Song-Ho Chong

Dynamics of ions in liquid water: An interaction-site-model description

We present a molecular theory for investigating the dynamics of ions in polar liquids. The theory is based on the interaction-site model for molecular liquids and on the generalized Langevin equation combined with the mode-coupling theory. The velocity autocorrelation function, diffusion and friction coefficients of ions in water at 25 °C and at infinite dilution are studied. The theoretical results for the velocity autocorrelation functions exhibit a gradual change from oscillatory to monotonic decay as the ion size increases. The diffusion (friction) coefficients of ions in aqueous solutions pass through a maximum (minimum) as a function of the ion size, with distinct curves and maxima (minima) for positive and negative ions. These trends are in complete accord with those of the molecular dynamics simulation results performed on the same system by Rasaiah and co-workers [J. Phys. Chem. B 102, 4193 (1998)]. It is worthwhile to mention that this is the first molecular theory that is capable of describing ...

DYNAMICS OF SOLVATED ION IN POLAR LIQUIDS : AN INTERACTION-SITE-MODEL DESCRIPTION

We present a molecular theory for the friction coefficient of an ion based on the interaction-site model for molecular liquids and on the mode-coupling theory. The ionic friction is described in terms of the response of collective excitations in solvent to a solute displacement, and the resultant formula expresses the friction as an amount of the energy dissipated during the relaxational processes after the solute perturbation. Utilizing this viewpoint, the ionic friction is shown to be naturally decomposed into hydrodynamic and dielectric contributions and their coupling. It is demonstrated from theoretical calculations of the ionic frictions that both classical pictures, the solventberg and dielectric friction pictures, are responsible for large frictions of small ions, and that the coupling term, which has been disregarded in theoretical considerations so far, should also be properly taken into account in dealing with small ions.

Nonlinear electrical potential fluctuations of solvent around solutes: An integral equation study

We present a theoretical formulation for the solvent force constant and the asymmetry parameter which characterize the free energy profile governing the electrical potential fluctuations of the solvent molecules around solutes. This formulation allows the calculation of these parameters via the extended reference interaction site method, an integral equation theory developed for molecular liquids and solutions. Since the method is inherently a nonlinear theory, we can shed some light on the possible nonlinear effects in the electrical potential fluctuations. The nonlinear effects in the solvent force constant and the validity of the linear approximation will be examined by a series of calculations for monatomic and diatomic solutes immersed in a simple dipolar solvent. The origin of the solute-dependent solvent force constant is discussed.

Interaction-site-model description of collective excitations in liquid water. I: Theoretical study

Collective excitations in liquid water are investigated using the recently developed theory for dynamics of molecular liquids which is based on the interaction-site model for polyatomic fluids, the projection-operator formalism of Zwanzig and Mori, and the simple approximation scheme for memory functions. It is shown that all the essential features of collective excitations in water, reported previously by neutron-scattering experiments, molecular dynamics simulations and dielectric theories, are well reproduced by the present theory.

Interaction-site-model description of collective excitations in liquid water. II: Comparison with simulation results

Theoretical results for dynamical correlation functions characterizing collective excitations in liquid water reported in a previous paper are compared with molecular dynamics simulation results performed on the same system. We also examine the validity and accuracy of the approximation scheme for memory functions employed in our theory by investigating whether the assumed form for memory functions and resulting expressions for dynamical correlation functions can be used as faithful models to reproduce the “experimental” data determined from the simulation.

Effect of molecular symmetry on electrical potential fluctuations of solvent around solute in polar liquid

Abstract The electrical potential fluctuations of solvent around solute in polar liquid are studied using the recently developed method based on an integral equation theory of molecular liquids. We focus on how the molecular symmetry of solvent and solute affects the free energy profile governing electrical potential fluctuations of solvent around solute and characteristic parameters relevant to electron transfer reactions in solutions.

The Free Energy Profiles for the Electron Transfer Reactions Calculated from the Integral Equation Method of Liquids

Abstract A microscopic description of the free energy profile for the electron tranfer reaction is presented based on the extended reference interaction site method (ex-RISM) in the statistical mechanical theory of molecular liquids. We employed the scheme proposed by Marcus to characterize the non-equilibrium solvent polarizations at the intermediate states of the reaction. The method is applied to two models of electron exchange reactions where the redox pairs have valence charges of ½+/½− and 2+ /3+, and to a model of a charge separation reaction. Due to the non-linear nature of the hypernetted chain (HNC) closure to solve the RISM equation, our method can shed light on the non-linearity of the free energy profiles.

Response to “Comment on ‘Dynamics of solvated ion in polar liquids: An interaction-site-model description’ ” [J. Chem. Phys. 110, 1833 (1999)]

We reply to the comments by Biswas and Bagchi on our paper concerning dynamics of solvated ion in polar liquids. Some of the disagreement between us is merely due to the ambiguity in definitions traditionally used in the field, and to the difference in our attitude in science. For those, we have made the definition and our stand point clear. We also clarify the relation between our microscopic results and the macroscopic expression for the Stokes friction.

Interaction-site representation for collective excitations in diatomic dipolar liquid

Abstract We describe a molecular theory for solvent dynamics which provides a method for calculating the dynamic structure factors of molecular fluids. The theory is based on the generalized Langevin equation and on the interaction-site model for molecular liquids. A simple approximation scheme for the memory function is developed by generalizing the conventional one which has been successfully applied to monatomic liquids. The theory is applied to the calculation of longitudinal current spectra of a diatomic dipolar liquid, and collective excitations in this solvent are studied. The paper clarifies how these excitations originate from the translational and rotational motions of molecules.

Non-linear effects in the free energy profile of electron transfer reaction: An integral equation approach

Abstract The free energy profile for the electron transfer reaction in a polar solvent is examined based on the extended reference interaction site method (ex-RISM) applying it to a simple model of a charge separation reaction which was previously studied by Carter and Hynes with molecular dynamics simulations. Due to the non-linear nature of the hypernetted chain (HNC) closure to solve the RISM equation, our method can shed light on the non-linearity of the free energy profiles, and we discuss these problems based on the obtained free energy profile.