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We revisit the role of the local solvent structure on the activity coefficient of electrolytes with a general non-local dielectric function approach. We treat the concentrated electrolyte as a dielectric medium and suggest an interpolated formula for the dielectric response. The pure water limit is calibrated based on MD simulations and experimental data. Solving our model around a central ion, we find strong over-screening and oscillations in the potential, which are absent in the standard "primitive model" predictions. We obtain mathematically tractable closed-form expressions for the activity coefficients and show reasonable agreement with experimental data.

International audience; Ion solvation process has been analysed for the spherically symmetrical system where an ion is located inside a cavity surrounded by an isotropic nonlocal dielectric medium. It has been proven that for any dielectric properties of the medium, the electric field outside the cavity as well as the ion solvation energy depend only on the total ion charge but not of the particular distribution of the ion charge density inside the cavity. These characteristics remain unchanged if the charge is displaced from the external boundary of the cavity into it. Analytical formulas for them have been derived for a particular model of the nonlocal dielectric function. Comparison of results for the solvation energy on the basis of this new theory and of the conventional approach (disregarding the existence of the cavity) shows a significant difference between their predictions if the ion charge is displaced inside the ion cavity.

Potentials of Mean Force (PMF) for the Na+Cl− ion pair in water–dimethyl sulfoxide (DMSO) mixtures for three DMSO mole fractions have been computed using constrained Molecular Dynamics (MD) simulations and confirmed by dynamical trajectories and residence times of the ion pair at various inter-ionic separations. The three ion-ion direct potentials used are 12-6-1, exp-6-1 and exp-8-6-1. The physical picture that emerges is that there is a strong contact ion pair (CIP) and strong to moderate solvent separated ion pair (SSIP) in these solutions. Analysis of local ion clusters shows that ions are dominantly solvated by water molecules. The 12-6-1 potential model predicts running coordination numbers closest to experimental data.

The structures of the solvation shells around each ion of the Na+-Cl- ion pair in liquid dimethyl sulfoxide (DMSO) have been studied in terms of the ion-solvent radial distribution functions (RDFs) and the ion-solvent orientational distribution functions (ODFs) at the three interionic separations of 2.6 Angstrom 4.9 Angstrom, and 7.2 Angstrom. The solvation shell around the sodium ion consists of only three DMSO molecules at the ion-ion separation of 2.6 W and this number grows to live DMSO molecules at interionic separation 4.9 K and beyond. These are in contrast with the octahedral solvation shells around sodium ion in water at all ion-ion separations, where the chloride ion replaces a molecule of water only at a short interionic distance of 2.7 A. The orientational structure of the solvent around the ion pair has been probed by dividing the DMSO solvent into five spatial regions and analyzing the angular distributions in each region. In the shell near the Naf ion, the orientation of the sulphur-oxygen vector in DMSO is sharply peaked about 155 degrees away from the sodium-sulphur vector for all the three interionic distances. Similarly, the orientation of the DMSO dipole vector is also sharply peaked about 155 degrees away from the sodium-DMSO center of mass (COM) vector. In the shell near the Cl- ion, the orientation of the sulphur-oxygen vector with respect to the chloride-sulphur vector shows broader peaks in the range 20 degrees-100 degrees. The solvent dipole vector gets oriented in a similar fashion with respect to the chloride-COM vector in this shell. In the regions far from the Na+ and Cl- solvation shells, both the sulphur-oxygen vector and the solvent dipole vector have broad distributions covering all the angles except the parallel and the antiparallel alignments. The angles between the Na+-S-O plane (or the Cl--S-O plane) and the S-Na+-C1(-) plane do not show a preference for any specific inclination, in any of the spatial regions around the ion pair. These broad distributions are indicative of a weaker second shell around the ion pair in DMSO than the second shell found in water and are a consequence of the near absence of hydrogen bonding in DMSO. (C) 1998

Structure and dynamics of potassium chloride ion pair in mixtures of dimethyl sulfoxide (DMSO) and water have been investigated using constrained molecular dynamics. We have obtained the potentials of mean force (PMFs) between the K+ and Cl- ions at 298 K over a wide concentration range of these mixtures. The results obtained are compared for relative stabilities of contact ion pair (CIP), solvent assisted ion pair (SAIP) and solvent separated ion pair (SSIP) in the mixtures with varying compositions. The stability of the CIPs decreases significantly as the mole fraction of water increases. The study of running coordination numbers at the first solvation shell suggests preferential solvation of ions by water in all the compositions and there is hardly any participation by DMSO molecules in the first solvation shell of the Cl- ion. The extent of association of potassium chloride ion pair in water. DMSO and their mixtures is determined by calculating the association constants. We have calculated the diffusion constants of ions and solvent molecules in the first solvation shell around K+ and Cl- in all the compositions. The diffusion constants of the shell molecules are the smallest in the DMSO mole fraction ranging from 0.2 to 0.5. (C) 2013 Elsevier B.V. All rights reserved.

We extend, via a reformulation in terms of Poisson brackets, the method developed previously (Rey et al., J. Phys. Chem. A, 2009, 113, 8949) allowing analysis of the pathways of an excited molecule's ultrafast vibrational relaxation in terms of intramolecular and intermolecular contributions. In particular we show how to ascertain, through the computation of power and work, which portion of an initial excess molecular energy (e.g. vibrational) is transferred to various degrees of freedom (e.g. rotational, translational) of the excited molecule itself and its neighbors. The particular case of bend excess energy relaxation in pure water is treated in detail, completing the picture reported in the work cited above. It is shown explicitly, within a classical description, that almost all of the initial water bend excitation energy is transferred—either indirectly, via Fermi resonance centrifugal coupling to the bend-excited water's rotation, or directly via intermolecular coupling— to local water librations, only involving molecules in the first two hydration shells of the vibrationally excited water molecule. Finally, it is pointed out that the Poisson bracket formulation can also be applied to elucidate the microscopic character of solvation and rotational dynamics, and should prove useful in developing a quantum treatment for energy flow in condensed phases.

A good example of macromolecular recognition is found in the interaction of the two monomers of the dimeric superoxide dismutase protein found in Photobacterium leiognathi. We have produced, by molecular dynamics simulation techniques, a specific path for the rupture of the dimer and calculated the effective force involved in the process by extending a well established free energy calculation scheme, the molecular dynamics blue moon approach to rare events. Within this picture we have generalized the approach to a vectorial reaction coordinate and performed a number of different simulations in function of the monomer-momomer separation, at fixed relative orientation. We find a deep minimum and we compute the height of the free energy barrier to break the dimer. As for the system characterization we have found that, when the separation distance increases, the protein structure is stable and the monomer-monomer interface is uniformly hydrated. Moreover, identifying the crucial contacts for the stabilization...

In a mixed solvent of water and dimethyl sulphoxide (DMSO) (with a mole fraction of DMSO = 0.21), the solute sodium chloride forms two distinct solvent separated ion pair (SSIP) species at interionic separations of 5.4 Angstrom and 7.1 Angstrom respectively. These SSIPs are separated by a free energy barrier at 6.6 Angstrom. Molecular Dynamics (MD) simulations are performed on the ion pair in the presence of the mixed solvent to study the solvation structure and the solvation dynamics through the friction kernels. The structural studies bring out the gradual evolution of the solvation of the individual ions as the interionic separation is increased and also the influence of both the components of the solvent mixture. The solvation shell of the ion pair has dominant contribution from water at all the three separations. Analysis of the computed normalised friction kernels of the ion pair at ion-ion separations of 5.4 Angstrom. (SSIP1), 6.6 Angstrom (TS) and at 7.1 Angstrom (SSIP2) reveals that the relative contribution to the net solvent friction by the individual solvents are different at these interionic distances. At all the ion-ion separations, the net dynamic friction experienced by the ion pair in the presence of the mixed solvent is similar to that in water. Intersolvent cross-correlated frictions on the ion pair are negative at short times and decay to zero within 0.7 ps. The dynamics of the barrier crossing reaction has also been studied using the Kramers and Grote-Hynes (GH) theories. The magnitude of the barrier frequency in the mixed solvent is larger than the corresponding barrier frequencies in water as well as in DMSO. The GH transmission coefficient approximately matches with the transmission coefficient obtained by the direct MD simulations. (C) 1998

The dynamics of association of Na+−Cl-, Na+−Na+, and Cl-−Cl- ion pairs in liquid dimethyl sulfoxide is studied by using the method of constrained molecular dynamics. Mean force potentials are employed to investigate the role of the solvent on the ion pairs. Friction kernels for the relative dynamics of the ion pairs have been evaluated at several interionic distances. Kramers and Grote−Hynes theories are applied to understand the passage of the ion pairs across the potential energy barrier existing between a contact ion pair and a solvent-separated ion pair. Transmission coefficients for the Na+−Cl- ion pair calculated from the above theories are in good agreement with the direct computer simulation results. The magnitudes of the squares of the nonadiabatic barrier frequencies are very large, and these confirm a polarization caging of the reactant ion pairs by the large solvent molecules.

Quantum molecular dynamics simulations have been performed for an alkali atom (sodium) diluted in a model dipolar fluid, at various solvent densities, and for the sodium/pyrazine ion pair in the same liquid. The excitonic state proposed by Logan, i.e., an hybrid s/p dipolar atom, is found to be the ground state of the solvated sodium. The Na atomic dipole increases with solvent density. The dipole moment of the Na/pyrazine pair increases with solvent density as well: In this case the Na self‐excitation is replaced by electron transfer from Na to pyrazine resulting in a larger dipole moment stabilized by the solvent. Our results compare favorably with available experimental data.

Computer simulations are performed to study the structure of the coordination shells of Fe+2 and Fe+3 ions fixed at a very close reactive separation. The simulations show that it is possible for the two octahedral aquo complexes, i.e., Fe(H2O)6(2+) and Fe(H2O)6(3+), to come as close as 5 angstrom without disrupting their coordination shells. The reorientational dynamics within the hydration shells of these ions at this separation is examined by studying the time correlation functions (TCFs) of the unit vectors on the water molecules and along the iron-oxygen vector. The quantities related to the solvent polarization relaxation during a change in the charge, the dipole moment, and the quadrupole moment located at the solute ions are examined by studying the corresponding TCFs in the system. The TCFs exhibited a bimodal response, with a very fast initial relaxation due to inertial motions of the solvent, followed by a long tail corresponding to a diffusive component. The polarization fluctuations are also estimated via the cavity field time correlation function (CFTCF), which is useful in the theory of electron transfer processes. The memory kernel or the time dependent friction (TDF) of the solvent is also estimated from the force-force time correlation function. The cross correlations between the Coulombic and the non-Coulombic components of the forces at the ions contribute significantly to the TDF.