Uranyl Speciation in Carbonate-Rich Hydrothermal Solutions: A Molecular Dynamics Study

In this study, we employed classical molecular dynamics (CMD) and first-principles molecular dynamics (FPMD) simulations to investigate the speciation of uranyl in carbonate-rich hydrothermal solutions. The association constants (log10KA) of uranyl carbonate complexes were derived from the potential of mean forces (PMFs) obtained from CMD simulations, and the acid constants (pKas) of uranyl aqua ions were calculated using the FPMD-based vertical energy gap method. The results showed that uranyl ions could form stable mono- and bi-carbonate complexes at elevated temperatures and that uranyl aqua ions strongly hydrolyzed in neutral solutions at temperatures exceeding 473 K. The speciation of uranyl in hydrothermal solutions was constructed based on the calculated thermodynamics data. It was found that uranyl carbonate complexes were predominant in aqueous solutions at temperatures below 373 K, and at higher temperatures, UO2(OH)2/UO2(OH)+ became the predominant species. These findings provide molecular-level insight into the speciation of uranyl in hydrothermal solutions and highlight the role of uranyl hydroxides in the transport and deposition of uranium in hydrothermal processes.

Quantum Chemical Insights into DNA Nucleobase Oxidation: Bridging Theory and Experiment

The oxidation free energies of DNA nucleobases in aqueous solution are still matter of extensive discussion because of the contrasting results reported so far. With the aim of settling a longstanding debate about the oxidation potentials of DNA constituents, herein we report the results of state-of-the-art DFT-based molecular dynamics simulations, in which the whole solvent environment is modeled at the atomistic level, by using DFT supercell calculations, with periodic boundary conditions. Calculated vertical ionization energies are very close to those observed by photoelectron spectroscopy both in the gas phase and in solution. One-electron oxidation free energies in aqueous solution agree well with the results of differential pulse voltammetry measurements and with those inferred by photoelectron spectroscopy with the aid of theoretical computations to estimate vibrational relaxation.

Molecular picture of electric double layers with weakly adsorbed water

Water adsorption energy, Eads, is a key physical quantity in sustainable chemical technologies such as (photo)electrocatalytic water splitting, water desalination, and water harvesting. In many of these applications, the electrode surface is operated outside the point (potential) of zero charge, which attracts counter-ions to form the electric double layer and controls the surface properties. Here, by applying density functional theory-based finite-field molecular dynamics simulations, we have studied the effect of water adsorption energy Eads on surface acidity and the Helmholtz capacitance of BiVO4 as an example of metal oxide electrodes with weakly chemisorbed water. This allows us to establish the effect of Eads on the coordination number, the H-bond network, and the orientation of chemisorbed water by comparing an oxide series composed of BiVO4, TiO2, and SnO2. In particular, it is found that a positive correlation exists between the degree of asymmetry ΔCH in the Helmholtz capacitance and the strength of Eads. This correlation is verified and extended further to graphene-like systems with physisorbed water, where the electric double layers (EDLs) are controlled by electronic charge rather than proton charge as in the oxide series. Therefore, this work reveals a general relationship between water adsorption energy Eads and EDLs, which is relevant to both electrochemical reactivity and the electrowetting of aqueous interfaces.

Physical adsorption of OH- causes anomalous charging at oxide-water interfaces

This study reveals a charging mechanism at oxide-water interfaces, solving the puzzle that challenges the traditional electrical double layer (EDL) model. We found that the experimentally measured zeta potential is caused by physically adsorbed OH-, instead of acidic dissociation of surface OHs and the first-layer water. This mechanism should apply for a wide range of material interfaces and could find applications in future.

Variations in proton transfer pathways and energetics on pristine and defect-rich quartz surfaces in water: Insights into the bimodal acidities of quartz

HYPOTHESIS

Understanding the mechanisms of proton transfer on quartz surfaces in water is critical for a range of processes in geochemical, environmental, and materials sciences. The wide range of surface acidities (>9 pKa units) found on the ubiquitous mineral quartz is caused by the structural variations of surface silanol groups. Molecular scale simulations provide essential tools for elucidating the origin of site-specific surface acidities.

SIMULATIONS

We used density-functional tight-binding-based molecular dynamics combined with rare-event metadynamics simulations to probe the mechanisms of deprotonation reactions from ten representative surface silanol groups found on both pristine and defect-rich quartz (101) surfaces with Si vacancies.

FINDINGS

The results show that deprotonation is a highly dynamic process where both the surface hydroxyls and bridging oxygen atoms serve as the proton acceptors, in addition to water. Deprotonation of embedded silanols through intrasurface proton transfer exhibited lower pKa values with less H-bond participation and higher energy barriers, suggesting a new mechanism to explain the bimodal acidity observed on quartz surface. Defect sites, recently shown to comprise a significant portion of the quartz (101) surface, diversify the coordination and local H-bonding environments of the surface silanols, changing both the deprotonation pathways and energetics, leading to a wider range of pKa values (2.4 to 11.5) than that observed on pristine quartz surface (10.4 and 12.1).

Accelerating Computation of Acidity Constants and Redox Potentials for Aqueous Organic Redox Flow Batteries by Machine Learning Potential-Based Molecular Dynamics

Due to the increased concern about energy and environmental issues, significant attention has been paid to the development of large-scale energy storage devices to facilitate the utilization of clean energy sources. The redox flow battery (RFB) is one of the most promising systems. Recently, the high cost of transition-metal complex-based RFB has promoted the development of aqueous RFBs with redox-active organic molecules. To expand the working voltage, computational chemistry has been applied to search for organic molecules with lower or higher redox potentials. However, redox potential computation based on implicit solvation models would be challenging due to difficulty in parametrization when considering the complex solvation of supporting electrolytes. Besides, although ab initio molecular dynamics (AIMD) describes the supporting electrolytes with the same level of electronic structure theory as the redox couple, the application is impeded by the high computation costs. Recently, machine learning molecular dynamics (MLMD) has been illustrated to accelerate AIMD by several orders of magnitude without sacrificing the accuracy. It has been established that redox potentials can be computed by MLMD with two separated machine learning potentials (MLPs) for reactant and product states, which is redundant and inefficient. In this work, an automated workflow is developed to construct a universal MLP for both states, which can compute the redox potentials or acidity constants of redox-active organic molecules more efficiently. Furthermore, the predicted redox potentials can be evaluated at the hybrid functional level with much lower costs, which would facilitate the design of aqueous organic RFBs.

Probing the self-ionization of liquid water with ab initio deep potential molecular dynamics

The chemical equilibrium between self-ionized and molecular water dictates the acid-base chemistry in aqueous solutions, yet understanding the microscopic mechanisms of water self-ionization remains experimentally and computationally challenging. Herein, Density Functional Theory (DFT)-based deep neural network (DNN) potentials are combined with enhanced sampling techniques and a global acid-base collective variable to perform extensive atomistic simulations of water self-ionization for model systems of increasing size. The explicit inclusion of long-range electrostatic interactions in the DNN potential is found to be crucial to accurately reproduce the DFT free energy profile of solvated water ion pairs in small (64 and 128 H2O) cells. The reversible work to separate the hydroxide and hydronium to a distance [Formula: see text] is found to converge for simulation cells containing more than 500 H2O, and a distance of [Formula: see text] 8 Å is the threshold beyond which the work to further separate the two ions becomes approximately zero. The slow convergence of the potential of mean force with system size is related to a restructuring of water and an increase of the local order around the water ions. Calculation of the dissociation equilibrium constant illustrates the key role of long-range electrostatics and entropic effects in the water autoionization process.

Protonation of Strained Epoxy Resin under Wet Conditions via First-Principles Calculations Using the H+-Shift Method

A significant challenge in adhesive bonding is the accelerated breaking of stretched adhesives under wet conditions, which is known as cohesive failure. One group of commonly used adhesives consists of the amine-cured epoxy resins. Based on deprotonation free-energy calculations of the unstrained resin in water, it has recently been proposed that these adhesives can undergo failure through breakage originating at the protonated amine group under neutral or acidic conditions [J. Phys. Chem. B 2021, 125, 8989-8996]. In this study, we comprehensively investigated the degree of protonation of the amine group under both stretched and compressed conditions by devising a robust first-principles protonation calculation method applicable to strained materials. It was found that the amine group was partially protonated in neutral water at 298 K and that the amine group was protonated when the epoxy resin was stretched to a greater extent in water, and vice versa. These findings support the physicochemical cause of cohesive failure due to protonation of the amine group in the stretched amine-cured epoxy resins.

Switching of Redox Levels Leads to High Reductive Stability in Water-in-Salt Electrolytes

Developing nonflammable electrolytes with wide electrochemical windows is of great importance for energy storage devices. Water-in-salt electrolytes (WiSE) have attracted great interests due to their widely opened electrochemical windows and high stability. Previous theoretical investigations have revealed changes in solvation shell of water molecules result in opening of HOMO-LUMO gaps of water, leading to the formation of an anion-derived solid-electrolyte-interphase (SEI) in WiSE. However, how solvation structures affect electrochemical windows at atomic level is still a puzzle, which hinders optimization and design of aqueous electrolytes. Herein, machine learning molecular dynamics and free energy calculation method are applied to compute redox potentials of anions of Li-salts and water of aqueous electrolytes at a range of salt concentrations. Furthermore, an analysis based on local solvation structures is employed to demonstrate the structure-property relations. Our calculation shows that the hydrogen evolution reaction is impeded in WiSE due to switching of the order of redox levels of the anion and H2O, leading to formation of SEI and high reductive stability. Level switching is caused by the special solvation environments of isolated water molecules. Our work provides new insight into the electrochemistry of aqueous electrolytes which would benefit the electrolyte design in energy storage devices.

Norm-Conserving 4f-in-Core Pseudopotentials and Basis Sets Optimized for Trivalent Lanthanides (Ln = Ce-Lu)

We present here a set of scalar-relativistic norm-conserving 4f-in-core pseudopotentials, together with complementary valence-shell Gaussian basis sets, for the lanthanide (Ln) series (Ce-Lu). The Goedecker, Teter, and Hutter (GTH) formalism is adopted with the generalized gradient approximation (GGA) for the exchange-correlation Perdew-Burke-Ernzerhof (PBE) functional. The 4f-in-core pseudopotentials are built through attributing 4f-subconfiguration 4fⁿ (n = 1-14) for Ln (Ln = Ce-Lu) into the atomic core region, making it possible to circumvent the difficulty of the description of the open 4fⁿ valence shell. A wide variety of computational benchmarks and tests have been carried out on lanthanide systems including Ln³⁺-containing molecular complexes, aqueous solutions, and bulk solids to validate the accuracy, reliability, and efficiency of the optimized 4f-in-core GTH pseudopotentials and basis sets. The 4f-in-core GTH pseudopotentials successfully replicate the main features of lanthanide structural chemistry and reaction energetics, particularly for nonredox reactions. The chemical bonding features and solvation shells, hydrolysis energetics, acidity constants, and solid-state properties of selected lanthanide systems are also discussed in detail by utilizing these new 4f-in-core GTH pseudopotentials. This work bridges the idea of keeping highly localized 4f electrons in the atomic core and efficient pseudopotential formalism of GTH, thus providing a highly efficient approach for studying lanthanide chemistry in multi-scale modeling of constituent-wise and structurally complicated systems, including electronic structures of the condensed phase and first-principles molecular dynamics simulations.

Unraveling the origin of reductive stability of super-concentrated electrolytes from first principles and unsupervised machine learning

Developing electrolytes with excellent electrochemical stability is critical for next-generation rechargeable batteries. Super-concentrated electrolytes (SCEs) have attracted great interest due to their high electrochemical performances and stability. Previous studies have revealed changes in solvation structures and shifts in lowest unoccupied molecular orbitals from solvents to anions, promoting the formation of an anion-derived solid-electrolyte-interphase (SEI) in SCE. However, a direct connection at the atomic level to electrochemical properties is still missing, hindering the rational optimization of electrolytes. Herein, we combine ab initio molecular dynamics with the free energy calculation method to compute redox potentials of propylene carbonate electrolytes at a range of LiTFSI concentrations, and moreover employ an unsupervised machine learning model with a local structure descriptor to establish the structure-property relations. Our calculation indicates that the network of TFSI- in SCE not only helps stabilize the added electron and renders the anion more prone to reductive decomposition, but also impedes the solvation of F- and favors LiF precipitation, together leading to effective formation of protective SEI layers. Our work provides new insights into the solvation structures and electrochemistry of concentrated electrolytes which are essential to electrolyte design in batteries.

Concentration dependent interfacial chemistry of the NaOH(aq): gibbsite interface

Caustic conditions are often employed for dissolution of a wide variety of minerals, where ion sorption, surface diffusion, and interfacial organization impact surface reactivity. In the case of gibbsite, γ-Al(OH)₃, the chemistry at the NaOH_(aq) interface is deeply intertwined with industrial processing of aluminum, including metal production and the disposition of Al-containing wastes. To date, little is known about the structure, speciation, and dynamic behavior of gibbsite interfaces (and that of many other minerals) with NaOH_(aq)-particularly as a function of ionic strength. Yet concentration-dependent interfacial organization and dynamics are a critical starting point to develop a fundamental understanding of the factors that influence dissolution. This work reports equilibrium molecular dynamics simulations of the γ-Al(OH)₃:NaOH_(aq) interface, revealing the sorption behavior and speciation of ions from 0.5-10 M [NaOH]. As inner-sphere complexes, Na⁺ primarily coordinates to the side of the gibbsite hexagonal cavities, while OH^- accepts hydrogen-bonding from the surface-OH groups. The mobility of inner-sphere Na⁺ and OH^- ions is significantly reduced due to a strong surface affinity in comparison to previous reports of NaCl, CaCl₂, or BaCl₂ electrolytes. At high [NaOH], contact ion pairing that is observed in the bulk solution is partially disrupted upon sorption to the gibbsite surface by the individual ion-surface interactions. The molecular-scale changes to surface speciation and competition between ion-surface vs. ion-ion interactions influence surface characterization of gibbsite and potential dissolution processes, providing a valuable baseline for starting conditions needed within future reactive molecular simulations.

Automated Workflow for Computation of Redox Potentials, Acidity Constants and Solvation Free Energies Accelerated by Machine Learning

Fast evolution of modern society stimulates intense development of new materials with novel functionalities in energy and environmental applications. Due to rapid progress of computer science, computational design of materials with target properties has recently attracted lots of interests. Accurate and efficient calculation of fundamental thermodynamic properties, including redox potentials, acidity constants, and solvation free energies, is of great importance for selection and design of desirable materials. Free energy calculation based on ab initio molecular dynamics (AIMD) can predict these properties with high accuracy at complex environments, however being impeded by high computational costs. To address this issue, this work develops an automated scheme that combines iterative training of machine learning potentials (MLPs) and free energy calculation, and deomonstrates that these thermodynamic properties can be computed by ML accelerated MD with ab initio accuracy and much longer time scale at cheaper costs, improving poor statistics and convergence of numerical integration by AIMD. Our automated scheme lays the foundation for computational chemistry-assisted materials design.

pH dependent reactivity of boehmite surfaces from first principles molecular dynamics

pH dependent interfacial chemistry depends upon the distribution and respective pK_a values of different surface active sites. This is highly relevant to the chemistry of nanoparticle morphologies that expose faces with varying surface termination. Recent synthetic advances for nanoparticles of various minerals, including AlO(OH) (boehmite), present an excellent opportunity to compare and contrast predicted surface pK_a on low Miller index planes so as to reinterpret reported interfacial properties (i.e., point of zero charge - PZC) and reactivity. This work employs ab initio molecular dynamics and empirical models to predict site-specific pK_a values of accurate (benchmarked) surface models of boehmite. Using the different surface site populations, the PZC is determined and the influence this has upon reported interfacial chemistry is described.

Lower degree of dissociation of pyruvic acid at water surfaces than in bulk

Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface.

Surface Acidity and As(V) Complexation of Iron Oxyhydroxides: Insights from First-Principles Molecular Dynamics Simulations

Iron hydroxides are ubiquitous in soils and aquifers and have been adopted as adsorbents for As(V) removal. However, the complexation mechanisms of As(V) have not been well understood due to the lack of information on the reactive sites and acidities of iron hydroxides. In this work, we first calculated the acidity constants (pK_as) of surface groups on lepidocrocite (010), (001), and (100) surfaces by using the first-principles molecular dynamics (FPMD)-based vertical energy gap method. Then, the desorption free energies of As(V) on goethite (110) and lepidocrocite (001) surfaces were calculated by using constrained FPMD simulations. The point of zero charges and reactive sites of individual surfaces were obtained based on the calculated pK_as. The structures, thermodynamics, and pH dependence for As(V) complexation were derived by integrating the pK_as and desorption free energies. The pK_a data sets obtained are fundamental parameters that control the charging and adsorption behavior of iron oxyhydroxides and will be very useful in investigating the adsorption processes on these minerals. The pH-dependent complexation mechanisms of As(V) derived in this study would be helpful for the development of effective adsorbent materials and the prediction of the long-term behavior of As(V) in natural environments.

The energetics of electron and proton transfer to CO2 in aqueous solution

The electrocatalytic reduction of CO₂ is considered an effective method to reduce CO₂ emissions and achieve electrical/chemical energy conversion. It is crucial to determine the reaction mechanism so that the key reaction intermediates can be targeted and the overpotential lowered. The process involves the interaction with the electrode surface and with species, including the solvent, at the electrode-electrolyte interface, and it is therefore not easy to separate catalytic contributions of the electrode from those of the electrolyte. We have used density functional theory-based molecular dynamics to calculate the Gibbs free energy of the proton and electron transfer reactions corresponding to each step in the electroreduction of CO₂ to HCOOH in aqueous media. The results show thermodynamic pathways consistent with the mechanism proposed by Hori. Since electrodes are not included in this work, differences between the calculated results and the experimental observations can help determine the catalytic contribution of the electrode surface.

First-Principles Calculations of the Protonation and Weakening of Epoxy Resin under Wet Conditions

In this study, we investigated the protonation of the amine group in epoxy resins prepared using amine-based curing agents by theoretical methods. Density functional theory (DFT)-based free-energy calculations of the corresponding deprotonation subreactions showed that the amine group of the epoxy resin is protonated at equilibrium depending on the location of the amine group when the epoxy resin is embedded in water under standard conditions. Additional DFT calculations demonstrate that the energetic barrier for breaking the ether bond of the epoxy resin is lowered by about 0.6 eV as a result of the cooperative effect of H₂O dissociation and that the transition-state energy for breaking the amine group bond is lowered by about 0.4 eV after the protonation of the amine group. Comparing the transition-state energies, we predict that the bond breakage of the protonated amine groups is the principal process causing the weakening of epoxy resins under wet conditions.

An Atomic-Scale Understanding of the Solution Chemistry of Antimony(V): Insights from First-Principles Molecular Dynamics Simulation

In this study, the structure, hydrolysis, and complexation of Sb(V) in aqueous solution has been elucidated by using first-principles molecular dynamics (FPMD) simulations. The results show that both antimonic acid and its deprotonated form have an octahedral configuration, with average Sb-OH₂ and Sb-OH distances of 2.25 and 2.05 Å, respectively. The computed pK_a of [Sb(OH)₅(OH₂)] is 1.8, while [Sb(OH)₆]^- has an extremely high pK_a. Consequently, [Sb(OH)₆]^- is the most dominant species of Sb(V) under common environmental conditions. A stable aqueous complex can form between [Sb(OH)₆]^- and common cations, and an Sb-Al bidentate complex has the largest dissociation free energy, followed by a Sb-Mg bidentate complex, indicating that they have significantly higher stabilities. For Na⁺ and Ca²⁺, their respective monodentate and bidentate complexes have similar dissociation free energies, indicating very close possibilities. These findings provide a comprehensive understanding of the solution chemistry of Sb(V) from a quantitative and microscopic perspective.

Computing Surface Acidity Constants of Proton Hopping Groups from Density Functional Theory-Based Molecular Dynamics: Application to the SnO2(110)/H2O Interface

Proton transfer at metal oxide/water interfaces plays an important role in electrochemistry, geochemistry, and environmental science. The key thermodynamic quantity to characterize this process is the surface acidity constant. An ab initio method that combines density functional theory-based molecular dynamics (DFTMD) and free energy perturbation theory has been established for computing surface acidity constants. However, it involves a reversible proton insertion procedure in which frequent proton hopping, e.g., for strong bases and some oxide surfaces (e.g., SnO₂), can cause instability issues in electronic structure calculation. In the original implementation, harmonic restraining potentials are imposed on all O-H bonds (denoted by the V_rH scheme) to prevent proton hopping and thus may not be applicable for systems involving spontaneous proton hopping. In this work, we introduce an improved restraining scheme with a repulsive potential V_rep to compute the surface acidities of systems in which proton hopping is spontaneous and fast. In this V_rep scheme, a Buckingham-type repulsive potential V_rep is applied between the deprotonation site and all other protons in DFTMD simulations. We first verify the V_rep scheme by calculating the pK_a values of H₂O and aqueous HS^- solution (i.e., strong conjugate bases) and then apply it to the SnO₂(110)/H₂O interface. It is found that the V_rep scheme leads to a prediction of the point of zero charge (PZC) of 4.6, which agrees well with experiment. The intrinsic individual pK_a values of the terminal five-coordinated Sn site (Sn_5cOH₂) and bridge oxygen site (Sn₂O_brH⁺) are 4.4 and 4.7, respectively, both being almost the same as the PZC. The similarity of the two pK_a values indicates that dissociation of terminal water has almost zero free energy at this proton hopping interface (i.e., partial water dissociation), as expected from the acid-base equilibrium on SnO₂.

Frontiers in molecular simulation of solvated ions, molecules and interfaces

This themed collection is a collection of articles on frontiers in molecular simulation of solvated ions, molecules and interfaces.

Rapid and accurate molecular deprotonation energies from quantum alchemy

We assess the applicability of alchemical perturbation density functional theory (APDFT) for quickly and accurately estimating deprotonation energies. We have considered all possible single and double deprotonations in one hundred small organic molecules drawn at random from QM9 [Ramakrishnan et al., JCTC, 2015]. Numerical evidence is presented for 5160 deprotonated species at both HF/def2-TZVP and CCSD/6-31G* levels of theory. We show that the perturbation expansion formalism of APDFT quickly converges to reliable results: using CCSD electron densities and derivatives, regular Hartree-Fock calculations are outperformed at the second or third order for ranking all possible doubly or singly deprotonated molecules, respectively. CCSD single deprotonation energies are reproduced within 1.4 kcal mol^-1 on average within third order APDFT. We introduce a hybrid approach where the computational cost of APDFT is reduced even further by mixing first order terms at a higher level of theory (CCSD) with higher order terms at a lower level of theory only (HF). We find that this approach reaches 2 kcal mol^-1 accuracy in absolute deprotonation energies compared to CCSD at 2% of the computational cost of third order APDFT.

First-Principles Calculation of Water pKa Using the Newly Developed SCAN Functional

Acid/base chemistry is an intriguing topic that still constitutes a challenge for computational chemistry. While estimating the acid dissociation constant (or pK_a) could shed light on many chemistry processes, especially in the fields of biochemistry and geochemistry, evaluating the relative stability between protonated and nonprotonated species is often very difficult. Indeed, a prerequisite for calculating the pK_a of any molecule is an accurate description of the energetics of water dissociation. Here, we applied constrained molecular dynamics simulations, a noncanonical sampling technique, to investigate the water deprotonation process by selecting the OH distance as the reaction coordinate. The calculation is based on density functional theory and the newly developed SCAN functional, which has shown excellent performance in describing water structure. This first benchmark of SCAN on a chemical reaction shows that this functional accurately models the energetics of proton transfer reactions in an aqueous environment. After taking Coulomb long-range corrections and nuclear quantum effects into account, the estimated water pK_a is only 1.0 pK_a unit different from the target experimental value. Our results show that the combination of SCAN and constrained MD successfully reproduces the chemistry of water and constitutes a good framework for calculating the free energy of chemical reactions of interest.

MUSIC Speciation of γ-Al2O3 at the Solid Liquid Interface: How DFT Calculations Can Help with Amorphous and Poorly Crystalline Materials

Transition aluminum oxides, such as γ-Al₂O₃ or alumina, are widely used in many different technical applications that rely on the surface reactivity of this material at the solid liquid interface. The speciation of surface sites of this material confronts several obstacles. On the one hand, alumina is a poorly crystalline oxide, thus allowing for a limited amount of empirical structural information for an important number of surface sites with different trends in reactivity and, on the other hand, it is a metastable material. In this work, we show several ways in which the multisite complexation model, combined with atomistic information from density functional theory and ab initio molecular dynamics (AIMD), can manage to perform speciation calculations of γ-Al₂O₃ surface sites at the solid liquid interface. Although the results are in good qualitative agreement with experimental titration curves, and they can serve as a guide for the interpretation of the reactivity of this material at the initial stages of an impregnation experiment and chemical weathering phenomena, this work highlights the need of more complex AIMD simulations to accurately model these phenomena in γ-Al₂O₃ surface/liquid interfaces.

Determination of pKa Values via ab initio Molecular Dynamics and its Application to Transition Metal-Based Water Oxidation Catalysts

The p K a values are important for the in-depth elucidation of catalytic processes, the computational determination of which has been challenging. The first simulation protocols employing ab initio molecular dynamics simulations to calculate p K a values appeared almost two decades ago. Since then several slightly different methods have been proposed. We compare the performance of various evaluation methods in order to determine the most reliable protocol when it comes to simulate p K a values of transition metal-based complexes, such as the here investigated Ru-based water oxidation catalysts. The latter are of high interest for sustainable solar-light driven water splitting, and understanding of the underlying reaction mechanism is crucial for their further development.

The multiple dissociation constants of glutathione disulfide: interpreting experimental pH-titration curves with ab initio MD simulations

The hexapeptide glutathione disulfide (GSSG) has six ionizable groups with six associated dissociation constants. The experimentally measured pH-titration curve, however, does not exhibit the six corresponding equivalence points and bears little resemblance to standard textbook examples of acid-base pH-titration curves. The curve highlights the difficulties in determining multiple pKa values of polyprotic acids - typically proteins and peptides - from experiment. The six pKa values of GSSG can, however, be estimated using Car-Parrinello molecular dynamics (CPMD) simulations in conjunction with metadynamics sampling of the underlying free energy landscape of the dissociation reactions. Ab initio MD simulations were performed on a GSSG molecule solvated by 200 water molecules. Using the estimated pKa values the theoretical titration curve was calculated and found to be in good agreement with experiment. The results clearly highlight how dissociation constants estimated from ab initio MD simulations can facilitate the interpretation of the pH-titration curves of complex chemical and biological systems.

Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions Using Theoretical Calculations and Raman Spectroscopy

The composition and structures of the two protonated species formed from uncharged molybdic acid, MoO₂(OH)₂(OH₂)₂⁰, in strongly acidic solutions have been investigated using a combination of density functional theory calculations, first-principles molecular dynamics simulations, and Raman spectroscopy. The calculations show that both protonated species maintain the original octahedral structure of molybdic acid. Computed p K_a values indicated that the ═O moieties are the proton acceptor sites and, therefore, that MoO(OH)₃(OH₂)₂⁺ and Mo(OH)₄(OH₂)₂²⁺ are the probable protonated forms of Mo(VI) in strong acid solutions, rather than the previously accepted MoO₂(OH)_{2- x}(OH₂)_{2+ x} ^x+ ( x = 1, 2) species. This finding is shown to be broadly consistent with the observed Raman spectra. Structural details of MoO(OH)₃(OH₂)₂⁺ and Mo(OH)₄(OH₂)₂²⁺ are reported.

Mechanistic Understanding of Uranyl Ion Complexation on Montmorillonite Edges: A Combined First-Principles Molecular Dynamics-Surface Complexation Modeling Approach

Systematic first-principles molecular dynamics (FPMD) simulations were carried out to study the structures, free energies, and acidity constants of UO₂²⁺ surface complexes on montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion (UO₂²⁺) on clay mineral edges at the atomic scale. Four representative complexing sites were investigated, that is, ≡Al(OH)₂ and ≡AlOHSiO on the (010) surface and ≡AlOHOa and ≡SiOOa on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites. All calculated binding free energies for these complexes are very similar. These bidentate complexes can be hydrolyzed, and their corresponding derived p K_a values (around 5.0 and 9.0 for p K_a1 and p K_a2, respectively) indicate that UO₂(OH)⁺ and UO₂(OH)₂ surface groups are the dominant surface species in the environmental pH range. The OH groups of UO₂(OH)₂ surface complexes can act as complexing sites for subsequent metals. Additional simulations showed that such multinuclear adsorption is feasible and can be important at high pH. Furthermore, FPMD simulation results served as input parameters for an electrostatic thermodynamic surface complexation model (SCM) that adequately reproduced adsorption data from the literature. Overall, this study provides an improved understanding of UO₂²⁺ complexation on clay mineral edge surfaces.

Atypical titration curves for GaAl12 Keggin-ions explained by a joint experimental and simulation approach

Although they have been widely used as models for oxide surfaces, the deprotonation behaviors of the Keggin-ions (MeAl₁₂⁷⁺) and typical oxide surfaces are very different. On Keggin-ions, the deprotonation occurs over a very narrow pH range at odds with the broad charging curve of larger oxide surfaces. Depending on the Me concentration, the deprotonation curve levels off sooner (high Me concentration) or later (for low Me concentration). The leveling off shows the onset of aggregation before which the Keggin-ions are present as individual units. We show that the atypical titration data previously observed for some GaAl₁₂ solutions in comparison to the originally reported data can be explained by the presence of Ga₂Al₁₁ ions. The pK_a value of aquo-groups bound to octahedral Ga was determined from ab initio molecular dynamics simulations relative to the pure GaAl₁₂ ions. Using these results within a surface complexation model, the onset of deprotonation of the crude solution is surprisingly well predicted and the ratio between the different species is estimated to be in the proportion 20 (Ga₂Al₁₁) : 20 (Al₁₃) : 60 (GaAl₁₂).

Uranyl Arsenate Complexes in Aqueous Solution: Insights from First-Principles Molecular Dynamics Simulations

In this study, the structures and acidity constants (p K_a's) of uranyl arsenate complexes in solutions have been revealed by using the first principle molecular dynamics technique. The results show that uranyl and arsenate form stable complexes with the U/As ratios of 1:1 and 1:2, and the bidentate complexation between U and As is highly favored. Speciation-pH distributions are derived based on free energy and p K_a calculations, which indicate that for the 1:1 species, UO₂(H₂AsO₄)(H₂O)₃⁺ is the major species at pH < 7, while UO₂(HAsO₄)(H₂O)₃⁰ and UO₂(AsO₄)(H₂O)₃^- dominate in acid-to-alkaline and extreme alkaline pH ranges. For the 1:2 species, UO₂(H₂AsO₄)₂(H₂O)⁰ is dominant under acid-to-neutral pH conditions, while UO₂(HAsO₄)(H₂AsO₄)(H₂O)^-, UO₂(HAsO₄)(HAsO₄)(H₂O)^2-, and UO₂(AsO₄)(HAsO₄)(H₂O)^3- become the major forms in the pH range of 7.2-10.7, 10.7-12.1, and >12.1, respectively.

Increased Acid Dissociation at the Quartz/Water Interface

As shown by a quite significant amount of literature, acids at the water surface tend to be "less" acid, meaning that their associated form is favored over the conjugated base. What happens at the solid/liquid interface? In the case of the silica/water interface, we show how the acidity of adsorbed molecules can instead increase. Using a free energy perturbation approach in combination with electronic structure-based molecular dynamics simulations, we show how the acidity of pyruvic acid at the quartz/water interface is increased by almost two units. Such increased acidity is the result of the specific microsolvation at the interface and, in particular, of the stabilization of the deprotonated form by the silanols on the quartz surface and the special interfacial water layer.

Lead and selenite adsorption at water-goethite interfaces from first principles

The complexation of toxic and/or radioactive ions on to mineral surfaces is an important topic in geochemistry. We apply periodic-boundary-conditions density functional theory (DFT) molecular dynamics simulations to examine the coordination of Pb(II), [Formula: see text], and their contact ion pairs to goethite (1 0 1) and (2 1 0) surfaces. The multitude of Pb(II) adsorption sites and possibility of Pb(II)-induced FeOH deprotonation make this a complex problem. At surface sites where Pb(II) is coordinated to three FeO and/or FeOH groups, and with judicious choices of FeOH surface group protonation states, the predicted Fe-Pb distances are in good agreement with EXAFS measurements. Trajectories where Pb(II) is in part coordinated to only two surface Fe-O groups exhibit larger fluctuations in Pb-O distances. Pb(II)/[Formula: see text] contact ion pairs are at least metastable on goethite (2 1 0) surfaces if the [Formula: see text] has a monodentate Se-O-Fe bond. Our DFT-based molecular dynamics calculations are a prerequisite for calculations of finite temperature equilibrium binding constants of Pb(II) and Pb(II)/[Formula: see text] ion pairs to goethite adsorption sites.

Complexation of carboxylate on smectite surfaces

We report a first principles molecular dynamics (FPMD) study of carboxylate complexation on clay surfaces. By taking acetate as a model carboxylate, we investigate its inner-sphere complexes adsorbed on clay edges (including (010) and (110) surfaces) and in interlayer space. Simulations show that acetate forms stable monodentate complexes on edge surfaces and a bidentate complex with Ca²⁺ in the interlayer region. The free energy calculations indicate that the complexation on edge surfaces is slightly more stable than in interlayer space. By integrating pK_as and desorption free energies of Al coordinated water calculated previously (X. Liu, X. Lu, E. J. Meijer, R. Wang and H. Zhou, Geochim. Cosmochim. Acta, 2012, 81, 56-68; X. Liu, J. Cheng, M. Sprik, X. Lu and R. Wang, Geochim. Cosmochim. Acta, 2014, 140, 410-417), the pH dependence of acetate complexation has been revealed. It shows that acetate forms inner-sphere complexes on (110) in a very limited mildly acidic pH range while it can complex on (010) in the whole common pH range. The results presented in this study form a physical basis for understanding the geochemical processes involving clay-organics interactions.

Dehydrogenation Free Energy of Co2+(aq) from Density Functional Theory-Based Molecular Dynamics

Electron and proton transfers are important steps occurring in chemical reactions. The often used approach of calculating the energy differences of those steps using methods based on geometry optimizations neglects the influence of dynamic effects. To further investigate this issue and inspired by research in water oxidation, we calculate in the present study the dehydrogenation free energy of aqueous Co²⁺, which is the free energy change associated with the first step of the water oxidation reaction mechanism of recently investigated model Co(II)-aqua catalysts. We employ a method based on a thermodynamic integration scheme with strong ties to Marcus theory to obtain free energy differences, solvent reorganization free energies, and dynamic structural information on the systems from density functional theory-based molecular dynamics. While this method is computationally orders of magnitude more expensive than a static approach, it potentially allows for predicting the validity of the approximation of neglecting dynamic effects.

Structures and Acidity Constants of Silver-Sulfide Complexes in Hydrothermal Fluids: A First-Principles Molecular Dynamics Study

In order to quantify the speciation and structures of silver-sulfide complexes in aqueous solutions, we have carried out systematic first-principles molecular dynamics (FPMD) simulations at three temperatures (25, 200, and 300 °C). It is found that monosulfide (i.e., Ag(HS)) and disulfide species (i.e., Ag(HS)₂^-) are the major silver-sulfide species over a wide T-P range, while Ag(HS)₃^2- can hold stably only at ambient temperatures, and Ag(HS)₄^3- does not exist even at the ambient conditions. Ag(H₂S)⁺ has a tetrahedral structure up to 300 °C (i.e., Ag(H₂S)(H₂O)₃⁺). Ag(H₂S)₂⁺ remains 4-coordinated to 200 °C (i.e., Ag(H₂S)₂(H₂O)₂⁺), but it transforms to 3-coordinated at 300 °C (i.e., Ag(H₂S)₂(H₂O)⁺). All of the other mono- and disulfide species (Ag(HS)(H₂O)⁰, Ag(HS)(OH)^-, Ag(HS)(H₂S)⁰, Ag(HS)₂^-, and AgS(HS)^2-) have 2-fold linear structures. For their solvation structures, the H₂S ligands donate weak H-bonds to water O; the HS^- ligands accept weak H-bonds from water H; the dangling S^2- form strong H-bonds with H of water molecules, and the OH^- ligands can form strong H-bonds as donors and weak H-bonds as acceptors. We further calculated the acidity constants (i.e., pK_as) of Ag(H₂S)⁺ and Ag(H₂S)₂⁺ complexes using FPMD based vertical energy gap method. Based on the calculated pK_as, the mono- and disulfide species distributions versus pH have been derived. We found that for monosulfide species, Ag(HS)(H₂O)⁰, is the major species in near neutral pH, while Ag(H₂S)(H₂O)₃⁺ and Ag(HS)(OH)^- exist in the acid and alkaline pH range at T ≤ 200 °C, respectively. At 300 °C, both Ag(HS)(OH)^- and Ag(HS)(H₂O)⁰ are dominant in the neutral pH range, and Ag(H₂S)(H₂O)₂⁺ only exists in acidic solutions. For disulfide species, Ag(HS)₂^- is dominative in near neutral pH condition at the three temperatures; Ag(HS)(H₂S)⁰ stays in mild acidic pH range only at 25 °C; AgS(HS)^2- and Ag(H₂S)₂(H₂O)₂⁺ (Ag(H₂S)₂(H₂O)⁺ at 300 °C) are trivial at the three conditions. The results of structures and acidity constants provide quantitative and microscopic basis for understanding the behavior of silver complexes in hydrothermal fluids.

Acidity Constant (pKa ) Calculation of Large Solvated Dye Molecules: Evaluation of Two Advanced Molecular Dynamics Methods

pH‐Sensitive dyes are increasingly applied on polymer substrates for the creation of novel sensor materials. Recently, these dye molecules were modified to form a covalent bond with the polymer host. This had a large influence on the pH‐sensitive properties, in particular on the acidity constant (pK_a). Obtaining molecular control over the factors that influence the pK_a value is mandatory for the future intelligent design of sensor materials. Herein, we show that advanced molecular dynamics (MD) methods have reached the level at which the pK_a values of large solvated dye molecules can be predicted with high accuracy. Two MD methods were used in this work: steered or restrained MD and the insertion/deletion scheme. Both were first calibrated on a set of phenol derivatives and afterwards applied to the dye molecule bromothymol blue. Excellent agreement with experimental values was obtained, which opens perspectives for using these methods for designing dye molecules.

Density Functional Theory Calculation of the Band Alignment of (101̅0) In(x)Ga(1-x)N/Water Interfaces

Conduction band edge (CBE) and valence band edge (VBE) positions of InxGa1-xN photoelectrodes were computed using density functional theory methods. The band edges of fully solvated GaN and InN model systems were aligned with respect to the standard hydrogen electrode using a molecular dynamics hydrogen electrode scheme applied earlier to TiO2/water interfaces. Similar to the findings for TiO2, we found that the Purdew-Burke-Ernzerhof (PBE) functional gives a VBE potential which is too negative by 1 V. This cathodic bias is largely corrected by application of the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional containing a fraction of Hartree-Fock exchange. The effect of a change of composition was investigated using simplified model systems consisting of vacuum slabs covered on both sides by one monolayer of H2O. The CBE was found to vary linearly with In content. The VBE, in comparison, is much less sensitive to composition. The data show that the band edges straddle the hydrogen and oxygen evolution potentials for In fractions less than 47%. The band gap was found to exceed 2 eV for an In fraction less than 54%.

Explicit solvent simulations of the aqueous oxidation potential and reorganization energy for neutral molecules: gas phase, linear solvent response, and non-linear response contributions

First principles simulations were used to predict aqueous one-electron oxidation potentials (Eox) and associated half-cell reorganization energies (λaq) for aniline, phenol, methoxybenzene, imidazole, and dimethylsulfide. We employed quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations of the oxidized and reduced species in an explicit aqueous solvent, followed by EOM-IP-CCSD computations with effective fragment potentials for diabatic energy gaps of solvated clusters, and finally thermodynamic integration of the non-linear solvent response contribution using classical MD. A priori predicted Eox and λaq values exhibit mean absolute errors of 0.17 V and 0.06 eV, respectively, compared to experiment. We also disaggregate Eox into several well-defined free energy properties, including the gas phase adiabatic free energy of ionization (7.73 to 8.82 eV), the solvent-induced shift in the free energy of ionization due to linear solvent response (-2.01 to -2.73 eV), and the contribution from non-linear solvent response (-0.07 to -0.14 eV). The linear solvent response component is further apportioned into contributions from the solvent-induced shift in vertical ionization energy of the reduced species (ΔVIEaq) and the solvent-induced shift in negative vertical electron affinity of the ionized species (ΔNVEAaq). The simulated ΔVIEaq and ΔNVEAaq are found to contribute the principal sources of uncertainty in computational estimates of Eox and λaq. Trends in the magnitudes of disaggregated solvation properties are found to correlate with trends in structural and electronic features of the solute. Finally, conflicting approaches for evaluating the aqueous reorganization energy are contrasted and discussed, and concluding recommendations are given.