, with its well-known value of 7 at ambient condition, is a most basic property of water, with wide implications in chemistry and biology. The value is determined by the tendency of autoionization of water molecules into ion pairs, and , and is expected to vary extensively with the water condition, which determines the stability of the ion pairs. When temperature rises from the normal to the supercritical region, the of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for , is very difficult to reproduce. In the present study a molecular simulation is performed to understand this peculiar temperature dependence. The imbalance between the ion-water and the water-water molecular interaction strengths and the concomitant water density enhancement in the hydration shell, observed in the supercritical liquids, serve to put a subtle balance to produce this temperature dependence of the value. It is found that the large charge transfers from and to the surrounding water molecules take place. In these transfers, not only water molecules in the neighboring hydration shell but also those in the outer hydration shell play a significant role. The coordination number of water molecules around is found to be 4.5 at , which decreases slowly with temperature, for example, 4 at , in the present calculation.
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Research Article| April 13 2005
A theoretical study on anomalous temperature dependence of of water
Takuma Yagasaki, Kensuke Iwahashi, Shinji Saito, Iwao Ohmine; A theoretical study on anomalous temperature dependence of of water. J. Chem. Phys. 8 April 2005; 122 (14): 144504. https://doi.org/10.1063/1.1878712
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