Potentials of Mean Force for Hydrophilic-Hydrophobic Solute Pairs in Water

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Published
2005
DOI
  • 10.1252/jcej.38.465
Publisher
The Society of Chemical Engineers, Japan

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We have carried out the molecular dynamics simulations to investigate not only the structural and dynamical properties of single solutes of the sodium cation (Na+), the chloride anion (Cl–), the tetramethylammonium cation (TMA+), and the methane molecule (CH4) in ambient water at infinite dilution, but the hydration structures and the potentials of mean force (PMFs) for the Na+–CH4, the Cl––CH4, the TMA+–CH4, and the Na+–TMA+ pairs. As a result, it is confirmed that the results of the diffusion coefficients of the four solutes are in quantitative agreement with the experimental data and that the monovalent ions in water exhibit three classes of hydrations, the structure-making hydration (Na+), the structure-breaking hydration (Cl–), and the hydrophobic structure-making hydration (TMA+), depending on the charge density determined by the ion size. A detailed comparison among the PMFs for the Na+–CH4, the Cl––CH4, and the TMA+–CH4 pairs indicates that, as for the monovalent ion–methane pairs in water, the increase of the ion size diminishes the instability of a contact solute pair due to the compactness of the hydrated ion and enhances the stability of a solvent-separated solute pair concurrently at the longer distances. The PMF and its solvent contribution for the TMA+–CH4 pair are quite similar to those for the apolar CH4–CH4 pair, probably because both the TMA+ and the CH4 exhibit hydrophobic hydration. For the same reason, the PMF for the Na+–TMA+ pair is qualitatively akin to that for the Na+–CH4 pair.

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