3,776 research outputs found
Development of intermolecular potential models for electrolyte solutions using an electrolyte SAFT-VR Mie equation of state
We present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimation. The inclusion of the Born free-energy contribution, together with appropriate estimates for the size of the ionic cavity, allows for accurate predictions of the Gibbs free energy of solvation of the ionic species considered. The solubility limits are also predicted for a number of salts; in cases where reliable reference data are available the predictions are in good agreement with experiment
Exploration of dynamical regimes of irradiated small protonated water clusters
We explore from a theoretical perspective the dynamical response of small
water clusters, (HO)HO with , to a short laser pulse
for various frequencies, from infrared (IR) to ultra-violet (UV) and
intensities (from W/cm to W/cm). To
that end, we use time-dependent local-density approximation for the electrons,
coupled to molecular dynamics for the atomic cores (TDLDA-MD). The
local-density approximation is augmented by a self-interaction correction (SIC)
to allow for a correct description of electron emission. For IR frequencies, we
see a direct coupling of the laser field to the very light H ions in the
clusters. Resonant coupling (in the UV) and/or higher intensities lead to fast
ionization with subsequent Coulomb explosion. The stability against Coulomb
pressure increases with system size. Excitation to lower ionization stages
induced strong ionic vibrations. These maintain rather harmonic pattern in
spite of the sizeable amplitudes (often 10% of the bond length).Comment: accepted in Eur. J. Phys.
Microscopic dynamics of charge separation at the aqueous electrochemical interface
We have used molecular simulation and methods of importance sampling to study
the thermodynamics and kinetics of ionic charge separation at a liquid
water-metal interface. We have considered this process using canonical examples
of two different classes of ions: a simple alkali-halide pair, NaI, or
classical ions, and the products of water autoionization, HOOH, or
water ions. We find that for both ion classes, the microscopic mechanism of
charge separation, including water's collective role in the process, is
conserved between the bulk liquid and the electrode interface. Despite this,
the thermodynamic and kinetic details of the process differ between these two
environments in a way that depends on ion type. In the case of the classical
ion pairs, a higher free energy barrier to charge separation and a smaller flux
over that barrier at the interface, results in a rate of dissociation that is
40x slower relative to the bulk. For water ions, a slightly higher free energy
barrier is offset by a higher flux over the barrier from longer lived hydrogen
bonding patters at the interface, resulting in a rate of association that is
similar both at and away from the interface. We find that these differences in
rates and stabilities of charge separation are due to the altered ability of
water to solvate and reorganize in the vicinity of the metal interface.Comment: 6 pages, 3 figures + S
Ambidentate coordination in hydrogen bonded dimethyl sulfoxide, (CH3)2SOH3O+, and in dichlorobis(dimethyl sulfoxide) palladium(II) and platinum(II) solid solvates, by vibrational and sulfur K-edge X-ray absorption spectroscopy
The strongly hydrogen bonded species (CH3)(2)SO center dot center dot center dot H3O+ formed in concentrated hydrochloric acid displays a new low energy feature in its sulfur K-edge X-ray absorption near edge structure (XANES) spectrum. Density Functional Theory-Transition Potential (DFT-TP) calculations reveal that the strong hydrogen bonding decreases the energy of the transition S(1s) -> LUMO, which has antibonding sigma*(S-O) character, with about 0.8 eV. Normal coordinate force. field analyses of the vibrational spectra show that the SO stretching force constant decreases from 4.72 N cm(-1) in neat liquid dimethyl sulfoxide to 3.73 N cm(-1) for the hydrogen bonded (CH3)(2)SO center dot center dot center dot H3O+ species. The effects of sulfur coordination on the ambidentate dimethyl sulfoxide molecule were investigated for the trans-Pd((CH3)(2)SO)(2)Cl-2, trans-Pd((CD3)(2)SO)(2)Cl-2 and cis-Pt((CH3)(2)SO)(2)Cl-2 complexes with square planar coordination of the chlorine and sulfur atoms. The XANES spectra again showed shifts toward low energy for the transition S(1 s) -> LUMO, now with antibonding sigma*(M-Cl, M-S) character, with a larger shift for M = Pt than Pd. DFT-TP calculations indicated that the differences between the XANES spectra of the geometrical cis and trans isomers of the M((CH3)(2)SO)(2)Cl-2 complexes are expected to be too small to allow experimental distinction. The vibrational spectra of the palladium(II) and platinum(II) complexes were recorded and complete assignments of the fundamentals were achieved. Even though the M-S bond distances are quite similar the high covalency especially of the Pt-S bonds induces significant increases in the S-O stretching force constants, 6.79 and 7.18 N cm(-1), respectively
Chemical evolution of turbulent protoplanetary disks and the Solar nebula
This is the second paper in a series where we study the influence of
transport processes on the chemical evolution of protoplanetary disks. Our
analysis is based on a flared alpha-model of the DM Tau system, coupled to a
large gas-grain chemical network. To account for production of complex
molecules, the chemical network is supplied with an extended set of surface
reactions and photo-processes in ice mantles. Our disk model covers a wide
range of radii, 10-800 AU (from a Jovian planet-forming zone to the outer disk
edge). Turbulent transport of gases and ices is implicitly modeled in full 2D
along with the time-dependent chemistry. Two regimes are considered, with high
and low efficiency of turbulent mixing. The results of the chemical model with
suppressed turbulent diffusion are close to those from the laminar model, but
not completely. A simple analysis for the laminar chemical model to highlight
potential sensitivity of a molecule to transport processes is performed. It is
shown that the higher the ratio of the characteristic chemical timescale to the
turbulent transport timescale for a given molecule, the higher the probability
that its column density will be affected by diffusion. We find that turbulent
transport enhances abundances and column densities of many gas-phase species
and ices, particularly, complex ones. For such species a chemical steady-state
is not reached due to long timescales associated with evaporation and surface
photoprocessing and recombination. In contrast, simple radicals and molecular
ions, which chemical evolution is fast and proceeds solely in the gas phase,
are not much affected by dynamics. All molecules are divided into three groups
according to the sensitivity of their column densities to the turbulent
diffusion. [Abridged]Comment: 42 pages, 13 figures, 16 tables, accepted for publication in ApJS
Spatially extended OH+ emission from the Orion Bar and Ridge
We report the first detection of a Galactic source of OH+ line emission: the
Orion Bar, a bright nearby photon-dominated region. Line emission is detected
over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the
Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission
close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most
hydrogen is in atomic form. Steady-state collisional and radiative excitation
models require unrealistically high OH+ column densities to match the observed
line intensity, indicating that the formation of OH+ in the Bar is rapid enough
to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2
is similar to that in previous absorption line studies, while our limits on the
ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before.
The column density of OH+ is consistent with estimates from a thermo-chemical
model for parameters applicable to the Orion Bar, given the current
uncertainties in the local gas pressure and the spectral shape of the ionizing
radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably
due to the high UV radiation field and electron density in this object. In the
Bar, photodissociation and electron recombination are more effective destroyers
of OH+ than the reaction with H2, which limits the production of H2O+. The
appearance of the OH+ lines in emission is the result of the high density of
electrons and H atoms in the Orion Bar, since for these species, inelastic
collisions with OH+ are faster than reactive ones. In addition, chemical
pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium
starlight contribute to the OH+ excitation. Similar conditions may apply to
extragalactic nuclei where OH+ lines are seen in emission.Comment: Accepted by A&A; 10 pages, 5 figure
Interstellar Hydrides
Interstellar hydrides -- that is, molecules containing a single heavy element
atom with one or more hydrogen atoms -- were among the first molecules detected
outside the solar system. They lie at the root of interstellar chemistry, being
among the first species to form in initially-atomic gas, along with molecular
hydrogen and its associated ions. Because the chemical pathways leading to the
formation of interstellar hydrides are relatively simple, the analysis of the
observed abundances is relatively straightforward and provides key information
about the environments where hydrides are found. Recent years have seen rapid
progress in our understanding of interstellar hydrides, thanks largely to
far-IR and submillimeter observations performed with the Herschel Space
Observatory. In this review, we will discuss observations of interstellar
hydrides, along with the advanced modeling approaches that have been used to
interpret them, and the unique information that has thereby been obtained.Comment: Accepted for publication in Annual Review of Astronomy and
Astrophysics 2016, Vol. 5
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