Hydration of Molecular Anions with Oxygen Sites – a Monte Carlo Study

Molecular modeling and simulation of aqueous solutions of alkali nitrates

A set of molecular models for the alkali nitrates (LiNO3, NaNO3, KNO3, RbNO3, and CsNO3) in aqueous solutions is presented and used for predicting the thermophysical properties of these solutions with molecular dynamics simulations. The set of models is obtained from a combination of a model for the nitrate anion from the literature with a set of models for the alkali cations developed in previous works of our group. The water model is SPC/E and the Lorentz–Berthelot combining rules are used for describing the unlike interactions. This combination is shown to yield fair predictions of thermophysical and structural properties of the studied aqueous solutions, namely the density, the water activity and the mean ionic activity coefficient, the self-diffusion coefficients of the ions, and radial distribution functions, which were studied at 298 K and 1 bar; except for the density of the solutions of all five nitrates and the activity properties of solutions of NaNO3, which were also studied at 333 K. For calculating the water the activity and the mean ionic activity coefficient, the OPAS ( osmotic pressure for the activity of selvents) method was applied. The new models extend an ion model family for the alkali halides developed in previous works of our group in a consistent way.

Hofmeister ion interactions with model amide compounds

Dissolved electrolytes interact with peptides and proteins in aqueous solution. Herein, we study small amide compounds in aqueous electrolyte solutions and link their salting-in and salting-out propensities to molecular-level structural details obtained with molecular simulations. Aqueous solutions of NaF, NaCl, NaBr, NaI, NaNO(3), and NaClO(4) with N-isopropylacrylamide (NiPAM) and N-methylacetamide (NMA) have been investigated. Our results show that NiPAM is salted-in by NaI, mediated through iodide interactions with nonpolar groups, while being salted-out by the other salts. Hydrogen-bonding interactions of anions with the amide group of NiPAM could not be identified, while in the systems with NMA all Hofmeister anions formed stable hydrogen bonds with the amide group. These results indicate that the immediate chemical environment of the backbone amide groups should be considered in studies of protein destabilization by dissolved electrolytes. We furthermore report that all salts but NaI provoke a hydrophobic collapse transition of poly(N-isopropylacrylamide) in water at 300 K, in qualitative agreement with experimentally measured salt effects on the lower critical solution temperature of this system.

Solution structure of NaNO3 in water: diffraction and molecular dynamics simulation study

The structure of a series of aqueous sodium nitrate solutions (1.9-7.6 M) was studied using a combination of experimental and theoretical methods. The results obtained from diffraction (X-ray, neutron) and molecular dynamics simulation have been compared and the capabilities and limitations of the methods in describing solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in description of hydration spheres of the sodium ion but do not yield detailed structural information on the anion's hydration structure. Molecular dynamics simulations proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions, ion pair formation, and bulk structure of solutions.

Is There an Anionic Hofmeister Effect on Water Dynamics? Dielectric Spectroscopy of Aqueous Solutions of NaBr, NaI, NaNO3, NaClO4, and NaSCN

A systematic study of the dielectric relaxation spectra of aqueous solutions of NaBr, NaI, NaNO(3), NaClO(4), and NaSCN has been made over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) and solute concentrations (0.05 < or = c/M < or = 1.5) at 25 degrees C. The spectra could be adequately described by a single Cole-Cole (CC) process, symmetrically broadened relative to that of pure water. However, similar quality fits were also obtained with a three-Debye-process (3D) model consisting of a small ion-pair contribution at lower frequencies and two solvent relaxations at higher frequencies. Assuming the ions to be solvent separated, the 3D model provided estimates of their association constants and their rate constants for formation and dissociation. The bulk water relaxation times obtained from both models showed almost no dependence on the nature of the anion. Nevertheless, there were subtle differences in the concentration dependences of the relaxation times which correlated with some, but not all, of the anion properties that are believed to be relevant for explaining the anionic Hofmeister series.