dorsal/arxiv
View SchemaThe hydration number of Na+ in liquid water
| Authors | Susan B. Rempe, Lawrence R. Pratt |
|---|---|
| Categories | |
| ArXiv ID | physics/0006026 |
| URL | https://arxiv.org/abs/physics/0006026 |
Abstract
An `ab initio' molecular dynamics simulation of a Na+ ion in aqueous solution is presented and discussed. The calculation treats a Na+ ion and 32 water molecules with periodic boundary conditions on a cubic volume determined by an estimate of zero partial molar volume for this solute in water at normal density and at a temperature of 344+/-24 K. Analysis of the last half of the 12 ps trajectory shows 4.6 water molecules occupying the inner hydration shell of the Na+ ion on average, with 5 being the most probable occupancy. The self-diffusion coefficient observed for the Na+ is 1.0 x 10$^{-5}$ cm$^2$/s. The quasi-chemical theory of solutions provides the framework for two more calculations. First a complementary calculation, based on electronic structure results for ion-water clusters and a dielectric continuum model of outer sphere hydration contributions, predicts an average hydration shell occupancy of 4.0. This underestimate is attributed to the harmonic approximation for the clusters in conjunction with the approximate dielectric continuum model treatment of outer sphere contributions. Finally, a maximum entropy fitting of inner sphere occupancies that leads to insensitive composite free energy approximations suggests a value in the neighborhood of -68 kcal/mol for the hydration free energy of Na+ (aq) under these conditions with no contribution supplied for packing or van der Waals interactions.
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"abstract": "An `ab initio\u0027 molecular dynamics simulation of a Na+ ion in aqueous solution\nis presented and discussed. The calculation treats a Na+ ion and 32 water\nmolecules with periodic boundary conditions on a cubic volume determined by an\nestimate of zero partial molar volume for this solute in water at normal\ndensity and at a temperature of 344+/-24 K. Analysis of the last half of the 12\nps trajectory shows 4.6 water molecules occupying the inner hydration shell of\nthe Na+ ion on average, with 5 being the most probable occupancy. The\nself-diffusion coefficient observed for the Na+ is 1.0 x 10$^{-5}$ cm$^2$/s.\nThe quasi-chemical theory of solutions provides the framework for two more\ncalculations. First a complementary calculation, based on electronic structure\nresults for ion-water clusters and a dielectric continuum model of outer sphere\nhydration contributions, predicts an average hydration shell occupancy of 4.0.\nThis underestimate is attributed to the harmonic approximation for the clusters\nin conjunction with the approximate dielectric continuum model treatment of\nouter sphere contributions. Finally, a maximum entropy fitting of inner sphere\noccupancies that leads to insensitive composite free energy approximations\nsuggests a value in the neighborhood of -68 kcal/mol for the hydration free\nenergy of Na+ (aq) under these conditions with no contribution supplied for\npacking or van der Waals interactions.",
"arxiv_id": "physics/0006026",
"authors": [
"Susan B. Rempe",
"Lawrence R. Pratt"
],
"categories": [
"physics.chem-ph",
"physics.bio-ph"
],
"title": "The hydration number of Na+ in liquid water",
"url": "https://arxiv.org/abs/physics/0006026"
},
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