Solvent structure, dynamics, and ion mobility in aqueous solutions at 25°C
Solvent structure, dynamics, and ion mobility in aqueous solutions at 25°C
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DOI：
10.1021/jp980642x
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发表时间：
1998-05-21
影响因子：
3.3
通讯作者：
Lee, SH
中科院分区：
文献类型：
--
作者：
Koneshan, S;Rasaiah, JC;Lee, SH
我们计算的迁移率u（i）的金属阳离子Li+，Na+，K+，Rb+，Cs+和Ca 2+和卤化物F-，Cl-，Br-和I-在无限稀释的分子动力学模拟使用SPC/E模型的水在25摄氏度和反应场的长程相互作用。离子迁移率的变化趋势与实验结果相同，阳离子和阴离子的迁移率具有明显的最大值。相应的不带电物种的迁移率（定义为u（i）= D-i/kT）也通过模拟确定，并且与Stokes定律定性一致。Li+、Na+、K+、Rb+和F-的迁移率增加了油排放，而Cl、Br和I的迁移率小于相应的阴离子。虚构的I+离子的迁移率，仅在电荷上不同于I-，以u（I）< u（I）（+）< u（I）-的顺序位于I-和I之间。当离子放电时，水在小阳离子（Li+和Na+）和Ca 2+的第一溶剂化壳层中的停留时间减少，而在中和I-时观察到相反的情况，这表明在大的不带电I周围形成溶剂笼，其在充电时部分破裂，增加相应离子的流动性。笼子的解体是更大的I-比I+，这与I-和I+的溶剂化熵的不对称性，在SPC/E水的电荷反转，提供了一个解释的趋势，在迁移率的I，I-，和I+。水在阳离子周围的初级水合壳层中的停留时间通过作为与相应溶剂化熵中的最大值相关的尺寸的函数的最小值，表明不同类型的水合，即，静电离子溶剂化（亲水性）和笼形成（疏水性）分别为小和大的阳离子。结果雅阁最近计算的溶剂化熵和自由能作为电荷和尺寸的连续函数（Lynden-Bell，R. M.; Rasaiah，J. C. 1997，107，1981）。亲水性和疏水性溶剂化反应反映在带电和不带电溶质周围的水合壳中水的交换动力学中。单个阳离子和阴离子的溶剂化动力学在短时间内是不同的，但在长时间内由溶剂表征。溶剂动力学，结构和笼调制的电荷和大小的离子强烈牵连在确定其迁移率。
We calculate the mobilities u(i) of the metal cations Li+, Na+, K+, Rb+, Cs+, and Ca2+ and the halides F-, Cl-, Br-, and I- at infinite dilution by molecular dynamics simulation using the SPC/E model for water at 25 degrees C and a reaction field for the long-range interactions. The ion mobilities show the same trends as the experimental results with distinct maxima for cations and anions. The mobilities (defined by u(i) = D-i/kT) of the corresponding uncharged species are also determined by simulation and are in qualitative agreement with Stokes' law. The mobilities of Li+, Na+, K+, Rb+ and F- increase oil discharge, whereas Cl, Br, and I have smaller mobilities than the corresponding anions. The mobility of the fictitious I+ ion, which differs from I- only in its charge, lies between that of I- and I in the order u(I) < u(I)(+) < u(I)-. The residence time of water in the first solvation shell of small cations (Li+ and Na+) and Ca2+ decreases when the ions are discharged, while the opposite is observed on neutralizing I-, suggesting the formation of a solvent cage around the large uncharged I which partially breaks up on charging, increasing the mobility of the corresponding ion. The cage breakup is greater for I- than for I+ which correlates with the asymmetry in the entropies of solvation of I- and I+, in SPC/E water on charge reversal, providing an explanation for the trends in the mobilities of I, I-, and I+. The residence times of water in the primary hydration shell around cations pass through a minimum as a function of size that correlates with the maximum in the corresponding solvation entropy, suggesting different types of hydration, i.e., electrostatic ion solvation (hydrophilic) and cage formation (hydrophobic) respectively for small and large cations. The results are in accord with recent calculations of the solvation entropy and free energy as continuous functions of the charge and size (Lynden-Bell, R. M.; Rasaiah, J. C. J. Chem. Phys. 1997, 107, 1981). Hydrophilic and hydrophobic solvation are reflected in the exchange dynamics of the water in the hydration shells around charged and uncharged solutes. The solvation dynamics of individual cations and anions are distinct at short times but characterized by the solvent at long times. Solvent dynamics, structure, and caging modulated by the charge and size of the ions are strongly implicated in determining their mobilities.