Concentration effects in aqueous NaCl solutions. A molecular dynamics simulation (original) (raw)
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Solvent Structure, Dynamics, and Ion Mobility in Aqueous Solutions at 25 °C
The Journal of Physical Chemistry B, 1998
Brand nd Iat infinite dilution by molecular dynamics simulation using the SPC/E model for water at 25°C and a reaction field for the long-range interactions. The ion mobilities show the same trends as the experimental results with distinct maxima for cations and anions. The mobilities (defined by u i) D i /kT) of the corresponding uncharged species are also determined by simulation and are in qualitative agreement with Stokes' law. The mobilities of Li + , Na + , K + , Rb + and Fincrease on discharge, whereas Cl, Br, and I have smaller mobilities than the corresponding anions. The mobility of the fictitious I + ion, which differs from Ionly in its charge, lies between that of Iand I in the order u I < u I + < u I-. The residence time of water in the first solvation shell of small cations (Li + and Na +) and Ca 2+ decreases when the ions are discharged, while the opposite is observed on neutralizing I-, suggesting the formation of a solvent cage around the large uncharged I which partially breaks up on charging, increasing the mobility of the corresponding ion. The cage breakup is greater for Ithan for I + which correlates with the asymmetry in the entropies of solvation of Iand I + , in SPC/E water on charge reversal, providing an explanation for the trends in the mobilities of I, I-, and I +. The residence times of water in the primary hydration shell around cations pass through a minimum as a function of size that correlates with the maximum in the corresponding solvation entropy, suggesting different types of hydration, i.e., electrostatic ion solvation (hydrophilic) and cage formation (hydrophobic) respectively for small and large cations. The results are in accord with recent calculations of the solvation entropy and free energy as continuous functions of the charge and size (
Condensed Matter Physics
Isothermal-isobaric molecular dynamics simulations are used to examine the microscopic structure and other properties of a model solution consisting of NaCl salt dissolved in water-methanol mixture. The SPC/E water model and the united atom model for methanol are combined with the force field for ions by Dang [J. Amer. Chem. Soc., 1995, 117, 6954] to describe the entire system. Our principal focus is to study the effects of two variables, namely, the solvent composition and ion concentrations on the solution's density, on the structural properties, self-diffusion coefficients of the species and the dielectric constant. Moreover, we performed a detailed analysis of the first coordination numbers of the species. Trends of the behaviour of the average number of hydrogen bonds between solvent molecules are evaluated.
A Molecular Dynamics Study of Aqueous Solutions
Zeitschrift für Naturforschung A, 1976
Results of a molecular dynamics study of a 0.55 molal aqueous NaCl solution are reported. The basic periodic box contained 200 water molecules, 2 sodium ions and 2 chloride ions. The calculated properties of this solution are compared with those obtained previously for a 2.2 molal NaCl solution. The formation of second hydration shells, an increase of the number of water molecules in the first hydration shells, and a release of internal pressure are the main changes connected with a decrease of the concentration.
The Journal of Physical Chemistry B, 2013
The association process of NaCl in aqueous solution was studied by a combination of quantum mechanical calculations on NaCl(H 2 O) n (n = 1−6) clusters and quantum mechanical/effective fragment potential−molecular dynamics (QM/EFP-MD) simulations for NaCl in 292 EFP waters. The interionic hydration structures (IHSs) were topologically classified as "ring" (R), "half-bridge" (H), and "full-bridge" (F) types on the basis of the quantum mechanical calculations. Subsequent IHS analysis on QM/EFP-MD simulations revealed that the NaCl contact ion pair (CIP) mainly involved R type hydration structures while the solvent-separated ion pair (SSIP) was composed of two different groups of F-type hydration structures. Our IHS analysis also discovered H type hydration even at large separation interionic distances (∼7 Å), which is denoted as a dissociating ion pair (DIP). The analysis was able to reveal the most complete interionic structures and their reorganizations of the association process. A strong correlation between the IHSs and interionic distance suggests that not only the solvent reorganization but also the local IHS changes are equally important. Mechanistically, it is suggested that the conversion between ring-type and full-bridge hydration structures is the main rate-determining step of ion-pair association.
2004
In this paper we compare different force fields that are widely used (Gromacs, Charmm-22/x-Plor, Charmm-27, Amber-1999, OPLS-AA) in biophysical simulations containing aqueous NaCl. We show that the uncertainties of the microscopic parameters of, in particular, sodium and, to a lesser extent, chloride translate into large differences in the computed radial-distribution functions. This uncertainty reflects the incomplete experimental knowledge of the structural properties of ionic aqueous solutions at finite molarity. We discuss possible implications on the computation of potential of mean force and effective potentials.
Molecular dynamics study of ion hydration under pressure
Journal of Molecular Liquids, 2011
We present results of a molecular dynamics simulation of pressure influence on ion hydration. Four different electrolyte aqueous solutions are considered: NaF, NaCl NaBr, NaI. These systems were modeled at two /pressure, temperature/ regimes: "normal" -10 5 Pa, 298 K and "high" -4·10 9 Pa, 500 K. Structural properties of the considered systems are shown in the form of instantaneous configurations of ionic hydration shells and distributions between the various atoms and/or ions. Dynamic properties are discussed in terms of the self-diffusion coefficients of the ions. We were interested in the pressure effect revealed in evolution of ionic hydration along the transition from "normal" to "high" regime. The results indicate strong changes in the hydration of considered ions as well as their ion-specific behavior.
Osmotic and activity coefficients from effective potentials for hydrated ions
Physical Review E, 1997
Based on a method we previously suggested ͓Phys. Rev. E 52, 3730 ͑1995͔͒, effective interaction potentials between Na ϩ and Cl Ϫ ions have been derived from interionic radial distribution functions ͑RDF͒ in molecular dynamics ͑MD͒ simulations of aqueous NaCl solution. The effective interaction potentials between the hydrated ions, which reproduce the original ion-ion RDF curves, can be used further to construct a corresponding ionic solution in a much larger scale and to calculate any properties dependent on the structure of the electrolyte solution. In a subsequent Monte Carlo ͑MC͒ simulation, using the effective potentials, the osmotic and activity coefficients are calculated for the ions. Calculation of these properties directly from atomic MD or MC simulations is beyond the capacity of the present computers due to the very large number of molecules required in the simulations to obtain reliable results. A very good agreement with the experimental results is obtained. Effects of three-body interactions and concentration dependencies of the effective potentials are discussed. ͓S1063-651X͑97͒01405-0͔
The Journal of Physical Chemistry B, 1998
Structural and dynamical properties of dilute aqueous solutions containing a trivalent cation have been determined by means of Molecular Dynamics simulations. The concept of hydrated ion has been used when considering aqueous solutions of Cr 3+ , [Cr(H 2 O) 6 ] 3+ being the cationic entity interacting in solution. An ab initio Cr 3+ hydrate-water interaction potential previously developed [J. Phys. Chem. 1996, 100, 11748] and a new one describing the Cr 3+ hydrate-Clinteractions have been used with a TIP4P water model to carry out simulations of the system Cr(H 2 O) 6 Cl 3 + 512H 2 O. To examine the role of anions, simulations without chloride ions were performed as well ([Cr(H 2 O) 6 ] 3+ + 512H 2 O). To investigate the influence of shape and size of the hydrated cation, two additional models of trivalent cation have been studied using the simplest concept of spherical ion. Ad hoc charged sphere-water interaction potentials for the latter situations were built. RDFs, hydration numbers, vibrational spectra of the intermolecular modes, translational self-diffusion coefficients for ions and water molecules in the different hydration shells, interdiffusion coefficients, mean residence times, and rotational diffusion coefficients and correlation times for the hexahydrate and water molecules are obtained and discussed. Comparison of dynamical properties of Cr 3+ aqueous solutions with those obtained from simulations of Cr 3+ hexahydrate strongly supports the validity of the hydrated ion model for this cation. The examination of rotational mobility leads to the conclusion that the hydrate ion rotates following Debye's rotational model. Advantages and drawbacks of the hydrated ion approach to deal with solvation of highly charged cations of transition metals are examined. The structural consequences of adopting a spherical shape for cation when developing potentials are quite different when either the bare or hydrated radius is considered; thus, whereas the small sphere overestimates the first shell coordination number, the big sphere overestimates the second hydration shell, promoting a clathrate structure. Specially designed EXAFS measurements of a set of Cr(NO 3 ) 3 aqueous solutions 0.1 M in hydrochloric and hydrobromic acids were carried out and analyzed to investigate the possibility of detecting the halide anion in the first or second hydration shell. Simulations agree with experimental results in the sense that the counterion of Cr 3+ hexahydrate is placed in dilute acidic solutions beyond the second hydration shell.
Molecular dynamics simulations of concentrated aqueous electrolyte solutions
Molecular Simulation, 2011
Transport properties of concentrated electrolytes have been analysed using classical molecular dynamics simulations with the algorithms and parameters typical of simulations describing complex electrokinetic phenomena. The electrical conductivity and transport numbers of electrolytes containing monovalent (KCl), divalent (MgCl2), a mixture of both (KCl+MgCl2) and trivalent (LaCl3) cations have been obtained from simulations of the electrolytes in electric fields of different magnitude. The results obtained for different simulation parameters have been discussed and compared with experimental measurements of our own and from the literature. The electroosmotic flow of water molecules induced by the ionic current in different cases has been calculated and interpreted with the help of the hydration properties extracted from the simulations.
Journal of Chemical Sciences, 2008
Molecular dynamics simulations of dilute and concentrated aqueous NaCl solutions are carried out to investigate the changes of the hydrogen bonded structures in the vicinity of ions for different ion concentrations. An analysis of the hydrogen bond population in the first and second solvation shells of the ions and in the bulk water is done. Although essentially no effect of ions on the hydrogen bonding is observed beyond the first solvation shell of the ions for the dilute solutions, for the concentrated solutions a noticeable change in the average number of water-water hydrogen bonds is observed in the second solvation shells of the ions and even beyond. However, the changes in the average number of hydrogen bonds are found to be relatively less when both water-water and ion-water hydrogen bonds are counted. Thus, the changes in the total number of hydrogen bonds per water are not very dramatic beyond the first solvation shell even for concentrated solutions.