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 ͑2009͔͒; this water model accommodates increased solvent polarizability ͑relative to the condensed phase͒ 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 ͑TI͒ 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 solvent͒, 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 Å 3 for TIP4P-QDP versus 1.1 Å 3 ͑TIP4P-FQ͒ and 0.87 Å 3 ͑POL3͒ ͓Ponder and Case, Adv. Protein Chem. 66, 27 ͑2003͔͖͒ 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 region͒ 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.

Solvation structure and energetics of single ions at the aqueous liquid–vapor interface

“…Results from 1M solutions NaX (X={Cl − ,Br − ,I − }) salts in TIP4P-QDP water showed a significant enhancement of anion density in the interfacial region, 20 whereas analogous enhancement in simulations of salts in TIP4P-FQ showed only enhancement for iodide (chloride showed no enhancement). 14 These differences in interfacial ion enhancement are reflected in the PMFs for single ions at the LV interface (Figure 1a).…”

“…These properties were also computed for ions in TIP4P-QDP water. 20 Unlike the TIP4P-FQ/ion sets, all anions in TIP4P-QDP show a pronounced minimum approximately 2 Å below the GDS. These ions have less favorable hydration free energies than analogous ions in TIP4P-FQ and also demonstrate a sharper decrease in coordination number as they move across the LV interface.…”

“…Parameters for simulations of ions in TIP4P-QDP were taken from Bauer and Patel. 20 A harmonic restoring potential was implemented to prevent unfavorable overpolarization of the TIP4P-QDP water. This potential with force constant 200.0 (kcal/mol)/esu 2 was turned on beyond upper and lower charge limits −0.6 e and −1.6 e for the M -site and 1.2 e and 0.3 e for hydrogen sites.…”

“…Additionally, we draw a qualitative comparison of the PMF and solvation properties of the TIP4P-FQ-ion systems with those of TIP4P-QDP. 20 These systems show differences in anionic enhancement at the liquid-vapor interface for 1M salt solutions, and such a comparison lends insight into the relationship between ion hydration and interfacial activity.…”

“…18 . We have previously studied inorganic ions at the water liquid-vapor interface using polarizable water and non-polarizable ion force fields 14,19,20 . We employ polarizable water force fields based on the charge equilibration (fluctuating charge) formalism.…”