11 research outputs found
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 included ( 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 , ions and , molecule , ...) 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 and ions and , 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 ( g/cm), 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