The short range anion-H interaction is the driving force for crystal formation of ions in water.

The crystal formation of NaCl in water is studied by extensive molecular dynamics simulations. Ionic solutions at room temperature and various concentrations are studied using the SPC/E and TIP4P/2005 water models and seven force fields of NaCl. Most force fields of pure NaCl fail to reproduce the experimental density of the crystal, and in solution some favor dissociation at saturated conditions, while others favor crystal formation at low concentration. A new force field of NaCl is proposed, which reproduces the experimental phase diagram in the solid, liquid, and vapor regions. This force field overestimates the solubility of NaCl in water at saturation conditions when used with standard Lorentz-Berthelot combining rules for the ion-water pair potentials. It is shown that precipitation of ions is driven by the short range interaction between Cl-H pairs, a term which is generally missing in the simulation of ionic solutions. The effects of intramolecular flexibility of water on the solubility of NaCl ions are analyzed and is found to be small compared to rigid models. A flexible water model, extending the rigid SPC/E, is proposed, which incorporates Lennard-Jones interactions centered on the hydrogen atoms. This force field gives liquid-vapor coexisting densities and surface tensions in better agreement with experimental data than the rigid SPC/E model. The Cl-H, Na-O, and Cl-O pair distribution functions of the rigid and flexible models agree well with experiment. The predicted concentration dependence of the electric conductivity is in fair agreement with available experimental data.

[1]  D. Frenkel,et al.  Rate of homogeneous crystal nucleation in molten NaCl. , 2005, The Journal of chemical physics.

[2]  I. R. Mcdonald,et al.  Theory of simple liquids , 1998 .

[3]  J. Alejandre,et al.  Thermodynamic and transport properties of simple fluids using lattice sums: bulk phases and liquid-vapour interface , 2002 .

[4]  N. Karasawa,et al.  Acceleration of convergence for lattice sums , 1989 .

[5]  L. Dang,et al.  Mechanism and Thermodynamics of Ion Selectivity in Aqueous Solutions of 18-Crown-6 Ether: A Molecular Dynamics Study , 1995 .

[6]  Mikko Karttunen,et al.  Systematic comparison of force fields for microscopic simulations of NaCl in aqueous solutions: Diffusion, free energy of hydration, and structural properties , 2004, J. Comput. Chem..

[7]  E. Spohr.  Molecular simulation of the electrochemical double layer , 1999 .

[8]  G. Ciccotti,et al.  Solubility of KF in water by molecular dynamics using the Kirkwood integration method , 2002 .

[9]  C. Vega,et al.  Surface tension of the most popular models of water by using the test-area simulation method. , 2007, The Journal of chemical physics.

[10]  Dean R. Haeffner,et al.  Electron distribution in water , 2000 .

[11]  C. Cavazzoni,et al.  Structure of NaCl and KCl concentrated aqueous solutions by ab initio molecular dynamics , 2004 .

[12]  M. Bellissent-Funel,et al.  Hydration of Ni2+ and Cl− in a concentrated nickel chloride solution at 100 °C and 300 °C , 1996 .

[13]  T. Straatsma,et al.  THE MISSING TERM IN EFFECTIVE PAIR POTENTIALS , 1987 .

[14]  T. Darden,et al.  A smooth particle mesh Ewald method , 1995 .

[15]  Neal T. Skipper,et al.  X-ray and neutron diffraction studies on concentrated aqueous solutions of sodium nitrate and silver nitrate , 1989 .

[16]  M. P. Tosi,et al.  Ionic sizes and born repulsive parameters in the NaCl-type alkali halides—II: The generalized Huggins-Mayer form☆ , 1964 .

[17]  D. Powell,et al.  The structure of Cl- in aqueous solution: an experimental determination of gClH(r) and gClO(r) , 1993 .

[18]  J. Alejandre,et al.  Effect of flexibility on surface tension and coexisting densities of water. , 2008, The Journal of chemical physics.

[19]  J. Ottino,et al.  Monte Carlo calculations of cluster statistics in continuum models of composite morphology , 1988 .

[20]  S. Neyertz,et al.  A GENERAL PRESSURE TENSOR CALCULATION FOR MOLECULAR DYNAMICS SIMULATIONS , 1995 .

[21]  C. Vega,et al.  A general purpose model for the condensed phases of water: TIP4P/2005. , 2005, The Journal of chemical physics.

[22]  D. Tildesley,et al.  Molecular dynamics simulation of the orthobaric densities and surface tension of water , 1995 .

[23]  A. Pilla,et al.  DENSITIES OF SOLID SALTS AT ELEVATED TEMPERATURES AND MOLAR VOLUME CHANGE ON FUSION , 1960 .

[24]  M. Tuckerman,et al.  A Liouville-operator derived measure-preserving integrator for molecular dynamics simulations in the isothermal–isobaric ensemble , 2006 .

[25]  B. Montgomery Pettitt,et al.  Alkali halides in water: Ion–solvent correlations and ion–ion potentials of mean force at infinite dilution , 1986 .

[26]  J. Israelachvili.  Intermolecular and surface forces , 1985 .

[27]  David E. Smith,et al.  Computer simulations of NaCl association in polarizable water , 1994 .

[28]  T. Darden,et al.  Liquid–vapour equilibrium of n -alkanes using interface simulations , 2006 .

[29]  Michele Parrinello,et al.  Water Molecule Dipole in the Gas and in the Liquid Phase , 1999 .

[30]  J. Rasaiah,et al.  Computer simulation studies of aqueous sodium chloride solutions at 298 K and 683 K , 2000 .

[31]  J. Hansen,et al.  Ions in water: from ion clustering to crystal nucleation. , 2007, Physical review. E, Statistical, nonlinear, and soft matter physics.

[32]  A. Lefohn,et al.  An improved Polarflex water model , 2003 .

[33]  C. Vega,et al.  Solubility of KF and NaCl in water by molecular simulation. , 2007, The Journal of chemical physics.

[34]  Bertrand Guillot,et al.  A reappraisal of what we have learnt during three decades of computer simulations on water , 2002 .

[35]  R. Lynden-Bell,et al.  Phase diagrams and surface properties of modified water models , 2007 .

[36]  W. L. Jorgensen,et al.  Comparison of simple potential functions for simulating liquid water , 1983 .

[37]  S. Weerasinghe,et al.  A Kirkwood–Buff derived force field for sodium chloride in water , 2003 .

[38]  Martin Mucha,et al.  Salt Crystallization from an Evaporating Aqueous Solution by Molecular Dynamics Simulations , 2003 .

[39]  B. Guillot,et al.  Coexisting phases and criticality in NaCl by computer simulation , 1994 .