Molecular dynamics simulations of nonpolarizable inorganic salt solution interfaces: NaCl, NaBr, and NaI in transferable intermolecular potential 4-point with charge dependent polarizability (TIP4P-QDP) water

Abstract

We present molecular dynamics simulations of the liquid-vapor interface of 1M salt solutions of nonpolarizable NaCl, NaBr, and NaI in polarizable transferable intermolecular potential 4-point with charge dependent polarizability water [B. A. Bauer et al., J. Chem. Theory Comput. 5, 359 (2009)]; this water model accommodates increased solvent polarizability (relative to the condensed phase) in the interfacial and vapor regions. We employ fixed-charge ion models developed in conjunction with the TIP4P-QDP water model to reproduce ab initio ion-water binding energies and ion-water distances for isolated ion-water pairs. The transferability of these ion models to the condensed phase was validated with hydration free energies computed using thermodynamic integration (TI) and appropriate energy corrections. Density profiles of Cl(-), Br(-), and I(-) exhibit charge layering in the interfacial region; anions and cation interfacial probabilities show marked localization, with the anions penetrating further toward the vapor than the cations. Importantly, in none of the cases studied do anions favor the outermost regions of the interface; there is always an aqueous region between the anions and vapor phase. Observed interfacial charge layering is independent of the strength of anion-cation interactions as manifest in anion-cation contact ion pair peaks and solvent separated ion pair peaks; by artificially modulating the strength of anion-cation interactions (independent of their interactions with solvent), we find little dependence on charge layering particularly for the larger iodide anion. The present results reiterate the widely held view of the importance of solvent and ion polarizability in mediating specific anion surface segregation effects. Moreover, due to the higher parametrized polarizability of the TIP4P-QDP condensed phase {1.31 A(3) for TIP4P-QDP versus 1.1 A(3) (TIP4P-FQ) and 0.87 A(3) (POL3) [Ponder and Case, Adv. Protein Chem. 66, 27 (2003)]} based on ab initio calculations of the condensed-phase polarizability reduction in liquid water, the present simulations highlight the role of water polarizability in inducing water molecular dipole moments parallel to the interface normal (and within the interfacial region) so as to favorably oppose the macrodipole generated by the separation of anion and cation charge. Since the TIP4P-QDP water polarizability approaches that of the experimental vapor phase value for water, the present results suggest a fundamental role of solvent polarizability in accommodating the large spatial dipole generated by the separation of ion charges. The present results draw further attention to the question of what exact value of condensed phase water polarizability to incorporate in classical polarizable water force fields.

Figure 1

RDFs specific for 1M solutions of NaCl (dashed line), NaBr (dashes and dots), and NaI (solid line) in TIP4P-QDP. (a) Oxygen-oxygen RDF (0.5 vertical offset). (b) Anion-anion RDF. (c) Anion-oxygen RDF. (d) Cation-anion RDF. (e) Cation-cation RDF (0.25 vertical offset). (f) Cation-oxygen RDF (0.5 vertical offset).

Figure 2

Density profiles for (a) NaCl, (b) NaBr, and (c) NaI solutions (approximately 1M). Water (solid line), anion (dashed line), and cation (dotted line) density profiles are included for each solution. Densities are normalized by the bulk density from computed from NPT simulations. The z-position is relative to the GDS, which is featured as the dotted vertical line.

Figure 3

Effects of anion-cation interactions on charge layering at the interface. The NaCl density profiles (a) and RDFs (c) for varying sets of Na–Cl interaction energies. Panels (b) and (d) are the analogous figures for NaI. In all panels, the solid lines indicate data in which the interaction between anion and cation is determined via combining rules (set 1 data, which is the data presented previously in this work); the dashed and dotted lines represent data in which the interaction has been made increasingly favorable.

Figure 4

Water contribution to the in-plane dielectric constant as a function of depth relative to the GDS for salt solutions (a) in TIP4P-QDP solvent and (b) in TIP4P-FQ solvent.

Figure 5

Orientational profiles of the water in each solution as a function of depth relative to the GDS. The upper panel (a) features the ⟨cos θ⟩ as a function of z, where θ is the angle made between the permanent dipole vector of the water molecule and the surface normal. The lower panel (b) features the ⟨|cos ϕ|⟩ as a function of z, where ϕ is the angle made between the molecular plane and the surface normal.

Figure 6

Average magnitude of the z-induced dipole moment of water in each salt solution relative to the GDS.

Figure 7

Interfacial potential for the water of each salt solution as a function of depth relative to the GDS.