Coupling chemical physics to continuum theories is a critical step to understanding multi-scale phenomena. This paper will connect non-equilibrium molecular dynamics simulations to a continuum-based Navier-Stokes equation that has relaxed the assumption of spatial uniformity in viscosity. Using a form for the viscosity based on spline interpolation, the viscosity as a function of position is obtained from a least squares fit of the velocity profile measured from molecular simulations of flow in a nanochannel. Viscosity can vary widely, particularly near the channel boundaries, indicating that a uniform viscosity is no longer appropriate. Variations of the viscosity near the channel surfaces implies that considering solution and surface chemistry could be necessary to rigorously understand molecular-scale flows in nanochannels.

The distribution of water molecules and ions comprising an electrical double layer at solid-liquid interfaces remains difficult to accurately predict, in part due to the limited ability to probe the interactions between surface sites, water molecules, and adsorbed ions from solution. We investigated the effect of solution composition on orthoclase-water interfaces using a combination of three-dimensional atomic force microscopy (AFM) and molecular dynamics (MD) simulations. In dilute solutions, water distribution is templated by the underlying lattice, with the first water layer adsorbing at cavity sites, followed by two layers of low and high degrees of structuring. At z = 0.6 nm, water molecules assemble into striped patterns with approximately 0.69 nm periodicity that connect the preferred packing locations above cavity sites. Corresponding MD simulations in combination with the solvent tip approximation yield three key oscillatory features separated by 0.21 and 0.27 nm in reasonable agreement with the AFM data. We further investigated the effect of specific ions; magnesium and calcium chloride showed oscillations with spacings of 0.41-0.45 nm, extending >1.5 nm from the surface even at dilute concentrations. High salt concentrations of 2 molal strongly influenced the solution structure and resulted in a larger "work of approach" as the incoming probe displaced the interfacial solution, which can be ascribed to a stronger hydration force. Our results demonstrate the effect of electrolyte type and concentration on the structure of the solution at the interface, with implications for a diversity of processes such as adsorption, dissolution/precipitation, particle aggregation, and self-assembly.

Understanding and describing reactivity at mineral-water interfaces, such as ion adsorption, the kinetics of dissolution, or surface charge development, depends on our ability to improve the accuracy of electrical double-layer (EDL) models. While molecular dynamics (MD) simulations are routinely used to investigate the structure and energetics of adsorbed ions comprising the EDL, less attention has been paid to their self-diffusion dynamics, which can uniquely inform coupling to interfacial reactions. Here, we use MD to investigate both the organization and diffusion dynamics of water and electrolyte ions (NaCl, KCl, CaCl2) on hydroxylated quartz (001) and (101) surfaces, which allow us to assess surface structural effects of corrugation and silanol density. Complementary atomic force microscopy measurements are also used to probe the interfacial solution structure. We found that the inner- versus outer-sphere complex formation depends on the cation size and charge but not necessarily on hydration energies. The participation of surface silanols in the hydration spheres of Na+ and K+ generally indicates their preference for inner-sphere complexation, but this is strongly dependent on the orientation of the surface considered through its influence on the organization and dynamics of adsorbed water layers. In particular, the surface orientation substantially affects the diffusive behavior of near-surface water. Na+ is found to decrease the mobility of water in the first layer, consistent with the increasing frequency of hydrolysis implied by the faster quartz dissolution rates observed in experiments via the well-known salt effect. Our results are also in good agreement with the observed dissolution rate of quartz vs the surface adsorption strength measured by Dove and Nix. This study sets the stage for a forthcoming study examining how the dynamics at quartz/electrolyte interfaces are influenced by externally applied electric fields.

Surface self-diffusion studies on metals under elevated reaction conditions are limited, as it is inherently challenging to unambiguously follow atomic transport across highly-reactive surfaces. Here, quantitative and mechanistic insight into thermally induced atomic transport processes in bcc α-iron at the sub-nanometer level was achieved using isotopic tracer techniques coupled with in situ atom probe tomography (APT) capabilities. Specifically, using a reactor directly connected to the APT, needle-shaped specimens fabricated from epitaxial thin films with an embedded 57Fe tracer layer were annealed in Ar at 500°C and 350°C for 1 hour. Furthermore, the tracer was positioned at various depths in the APT specimen by field evaporation, enabling targeted and simultaneous analysis of lattice and surface diffusion. 57Fe concentration profiles reveal lattice self-diffusion occurs at 500°C on the order of ∼7 – 9 monolayers, while lattice diffusion is not resolvable at 350°C. Considerable surface transport was, however, observed at both conditions, where atomic transport over the specimen surface led to the formation of a thin (≤1 nm), isotopically-intermixed layer at the surface. The observed isotopic redistributions at 500°C were convoluted by additional processes occurring in the subsurface, such as atomic intermixing in correlation with lattice diffusion. However, surface diffusion was determined to be the primary transport process at 350°C and was thereby quantified. Ultimately, these results demonstrate the significance of surface self-diffusion as a short circuit pathway. More broadly, this approach has the potential to provide detailed insight into (self-)diffusion mechanisms across various materials while targeting site-specific reactions under elevated reaction conditions.

Surface passivation, a desirable natural consequence during initial oxidation of alloys, is the foundation for functioning of corrosion and oxidation resistant alloys ranging from industrial stainless steel to kitchen utensils. This initial oxidation has been long perceived to vary with crystal facet, however, the underlying mechanism remains elusive. Here, using in situ environmental transmission electron microscopy, we gain atomic details on crystal facet dependent initial oxidation behavior in a model Ni-5Cr alloy. We find the (001) surface shows higher initial oxidation resistance as compared to the (111) surface. We reveal the crystal facet dependent oxidation is related to an interfacial atomic sieving effect, wherein the oxide/metal interface selectively promotes diffusion of certain atomic species. Density functional theory calculations rationalize the oxygen diffusion across Ni(111)/NiO(111) interface, as contrasted with Ni(001)/NiO(111), is enhanced. We unveil that crystal facet with initial fast oxidation rate could conversely switch to a slow steady state oxidation. Self-passivation in metallic alloys is a fundamental issue for corrosion and oxidation resistance. Here, authors demonstrate crystal facet-dependent initial oxidation behavior in a model Ni-Cr alloy, using in situ environmental TEM and DFT calculations. They show that the (001) facet exhibits incipient oxidation resistance compared to the (111) facet.

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (similar to 0.8 eV for Cr and 0.7 eV for Ni within 1300-1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

Understanding the electrochemical properties of mineral-water interfaces tends to rely upon electrical double layer (EDL) models, but these models are based on the assumption that electrostatic equilibrium is constantly maintained. In reality, interfacial reactions, ion diffusion, and their electrochemical signatures are based in nonequilibrium conditions of locally or globally imbalanced electrical fields where current EDL models have limited purview. Here, we performed molecular dynamics (MD) simulations of the orthoclase (001) surface in contact with a 1 M NaCl aqueous solution under various electric fields, to explore the interplay between EDL structure and dynamics when perturbed by electric fields of different directions and strengths, by confinement, and by different distributions of structural surface charge. The simulations showed that confinement between two opposing (001) surfaces led to the development of an induced field when the applied field was perpendicular to the surfaces and, as a result, to ionic diffusion coefficients that were independent of electric field strength. In contrast, when the applied field was parallel to the surfaces, confinement resulted in ionic diffusion coefficients that were more strongly dependent on the magnitude of the electric field than in bulk water. Differences in the density and distribution of aluminol groups on the two surfaces had a significant impact on how the interfacial structure and dynamics varied in the presence of an electric field. Notably, these differences resulted in an electro-osmotic flow with opposite directions at the two surfaces under parallel applied electric field. Overall, the MD simulations highlighted the importance of considering atomic-level structure and heterogeneities when developing models of the electrochemical properties of mineral-water interfaces.

Developing an understanding of the response of mineral/water interfaces to applied electric fields is central to detecting and interpreting signatures of interfacial processes in the subsurface. Here, we focus density functional theory calculations on understanding K+ cation binding and migration across the (001) surface of orthoclase feldspar under various applied electric fields with and without surface hydration. The calculations reveal how water ligands labilize surface K+ cations for migration while also increasing their sensitivity to electric field effects on the binding energy at different surface sites. The calculations also show how the direction and strength of the electric field systematically affect surface cation mobility, sorption, and hydration behavior. Specifically, electric fields directed toward the surface reduce the energy gap between the different surface potassium sites, favor hydration, and shorten K+ residence time at their crystallographic site. The findings help fill a major knowledge gap in the impact of electric fields on mineral/water interface structure and dynamics more generally, featuring, in this case, a commonly found type of feldspar involved in a multitude of atmospheric and geochemical processes.

Interactions of selenium (Se), a trace element bioessential at low concentrations but highly toxic at high concentrations, with the most abundant sulfide mineral in the earth's crust, namely, pyrite, were investigated over a wide range of time scales from nanoseconds to days. At the nanosecond scale, selenate Se(VI)O-4(2-) adsorption onto the neat pyrite surface is shown by ab initio computations to proceed via the formation of a chemical bond between an oxyanion oxygen atom and a surface Fe atom, weakening the other Se-O bonds and reducing the Se atom oxidation state. At the hour-to-day scale, the adsorption and coprecipitation of selenate Se(VI)O-4(2-) and selenite, Se(IV)O-3(2-), were investigated through wet chemical batch experiments at various pH values at different sulfide concentrations. Selenium removal from solution is slower and weaker for selenate than for selenite. After 24 h, only 10% of selenate, against 60% of selenite (up to 100% in the presence of sulfide), is removed by the pyrite surface. Independently of its original oxidation state, adsorbed Se is completely reduced to elemental trigonal selenium via adsorption, precipitation, or coprecipitation, as shown by XANES spectroscopy. Our EXAFS results, compared to published data on Se-rich pyrite, show a Se to S substitution within the pyrite structure. The reductive coprecipitation mechanism of selenium with pyrite represents valuable new insights for improving our understanding of modern and ancient biogeochemical cycles involving Se. In addition, several industries can benefit from direct applications of our findings, such as water treatment, green technologies, and sustainable mining.

Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

Natural sulfidation of silver nanomaterials can passivate the surface, while preserving desirable optical and electrical properties, which is beneficial for limiting Ag+ release and cytotoxicity. But little is known at the atomic scale about silver sulfidation mechanisms, particularly on different crystallographic terminations. Using density functional theory (DFT) calculations, we examined the process of H2S sorption and reaction on Ag(100) surfaces relevant to Ag nanowires (AgNWs). DFT energy minimizations predict a strong dissociative chemisorption of H2S on the surface yielding co-adsorbed sulfide and hydrogen atoms in specific surface sites. However, nudged elastic band (NEB) calculations suggest relatively large activation energies for both the first and second dissociation steps, due in part to overcoming the energy to cleave the S-H bond and attendant site migration from an on-top Ag site position to a hollow site position of the bound S atom. The large barriers associated with the dissociative chemisorption reaction for gas-phase H2S points to the importance of including thermochemical contributions and the influence of other components in more complex environmental media such as air or water to help complete the mechanistic picture of silver sulfidation and passivation for realistic systems.

Fe-bearing clay minerals are important redox-active components of the subsurface and engineered barriers. Their interfacial reactivity plays an essential role in environmental processes such as biogeochemical cycling of various elements and contaminants. A detailed mechanism of Fe(II) surface speciation and interfacial electron transfer (ET) to Fe(III) in the octahedral sheet is still under debate despite its well-established consequence as one of the most effective reductants in anoxic environments. Recent developments have shown that edge-bound Fe(II) adsorption complexes at different surface sites may coexist on different edge facets. It has also been shown that complexes at ferrinol FeO(H) edge sites are the most energetically favorable and ET at these sites is facile and coupled to proton exchange. However, ET from Fe(II) sorbed to the external basal surface to octahedral Fe(III) is predicted to be predominantly thermodynamically uphill. A major enduring uncertainty in experiments is the extent to which Fe(II) can displace interlayer cations through cation exchange and there become a more effective reductant for Fe(III) in the octahedral sheet. Herein, we apply density functional theory (DFT) calculations to provide atomistic insights into the valence-interchange ET energetics and kinetics between Fe(II) in the interlayer site and an Fe(III) in the octahedral sheet, which reveals a lower ET barrier due to desolvation of interlayer Fe(II).

Defect-mediated mass transport is a critical phenomenon for many material performance and degradation issues. In the area of stress corrosion cracking, a predictive model of mass transport under realistic, elevated temperatures would be instrumental in predicting crack initiation and propagation tendencies. Molecular simulations can address some of these gaps about underlying processes. In that context, this study presents a robust model that describes self-diffusion at high temperatures in Ni-xCr alloys with varying Cr concentrations. We show that by using a vacancy concentration rescaling factor, it is possible to properly estimate diffusion coefficients in Ni5Cr, Ni-10Cr and Ni-20Cr. The model reaches its upper limit at 1600 K for Ni-20Cr, because it has been fitted on dilute solute concentrations. Extrapolations at low temperature match experimental trend. Simulations with a theoretical number of vacancies give results comparable to the diffusion coefficients with the rescaling factor. In further simulations of more complex systems, efforts should be made in interstitial formation and migration barrier energy description.

Superionic conductivity in certain polymorphs of Ag2S has inspired numerous concepts for materials applications, but the relationship between the structure and the mobility of silver ions remains poorly explored. Here, we report ab initio molecular dynamics simulations for low- (acanthite) and high-temperature (argentite) Ag2S polymorphs that reveal the dynamical processes giving rise to the superionic behavior in the latter. Similarities between their sulfur sublattices enable simulations of silver ion diffusivities and pathways on essentially an equal footing. For the higher temperature polymorph, calculated temperature-dependent mean square displacements and activation energies by the nudged elastic band method show good correspondence with expectations from the experiment. In the superionic state, silver atoms diffuse in a liquid-like behavior with no preferred diffusion pathways, within the relatively stable body-centered cubic sulfur framework. In contrast, conduction in acanthite appears to depend more on the mobilities of electronic charge carriers.

We use molecular dynamics to investigate how the structure, diffusion, and hydrodynamic properties of clay interfaces with aqueous solutions depend on the nature of the clay, the nature of the counterions, and the salt concentration in the solution. Specifically, we study water-filled nanopores between uncharged (pyrophyllite) and charged (montmorillonite and beidellite, with substitutions located in the octahedral and tetrahedral layers, respectively) clays, with sodium or cesium as counterions, in the absence and in the presence of added salt. We discuss how the balance between solvation and attraction of the cations to the surface results in various distributions between inner- and outer-sphere complexes, and how this influences the dynamics of water near the surface, as well as the hydrodynamic flow in the presence of an external force. In the latter case, the discussion based on mapping the molecular velocity profiles to a continuous description (parabolic Poiseuille flow) shows that the larger effects come from the presence/absence of charge in the mineral, as well as the localization of substitutions within the clay layer. The salt concentration and the nature of the counterions have a comparatively less important impact far from the surface—even though some differences are observed in its close vicinity, which are not properly captured by the continuous description.

We investigate finite-size effects on diffusion in confined fluids using molecular dynamics simulations and hydrodynamic calculations. Specifically, we consider a Lennard-Jones fluid in slit pores without slip at the interface and show that the use of periodic boundary conditions in the directions along the surfaces results in dramatic finite-size effects, in addition to that of the physically relevant confining length. As in the simulation of bulk fluids, these effects arise from spurious hydrodynamic interactions between periodic images and from the constraint of total momentum conservation. We derive analytical expressions for the correction to the diffusion coefficient in the limits of both elongated and flat systems, which are in excellent agreement with the molecular simulation results except for the narrowest pores, where the discreteness of the fluid particles starts to play a role. The present work implies that the diffusion coefficients for wide nanopores computed using elongated boxes suffer from finite-size artifacts which had not been previously appreciated. In addition, our analytical expression provides the correction to be applied to the simulation results for finite (possibly small) systems. It applies not only to molecular but also to all mesoscopic hydrodynamic simulations, including Lattice-Boltzmann, Multiparticle Collision Dynamics or Dissipative Particle Dynamics, which are often used to investigate confined soft matter involving colloidal particles and polymers.

La récupération d'hydrocarbures non conventionnels fait partie des enjeux énergétiques majeurs. Ils ne peuvent être extraits par simple forage car la roche qui les contient, constituée essentiellement de nanopores, présente une très faible perméabilité. A l'échelle macroscopique, c'est-à-dire à l'échelle du bassin, les écoulements de fluides sont décrits par la loi de Darcy qui relie le flux à la perméabilité, au gradient de pression et à la viscosité. La perméabilité d'un matériau peut être mesurée expérimentalement ou théoriquement par homogénéisation à partir de l’hydrodynamique continue. Cependant, lorsque la taille des pores devient comparable à celle des molécules de fluide, une telle description n'est pas satisfaisante. D’une part l’hydrodynamique continue, où la nature du fluide n’intervient qu’à travers la viscosité, ne suffit pas forcément pour décrire l’écoulement. D’autre part les interactions au niveau moléculaire entre le fluide et le solide jouent un rôle important. Cette thèse porte sur le transport de fluides à l'échelle moléculaire et revisite la description traditionnelle qui sert de point de départ pour des écoulements à l'échelle macroscopique, en particulier dans le cas des écoulements multiphasiques. Par des simulations de dynamique moléculaire classique, nous avons étudié l'écoulement de systèmes monophasiques et diphasiques, précisant l’influence de la nature des surfaces, ainsi que de la nature et de la concentration des espèces dissoutes. Nous avons également apporté une contribution méthodologique originale pour le calcul des coefficients de diffusion d'espèces.