Neutral and charged interfaces between molten alkali carbonates M2CO3 (M = Li, Na, and K) and planar solid walls have been investigated by molecular dynamics based on a rigid-ions force field. ...Simulations cover the temperature range 1200 K ≤ T ≤ 1500 K at a moderate (∼15 kbar) overpressure to compensate for the slight overestimate of the system volume by the force field model. The results provide an intriguing view of the interplay among ion packing, oscillating screening, anisotropic correlations, and ion dynamics at the interface. The mass and charge density profiles display prominent peaks at contact, and tend to their constant bulk value through several oscillations, whose amplitude decays exponentially moving away from the interface. Oscillations in the charge density profile extend screening to longer distances and limit the capacitance of the interface. Ion–ion correlations are enhanced in proximity of the interface but retain the exponentially decaying oscillatory form of their bulk counterpart. Diffusion is slower in the molecularly thin layer of ions next to the interface than in the bulk. The analysis of interfaces is completed by the computation of structural properties of bulk phases, and by the estimate of transport coefficients such as self-diffusion, electrical conductivity, and especially thermal conductivity, which is seldom computed by simulation. All together, the results of our simulations for homogeneous and inhomogeneous molten carbonates provide crucial insight on systems and properties relevant for advanced devices such as fuel cells, that, in turn, might play a prominent role in future power generation strategies.
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The modelling of thermodynamic properties of liquids from local density fluctuations is relevant to many chemical and biological processes. The Kirkwood-Buff (KB) theory connects the microscopic ...structure of isotropic liquids with macroscopic properties such as partial derivatives of activity coefficients, partial molar volumes and compressibilities. Originally, KB integrals were formulated for open and infinite systems which are difficult to access with standard Molecular Dynamics (MD) simulations. Recently, KB integrals for finite and open systems were formulated (J Phys Chem Lett. 2013;4:235). From the scaling of KB integrals for finite subvolumes, embedded in larger reservoirs, with the inverse of the size of these subvolumes, estimates for KB integrals in the thermodynamic limit are obtained. Two system size effects are observed in MD simulations: (1) effects due to the size of the simulation box and the size of the finite subvolume embedded in the simulation box, and (2) effects due to computing radial distribution functions (RDF) from a closed and finite system. In this study, we investigate the two effects in detail by computing KB integrals using the following methods: (1) Monte Carlo simulations of finite subvolumes of a liquid with an analytic RDF and (2) MD simulations of a WCA mixture for various simulation box sizes, but at the same thermodynamic state. We investigate the effect of the size of the simulation box and quantify the differences compared to KB integrals computed in the thermodynamic limit. We demonstrate that calculations of KB integrals should not be extended beyond half the size of the simulation box. For finite-size effects related to the RDF, we find that the Van der Vegt correction (J Chem Theory Comput. 2013;9:1347) yields the most accurate results.
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We study the adsorption of carbon dioxide at a graphite surface using the new Small System Method, and find that for the temperature range between 300 K and 550 K most relevant for CO2 separation; ...adsorption takes place in two distinct thermodynamic layers defined according to Gibbs. We calculate the chemical potential and the activity coefficient of both layers directly from the simulations. Based on thermodynamic relations, the entropy and enthalpy of the CO2 adsorbed layers are also obtained. Their values indicate that there is a trade-off between entropy and enthalpy when a molecule chooses for one of the two layers. The first layer is a densely packed monolayer of relatively constant excess density with relatively large repulsive interactions and negative enthalpy. The second layer has an excess density varying with the temperature, an activity coefficient, which also indicates repulsion, but to a much smaller degree than in the first layer. Results for activity coefficients, entropies and enthalpies can be used to model transport through and along graphitic membranes for carbon dioxide separation purposes.
In this article, we present a theory for the dielectric behavior of a colloidal spheroid, based on an improved version of a previously published analytical theory C. Chassagne, D. Bedeaux, G.J.M. ...Koper, Physica A 317 (2003) 321–344. The theory gives the dipolar coefficient of a dielectric spheroid in an electrolyte solution subjected to an oscillating electric field. In the special case of the sphere, this theory is shown to agree rather satisfactorily with the numerical solutions obtained by a code based on DeLacey and White's E.H.B. DeLacey, L.R. White, J. Chem. Soc. Faraday Trans. 2 77 (1981) 2007 for all zeta potentials, frequencies and
κ
a
⩾
1
where
κ is the inverse of the Debye length and
a is the radius of the sphere. Using the form of the analytical solution for a sphere we were able to derive a formula for the dipolar coefficient of a spheroid for all zeta potentials, frequencies and
κ
a
⩾
1
. The expression we find is simpler and has a more general validity than the analytical expression proposed by O'Brien and Ward R.W. O'Brien, D.N. Ward, J. Colloid Interface Sci. 121 (1988) 402 which is valid for
κ
a
≫
1
and zero frequency.
We derived the dipolar coefficients
β
i
for an ellipsoidal colloid as a function of the different complex conductivities (
K
i
) and depolarization factors (
L
i
).
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