a general trend, a competition between the effects of confinement and temperature is observed on the local hydrogen-bonded structures around the lone proton and, consequently, on the mean force exerted by its environment on the water molecule carrying the proton. Free energy barriers estimated from the computed potentials of mean force tend to rise with the combined effect of increasing temperatures and the packing effect due to a larger extent of hydrophobic confinement. The main reason observed for such enhancement of the free energy barriers was the breaking of the second coordination shell around the lone proton.

Positional Isotope Exchange in HX·(H2O)n (X = F, I) Clusters at Low Temperatures.

We present molecular dynamics simulation results describing proton/deuteron exchange equilibria along hydrogen bonds at the vicinity of HX acids (X = F, I) in aqueous clusters at low temperatures. To allow for an adequate description of proton transfer processes, our simulation scheme resorted on the implementation of a multistate empirical valence bond hamiltonian coupled to a path integral scheme to account for effects derived from nuclear quantum fluctuations. We focused attention on clusters comprising a number of water molecules close to the threshold values necessary to stabilize contact-ion-pairs. For X = F, our results reveal a clear propensity of the heavy isotope to lie at the bond bridging the halide to the nearest water molecule. Contrasting, for X = I, the thermodynamic stability is reversed and the former connectivity is preferentially articulated via the light isotope. These trends remain valid for undissociated and ionic descriptions of the stable valence bond states. The preferences are rationalized in terms of differences in the quantum kinetic energies of the isotopes which, in turn, reflect the extent of the local spatial confinements prevailing along the different hydrogen bonds in the clusters. In most cases, these features are also clearly reflected in the characteristics of the corresponding stretching bands of the simulated infrared spectra. This opens interesting possibilities to gauge the extent of the isotopic thermodynamic stabilizations and the strengths of the different hydrogen bonds by following the magnitudes and shifts of the spectral signals in temperature-controlled experiments, performed on mixed clusters combining H2O and HOD.

Multistate empirical valence bond study of temperature and confinement effects on proton transfer in water inside hydrophobic nanochannels.

Microscopic characteristics of an aqueous excess proton in a wide range of thermodynamic states, from low density amorphous ices (down to 100 K) to high temperature liquids under the critical point (up to 600 K), placed inside hydrophobic graphene slabs at the nanometric scale (with interplate distances between 3.1 and 0.7 nm wide) have been analyzed by means of molecular dynamics simulations. Water-proton and carbon-proton forces were modeled with a multistate empirical valence bond method. Densities between 0.07 and 0.02 Å(-3) have been considered. As a general trend, we observed a competition between effects of confinement and temperature on structure and dynamical properties of the lone proton. Confinement has strong influence on the local structure of the proton, whereas the main effect of temperature on proton properties is observed on its dynamics, with significant variation of proton transfer rates, proton diffusion coefficients, and characteristic frequencies of vibrational motions. Proton transfer is an activated process with energy barriers between 1 and 10 kJ/mol for both proton transfer and diffusion, depending of the temperature range considered and also on the interplate distance. Arrhenius-like behavior of the transfer rates and of proton diffusion are clearly observed for states above 100 K. Spectral densities of proton species indicated that in all states Zundel-like and Eigen-like complexes survive at some extent. © 2016 Wiley Periodicals, Inc.

Importance of hydrophobic traps for proton diffusion in lyotropic liquid crystals.

The diffusion of protons in self-assembled systems is potentially important for the design of efficient proton exchange membranes. In this work, we study proton dynamics in a low-water content, lamellar phase of a sodium-carboxylate gemini surfactant/water system using computer simulations. The hopping of protons via the Grotthuss mechanism is explicitly allowed through the multi-state empirical valence bond method. We find that the hydronium ion is trapped on the hydrophobic side of the surfactant-water interface, and proton diffusion then proceeds by hopping between surface sites. The importance of hydrophobic traps is surprising because one would expect the hydronium ions to be trapped at the charged headgroups. The physics illustrated in this system should be relevant to the proton dynamics in other amphiphilic membrane systems, whenever there exist exposed hydrophobic surface regions.

Role of Presolvation and Anharmonicity in Aqueous Phase Hydrated Proton Solvation and Transport

Results from condensed phase ab initio molecular dynamics (AIMD) simulations suggest a proton transfer reaction is facilitated by "presolvation" in which the hydronium is transiently solvated by four water molecules, similar to the typical solvation structure of water, by accepting a weak hydrogen bond from the fourth water molecule. A new version 3.2 multistate empirical valence bond (MS-EVB 3.2) model for the hydrated excess proton incorporating this presolvation behavior is therefore developed. The classical MS-EVB simulations show similar structural properties as those of the previous model but with significantly improved diffusive behavior. The inclusion of nuclear quantum effects in the MS-EVB also provides an even better description of the proton diffusion rate. To quantify the influence of anharmonicity, a second model (aMS-EVB 3.2) is developed using the anharmonic aSPC/Fw water model, which provides similar structural properties but improved spectroscopic responses at high frequencies.