A theoretical study on anomalous temperature dependence of pK w of water

Takuma Yagasaki, Kensuke Iwahashi, Shinji Saito, Iwao Ohmine

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

pH, 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 pH value is determined by the tendency of autoionization of water molecules into ion pairs, H+ and O H-, 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 pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for O H-, 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 pH value. It is found that the large charge transfers from H+ and O H- 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 O H- is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.

Original languageEnglish
Article number144504
JournalThe Journal of Chemical Physics
Volume122
Issue number14
DOIs
Publication statusPublished - 2005
Externally publishedYes

Fingerprint

temperature dependence
Water
water
Hydration
hydration
Temperature
Ions
Molecules
molecules
ions
Molecular interactions
autoionization
molecular interactions
biology
coordination number
Charge transfer
tendencies
charge transfer
chemistry
temperature

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics

Cite this

A theoretical study on anomalous temperature dependence of pK w of water. / Yagasaki, Takuma; Iwahashi, Kensuke; Saito, Shinji; Ohmine, Iwao.

In: The Journal of Chemical Physics, Vol. 122, No. 14, 144504, 2005.

Research output: Contribution to journalArticle

Yagasaki, Takuma ; Iwahashi, Kensuke ; Saito, Shinji ; Ohmine, Iwao. / A theoretical study on anomalous temperature dependence of pK w of water. In: The Journal of Chemical Physics. 2005 ; Vol. 122, No. 14.
@article{477b4506686547ac972c679cb900dc1a,
title = "A theoretical study on anomalous temperature dependence of pK w of water",
abstract = "pH, 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 pH value is determined by the tendency of autoionization of water molecules into ion pairs, H+ and O H-, 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 pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for O H-, 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 pH value. It is found that the large charge transfers from H+ and O H- 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 O H- is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.",
author = "Takuma Yagasaki and Kensuke Iwahashi and Shinji Saito and Iwao Ohmine",
year = "2005",
doi = "10.1063/1.1878712",
language = "English",
volume = "122",
journal = "Journal of Chemical Physics",
issn = "0021-9606",
publisher = "American Institute of Physics Publising LLC",
number = "14",

}

TY - JOUR

T1 - A theoretical study on anomalous temperature dependence of pK w of water

AU - Yagasaki, Takuma

AU - Iwahashi, Kensuke

AU - Saito, Shinji

AU - Ohmine, Iwao

PY - 2005

Y1 - 2005

N2 - pH, 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 pH value is determined by the tendency of autoionization of water molecules into ion pairs, H+ and O H-, 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 pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for O H-, 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 pH value. It is found that the large charge transfers from H+ and O H- 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 O H- is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.

AB - pH, 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 pH value is determined by the tendency of autoionization of water molecules into ion pairs, H+ and O H-, 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 pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for O H-, 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 pH value. It is found that the large charge transfers from H+ and O H- 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 O H- is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.

UR - http://www.scopus.com/inward/record.url?scp=20944432472&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=20944432472&partnerID=8YFLogxK

U2 - 10.1063/1.1878712

DO - 10.1063/1.1878712

M3 - Article

AN - SCOPUS:20944432472

VL - 122

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 14

M1 - 144504

ER -