Flots microscopiques et mécanisme de la diffusion en phase liquide

This work proposes to reconcile the points of view developed by microscopic theories and hydrodynamics on diffusion. We test the hydrodynamics hypothesis and try reformulate them from microscopic arguments. In particular, for a laminar viscous liquid, the exerced force on a spherical particle or the velocity field around the spherical particle with a prescribed velocity, respectively described by the Basset-Boussinesq force and the Stokes flow, will be the object of our investigation. Is their form relevant at microscopic scale ? If so, how can we express them at the molecular scale ? To do so, we realized molecular simulations and based our developement thanks to the Mori-Zwanzig theory which gives us exact microscopic relations. Initally, we study the evolution of fluctuations of the particle in order to interpret the behaviour at long times and link it with inertia, diffusion and hydrodynamics. Then we propose a novel approch to extract the microscopic flow of Lennard-Jones or granular fluids in order to compare it directly with the hydrodynamic solutions, which allows us to study boundary conditions. We were able to develop an analytical expression for boundary conditions of the flow where fluctuations play a key role. Thanks to our developed analysis tools, we also investigate the coupling between the translational and rotational movement. Finally, we extend our investigation to more realistic fluids such as aqueous solutions and water.Le manuscrit de thèse se propose de concilier les points de vue développés par les théories microscopiques et la théorie hydrodynamique sur la diffusion. Nous testons les hypothèses hydrodynamiques et tentons d'obtenir une reformulation de celles-ci à partir d'arguments microscopiques. En particulier, pour un fluide laminaire visqueux, la force exercée sur une particule sphérique ou encore le champ de vitesse autour d'une particule sphérique en mouvement, respectivement décrits par la force de Basset-Boussinesq et la solution de Stokes, seront l'objet de notre investigation. Leurs formes sont-elles pertinentes à l'échelle microscopique ? Si oui, comment peut-on les exprimer à l'échelle moléculaire ? Pour ce faire, nous avons réalisé des simulations moléculaires et basé notre développement à l'aide de la théorie de Mori-Zwanzig. Dans un premier temps, nous étudions l'évolution des fluctuations de la particule afin d'interpréter le comportement aux temps longs et réalisons le lien avec l'inertie, la diffusion et l'hydrodynamique. Puis nous proposons une nouvelle approche pour extraire le flot microscopique de fluides Lennard-Jones ou granulaires afin de le comparer directement aux solutions hydrodynamiques, ce qui nous permet d'étudier les conditions aux bords. Nous avons pu développer une expression analytique pour les conditions aux bords du flot où les fluctuations jouent un rôle clef. A l'aide des outils d'analyse développés, nous pouvons aussi investiguer le couplage entre le mouvement translationnel et rotationnel. Enfin nous étendons notre analyse à des systèmes tels que les ions aqueux et l'eau surfondue

Molecular hydrodynamics from memory kernels

The memory kernel for a tagged particle in a fluid, computed from molecular dynamics simulations, decays algebraically as $t^{-3/2}$. We show how the hydrodynamic Basset-Boussinesq force naturally emerges from this long-time tail and generalize the concept of hydrodynamic added mass. This mass term is negative in the present case of a molecular solute, at odds with incompressible hydrodynamics predictions. We finally discuss the various contributions to the friction, the associated time scales and the cross-over between the molecular and hydrodynamic regimes upon increasing the solute radius.Comment: 5 pages, 4 figure

Field-dependent ionic conductivities from generalized fluctuation-dissipation relations

We derive a relationship for the electric field dependent ionic conductivity in terms of fluctuations of time integrated microscopic variables. We demonstrate this formalism with molecular dynamics simulations of solutions of differing ionic strength with implicit solvent conditions and molten salts. These calculations are aided by a novel nonequilibrium statistical reweighting scheme that allows for the conductivity to be computed as a continuous function of the applied field. In strong electrolytes, we find the fluctuations of the ionic current are Gaussian and subsequently the conductivity is constant with applied field. In weaker electrolytes and molten salts, we find the fluctuations of the ionic current are strongly non-Gaussian and the conductivity increases with applied field. This nonlinear behavior, known phenomenologically for dilute electrolytes as the Onsager-Wien effect, is general and results from the suppression of ionic correlations at large applied fields, as we elucidate through both dynamic and static correlations within nonequilibrium steady-states.Comment: 6 pages, 3 figure

Microscopic flows and mechanism of diffusion in liquid phase

Le manuscrit de thèse se propose de concilier les points de vue développés par les théories microscopiques et la théorie hydrodynamique sur la diffusion. Nous testons les hypothèses hydrodynamiques et tentons d'obtenir une reformulation de celles-ci à partir d'arguments microscopiques. En particulier, pour un fluide laminaire visqueux, la force exercée sur une particule sphérique ou encore le champ de vitesse autour d'une particule sphérique en mouvement, respectivement décrits par la force de Basset-Boussinesq et la solution de Stokes, seront l'objet de notre investigation. Leurs formes sont-elles pertinentes à l'échelle microscopique ? Si oui, comment peut-on les exprimer à l'échelle moléculaire ? Pour ce faire, nous avons réalisé des simulations moléculaires et basé notre développement à l'aide de la théorie de Mori-Zwanzig. Dans un premier temps, nous étudions l'évolution des fluctuations de la particule afin d'interpréter le comportement aux temps longs et réalisons le lien avec l'inertie, la diffusion et l'hydrodynamique. Puis nous proposons une nouvelle approche pour extraire le flot microscopique de fluides Lennard-Jones ou granulaires afin de le comparer directement aux solutions hydrodynamiques, ce qui nous permet d'étudier les conditions aux bords. Nous avons pu développer une expression analytique pour les conditions aux bords du flot où les fluctuations jouent un rôle clef. A l'aide des outils d'analyse développés, nous pouvons aussi investiguer le couplage entre le mouvement translationnel et rotationnel. Enfin nous étendons notre analyse à des systèmes tels que les ions aqueux et l'eau surfondue.This work proposes to reconcile the points of view developed by microscopic theories and hydrodynamics on diffusion. We test the hydrodynamics hypothesis and try reformulate them from microscopic arguments. In particular, for a laminar viscous liquid, the exerced force on a spherical particle or the velocity field around the spherical particle with a prescribed velocity, respectively described by the Basset-Boussinesq force and the Stokes flow, will be the object of our investigation. Is their form relevant at microscopic scale ? If so, how can we express them at the molecular scale ? To do so, we realized molecular simulations and based our developement thanks to the Mori-Zwanzig theory which gives us exact microscopic relations. Initally, we study the evolution of fluctuations of the particle in order to interpret the behaviour at long times and link it with inertia, diffusion and hydrodynamics. Then we propose a novel approch to extract the microscopic flow of Lennard-Jones or granular fluids in order to compare it directly with the hydrodynamic solutions, which allows us to study boundary conditions. We were able to develop an analytical expression for boundary conditions of the flow where fluctuations play a key role. Thanks to our developed analysis tools, we also investigate the coupling between the translational and rotational movement. Finally, we extend our investigation to more realistic fluids such as aqueous solutions and water

Field-Dependent Ionic Conductivities from Generalized Fluctuation-Dissipation Relations.

We derive a relationship for the electric field dependent ionic conductivity in terms of fluctuations of time integrated microscopic variables. We demonstrate this formalism with molecular dynamics simulations of solutions of differing ionic strength with implicit solvent conditions and molten salts. These calculations are aided by a novel nonequilibrium statistical reweighting scheme that allows for the conductivity to be computed as a continuous function of the applied field. In strong electrolytes, we find the fluctuations of the ionic current are Gaussian, and subsequently, the conductivity is constant with applied field. In weaker electrolytes and molten salts, we find the fluctuations of the ionic current are strongly non-Gaussian, and the conductivity increases with applied field. This nonlinear behavior, known phenomenologically for dilute electrolytes as the Onsager-Wien effect, is general and results from the suppression of ionic correlations at large applied fields, as we elucidate through both dynamic and static correlations within nonequilibrium steady states

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

On the molecular correlations that result in field-dependent conductivities in electrolyte solutions

Employing recent advances in response theory and nonequilibrium ensemble reweighting, we study the dynamic and static correlations that give rise to an electric field-dependent ionic conductivity in electrolyte solutions. We consider solutions modeled with both implicit and explicit solvents, with different dielectric properties, and at multiple concentrations. Implicit solvent models at low concentrations and small dielectric constants exhibit strongly field-dependent conductivities. We compare these results to Onsager-Wilson theory of the Wien effect, which provides a qualitatively consistent prediction at low concentrations and high static dielectric constants but is inconsistent away from these regimes. The origin of the discrepancy is found to be increased ion correlations under these conditions. Explicit solvent effects act to suppress nonlinear responses, yielding a weakly field-dependent conductivity over the range of physically realizable field strengths. By decomposing the relevant time correlation functions, we find that the insensitivity of the conductivity to the field results from the persistent frictional forces on the ions from the solvent. Our findings illustrate the utility of nonequilibrium response theory in rationalizing nonlinear transport behavior

Carbon Species Solvated in Molten Carbonate Electrolyser Cell from First-Principles Simulations

We study the solvation of molecules and ions that are key in the context of Molten Carbonate Electrolyser Cells using first-principles simulations. Focusing on the electroreduction of CO2 to CO in a molten carbonate medium, we investigate the solvation of both the reactant CO2 and the product CO in the eutectic LiKCO3, (containing 62 % Li2CO3, 38 % K2CO3,). CO2 is found to spontaneously react with the carbonate ions to form the transient pyrocarbonate species, C2O52-. To investigate the similar reaction that could occur with CO and CO32- to form an oxalate, we simulated that species and found it to be stable in the melt, supporting this hypothesis. We further present the solvation of O2-, finding that it shows preferential formation of a complex with four lithium cations in a tetrahedral arrangement. Estimates of the diffusion coefficients of these species are then reported, showing that CO has the faster diffusion of all the molecules and ions studied.<br /