A theoretical study on anomalous temperature dependence of pKw of water.
A theoretical study on anomalous temperature dependence of pKw of water.
复制标题
DOI：
10.1063/1.1878712
复制
发表时间：
2005-04
期刊：
影响因子：
--
通讯作者：
T. Yagasaki;Kensuke Iwahashi;S. Saito;I. Ohmine
中科院分区：
文献类型：
--
作者：
T. Yagasaki;Kensuke Iwahashi;S. Saito;I. Ohmine
pH 值在环境条件下众所周知为 7，是水最基本的属性，在化学和生物学领域具有广泛的影响。 pH 值由水分子自电离成离子对 H(+) 和 OH(-) 的倾向决定，并且预计会随着水的状况而发生很大变化，这决定了离子对的稳定性。当温度从常温升至超临界区时，水的pH在实验上表现出复杂的、非单调的温度依赖性，即它首先从7下降，然后迅速上升。对 pH 值的准确理论评估和对这种异常行为的微观理解已被证明是一项具有挑战性的任务，因为这些离子的水合，特别是 OH(-)，很难重现。在本研究中，进行分子模拟来了解这种特殊的温度依赖性。在超临界液体中观察到，离子-水和水-水分子相互作用强度之间的不平衡以及水合壳中随之而来的水密度增强，有助于实现微妙的平衡，从而产生这种 pH 值的温度依赖性。研究发现，大量电荷从H(+)和OH(-)转移到周围的水分子。在这些转移中，不仅邻近水化层中的水分子，而且外部水化层中的水分子也发挥着重要作用。发现 OH(-) 周围水分子的配位数在 300 K 时为 4.5，随着温度的升高而缓慢降低，例如在本计算中，在 800 K 时为 4。
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 OH(-), 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 OH(-), 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 OH(-) 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 OH(-) is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.