title: A continuum solvent model of ion-ion interactions in water.
abstract: The calculation of ion-ion interactions in water is a problem of long standing importance. Modelling these interactions is a prerequisite to explaining Hofmeister (specific ion) effects. We here generalize our solvation model of ions to calculate the free energy of two ions in water as a function of separation. The same procedure has previously been applied to calculate ion interactions with the air-water interface successfully. The Conductor like Screening Model (COSMO) is used. This treats the ions on a quantum mechanical level and calculates numerically the electrostatic response of the surrounding solvent. Estimates of the change in the cavity formation energy and the change in the ion-water dispersion energy as the ions approach are included separately. The calculated interaction potentials are too attractive and this is a significant issue. However, they do reproduce the affinity of similarly sized ions for each other, which is a crucial property of these potentials. They are also oscillatory, another important property. We normalize the potentials to reduce the over-attraction, and good correlation with experimental values is achieved. We identify the driving contributions to this like-prefers-like behaviour. We then put forward a plausible hypothesis for the over-attraction of the potentials. An agreeable feature of our approach is that it does not rely on salt specific parameters deliberately adjusted to reproduce experimental values.
authors.affiliation: Applied Mathematics Department, Australian National University, Canberra, ACT 0200, Australia. tim.duignan@anu.edu.au.
authors.forename: Timothy T
authors.initials: TT
authors.lastname: Duignan
authors.forename: Drew F
authors.initials: DF
authors.lastname: Parsons
authors.forename: Barry W
authors.initials: BW
authors.lastname: Ninham
mesh_terms: D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D014867:Water / Q000737:chemistry*
chemical_list: D007477:Ions; D012997:Solvents; D014867:Water