Onsager model for a variable dielectric permittivity near an interface

Using a generalisation of an Onsager type approach, we are able to predict a dielectric permittivity profile of an inhomogeneous dipolar fluid in the presence of a dielectric interface. The reaction and cavity fields are calculated semi-analytically using bispherical coordinates. An asymptotic expression for the local permittivity is derived as a function of distance from the interface.Comment: 20 pages, 4 figures, submitted to Molecular Physic

Thermodynamics of atomic and ionized hydrogen : analytical results versus OPAL tables and Monte Carlo data

We compute thermodynamical properties of a low-density hydrogen gas within the physical picture, in which the system is described as a quantum electron-proton plasma interacting via the Coulomb potential. Our calculations are done using the exact Scaled Low-Temperature (SLT) expansion, which provides a rigorous extension of the well known virial expansion -- valid in the fully ionized phase -- into the Saha regime where the system is partially or fully recombined into hydrogen atoms. After recalling the SLT expansion of the pressure [A. Alastuey et al, J. Stat. Phys. {\bf 130}, 1119 (2008)], we obtain the SLT expansions of the chemical potential and of the internal energy, up to to order $\exp(|E_H|/kT)$ included ($E_H \simeq -13.6$ eV). Those truncated expansions describe the first five non-ideal corrections to the ideal Saha law. They account exactly, up to the considered order, for all effects of interactions and thermal excitations, including the formation of bound states (atom $H$, ions $H^-$ and $H_2^+$, molecule $H_2$, ...) and atom-charge and atom-atom interactions. Among the five leading corrections, three are easy to evaluate, while the remaining ones involve well-defined internal partition functions for molecule $H_2$ and ions $H^-$ and $H_2^+$, for which no closed-form analytical formula exist currently. We provide accurate low-temperature approximations for those partition functions by using known values of rotational and vibrational energies. We compare then the predictions of the SLT expansion with the OPAL EOS and data of path integral quantum Monte Carlo (PIMC) simulations. In general, a good agreement is found. At low densities, the simple analytical SLT formulae reproduce the values of the OPAL tables up to the last digit in a large range of temperatures, while at higher densities ($\rho\sim10^{-2}$ g/cm$^3$), some discrepancies between the SLT, OPAL and PIMC results are observed

Etude des phénomènes d'écran et de polarisation dans un plasma quantique par la méthode des graphes de Mayer

This thesis is devoted to the study of screening and polarisation effects in a quantum plasma of electrons and protons, when the system is close to a dilute gas of hydrogen atoms. This atomic phase is obtained by considering a coupled low-density and low-temperature limit, in which the binding of charges into hydrogen atoms is favored. We study the electrical susceptibility of this plasma using a fugacity expansion of this function obtained from a resummed Mayer diagrammatic series. This expansion -- which is non-perturbative with respect to electric charge and Planck's constant -- allows to take into account systematically, at low densities, all phenomena induced by the Coulomb interactions, among which atomic and molecular binding and screening effects. We exhibit in particular a regime where the plasma's susceptibility measures the dielectric screening effect due to the polarisability of the hydrogen atoms. We consider also in this thesis the problem of dealing with the boundary effect that occurs when a finite dielectric sample is polarized under the influence of a static electric field. We describe the dielectric material as a classical dipolar fluid confined to a certain region, and we calculate its mean polarisation using statistical mechanics. We show that in the thermodynamical limit, this polarisation satisfies the local dielectric law of macroscopic electrostatics, with a dielectric constant that is a bulk property, independent of the sample's shape

Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere

We describe a scenario of Titan's formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titan's atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding 50 K during their migration within Saturn's subnebula. The observed deficiencies of Titan's atmosphere in krypton and xenon could result from other processes that may have occurred both prior or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH3+ complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellite's surface or in its atmospheric haze.Comment: Accepted for publication in The Astrophysical Journa

Variability of the methane trapping in martian subsurface clathrate hydrates

Recent observations have evidenced traces of methane CH4 heterogeneously distributed in the martian atmosphere. However, because the lifetime of CH4 in the atmosphere of Mars is estimated to be around 300-600 years on the basis of photochemistry, its release from a subsurface reservoir or an active primary source of methane have been invoked in the recent literature. Among the existing scenarios, it has been proposed that clathrate hydrates located in the near subsurface of Mars could be at the origin of the small quantities of the detected CH4. Here, we accurately determine the composition of these clathrate hydrates, as a function of temperature and gas phase composition, by using a hybrid statistical thermodynamic model based on experimental data. Compared to other recent works, our model allows us to calculate the composition of clathrate hydrates formed from a more plausible composition of the martian atmosphere by considering its main compounds, i.e. carbon dioxyde, nitrogen and argon, together with methane. Besides, because there is no low temperature restriction in our model, we are able to determine the composition of clathrate hydrates formed at temperatures corresponding to the extreme ones measured in the polar caps. Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH4-rich atmosphere has existed or if a subsurface source of CH4 has been (or is still) present.Comment: Accepted for publication in Planetary and Space Scienc

Cage Occupancies in Nitrogen Clathrate Hydrates from Monte Carlo Simulations

International audienc

Quantum partition functions of composite particles in a hydrogen-helium plasma via path integral Monte Carlo

International audienc