Ionic interactions, molecular dynamics

In periodic boimdary conditions, one possible way to avoid truncation of electrostatic interaction is to apply the so-called Particle Mesh Ewald (PME) method, which follows the Ewald summation method of calculating the electrostatic energy for a number of charges [27]. It was first devised by Ewald in 1921 to study the energetics of ionic crystals [28]. PME has been widely used for highly polar or charged systems. York and Darden applied the PME method already in 1994 to simulate a crystal of the bovine pancreatic trypsin inhibitor (BPTI) by molecular dynamics [29]. 

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

Finally, it must be recalled that the transport properties of any material are strongly dependent on the molecular or ionic interactions, and that the dynamics of each entity are narrowly correlated with the neighboring particles. This is the main reason why the theoretical treatment of these processes often shows similarities with models used for thermodynamic properties. The most classical example is the treatment of dilute electrolyte solutions by the Debye-Hiickel equation for thermodynamics and by the Debye-Onsager equation for conductivity. 

This volume of Modem Aspects covers a wide spread of topics presented in an authoritative, informative and instructive manner by some internationally renowned specialists. Professors Politzer and Dr. Murray provide a comprehensive description of the various theoretical treatments of solute-solvent interactions, including ion-solvent interactions. Both continuum and discrete molecular models for the solvent molecules are discussed, including Monte Carlo and molecular dynamics simulations. The advantages and drawbacks of the resulting models and computational approaches are discussed and the impressive progress made in predicting the properties of molecular and ionic solutions is surveyed. 

In an early study, Mauritz et al. investigated anion—cation interactions within Nation sulfonate membranes versus degree of hydration using FTIR/ ATR and solid state NMR (SSNMR) spectroscopies. An understanding of the dynamic ionic—hydrate molecular structures within and between the sulfonate clusters is essential for a fundamental understanding of the action of these membranes in ion transport. This information can be directly related to the equilibrium water swelling that, in turn, influences molecular migration. 

On longer times one nevertheless needs an explicit account of ionic motion. We then treat the ions as classical particles described within the standard framework of molecular dynamics. The force acting on the ions originates from the electrons (through pseudopotentials), from ion-ion interactions (treated as point charges) and from the external field (laser, projectile). For ion number 7, it thus reads... 

To summarize briefly the use of nmr methods for the structural and dynamic problems at hand, a first class of applications is the direct observation of the nuclei present in the solid support (29Si, 27A1, 170, in the first place). Alternatively, one may examine nuclei of ionic species (23Na, 35C1, 39K, etc.) or of neutrals ( H, 13C, etc.) interacting with the clay surface. In this second mode, one reaps information about restriction in ionic or molecular mobility, translational and reorientational, due to the proximity of the aluminosilicate solid surface. 

We showed previously that a simple model for the ion-hydration interactions, which separates the ion-hydration forces in a long-range term due to the behavior of water as a continuous dielectric (the screened image force) and a short-range term due to the discreetness of the water molecules (SM/SB), can explain almost quantitatively a number of phenomena related to the electrolyte interfaces.6 In this article, we examined the limitations of the model in predicting the distributions of ions near the air/water interface, by comparison with molecular dynamics simulations. It is clear that the real ion-hydration forces are more complicated than the simple model employed here however, the interfacia] phenomena (including specific ionic effects) can be understood, at least qualitatively, in terms of this simple approach. 

The physical chemistry of ionic solutions deals with ions and solvents and how ions interact dynamically with water as they move about in solutions. The study of ion-ion interactions tells us how ions associate, sometimes even forming polymers in solution. These interactions are important for the new spectroscopic techniques, neutron diffraction and infrared spectroscopy and for molecular dynamics (MD). 

Finally, there is the theoretical method of approaching ionic solvation including the molecular dynamics simulations. These have become increasingly used because they are cheap and quick. However, MD methods use two-body interaction equations and the parameters used here need experimental data to act as a guide for the determination of parameters that fit. 

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