稀有气体在水中的溶解度平衡同位素效应：理论与观察,The Journal of Physical Chemistry B

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稀有气体在水中的溶解度平衡同位素效应：理论与观察
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2023-11-08 , DOI: 10.1021/acs.jpcb.3c05651
Alan M Seltzer ₁ , Sarah A Shackleton ₂ , Ian C Bourg ₃

Affiliation

溶解在水中的稀有气体的丰度和同位素组成在地球科学中有许多应用。近年来，新的分析技术为使用稀有气体同位素的高精度测量作为地下水水文学、海洋学、地幔地球化学和古气候学的示踪剂打开了大门。这些分析进展带来了水中溶解度平衡同位素效应（SEIE）（即稀有气体同位素的相对溶解度）及其对温度和盐度敏感性的新测量。在这里，我们进行了一系列经典分子动力学 (MD) 模拟，并采用量子校正的理论方法来估计 SEIE，以便与实验观察结果进行比较。我们发现经典的 MD 模拟可以在分析不确定性的范围内准确预测 Ar、Kr 和 Xe 同位素的 SEIE，精度可达 0.01‰。然而，MD 模拟始终高估 Ne 和 He 的 SEIE，高出观测值高达 40%。我们在不同温度、盐度和压力下进行敏感性测试，并采用不同组的原子间势参数和水模型。对于所有稀有气体同位素，TIP4P 水模型比 SPC/E 和 TIP4P/冰模型更准确地重现观察到的 SEIE。经典 MD 模拟还可以准确捕获重惰性气体 SEIE 的温度和盐度敏感性的符号和大致大小。我们发现实验和建模的 SEIE 通常遵循反平方质量依赖性，这意味着溶剂化壳层内的惰性气体原子所经历的均方力对于所有惰性气体都是相似的。这种平方反比质量比例对于 Ar、Kr 和 Xe 同位素来说几乎是精确的，但 He 和 Ne 表现出稍弱的质量依赖性。我们假设 He-Ne 和 Ar-Kr-Xe SEIE 之间的明显二分性可能是由于原子尺寸差异造成的，因此较小的惰性气体更有可能自发地融入水的空腔中，而不会破坏水-水氢键，从而经历溶剂化壳内平移过程中的碰撞更柔和。我们进一步推测，模拟 He 和 Ne SEIE 的过度预测可能是由于忽略了高阶量子校正或广泛使用的 Lennard-Jones 6-12 势模型对范德华排斥力的过度僵化表示造成的。我们认为，重质和轻质稀有气体 SEIE 的新测量可能代表了一组新的约束，通过这些约束可以改进疏水溶剂化理论并优化用于水和稀有气体 MD 模拟的原子间势模型集。

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Solubility Equilibrium Isotope Effects of Noble Gases in Water: Theory and Observations

The abundance and isotopic composition of noble gases dissolved in water have many applications in the geosciences. In recent years, new analytical techniques have opened the door to the use of high-precision measurements of noble gas isotopes as tracers for groundwater hydrology, oceanography, mantle geochemistry, and paleoclimatology. These analytical advances have brought about new measurements of solubility equilibrium isotope effects (SEIEs) in water (i.e., the relative solubilities of noble gas isotopes) and their sensitivities to the temperature and salinity. Here, we carry out a suite of classical molecular dynamics (MD) simulations and employ the theoretical method of quantum correction to estimate SEIEs for comparison with experimental observations. We find that classical MD simulations can accurately predict SEIEs for the isotopes of Ar, Kr, and Xe to order 0.01‰, on the scale of analytical uncertainty. However, MD simulations consistently overpredict the SEIEs of Ne and He by up to 40% of observed values. We carry out sensitivity tests at different temperatures, salinities, and pressures and employ different sets of interatomic potential parameters and water models. For all noble gas isotopes, the TIP4P water model is found to reproduce observed SEIEs more accurately than the SPC/E and TIP4P/ice models. Classical MD simulations also accurately capture the sign and approximate magnitude of temperature and salinity sensitivities of SEIEs for heavy noble gases. We find that experimental and modeled SEIEs generally follow an inverse-square mass dependence, which implies that the mean-square force experienced by a noble gas atom within a solvation shell is similar for all noble gases. This inverse-square mass proportionality is nearly exact for Ar, Kr, and Xe isotopes, but He and Ne exhibit a slightly weaker mass dependence. We hypothesize that the apparent dichotomy between He–Ne and Ar–Kr–Xe SEIEs may result from atomic size differences, whereby the smaller noble gases are more likely to spontaneously fit within cavities of water without breaking water–water H-bonds, thereby experiencing softer collisions during translation within a solvation shell. We further speculate that the overprediction of simulated He and Ne SEIEs may result from the neglection of higher-order quantum corrections or the overly stiff representation of van der Waals repulsion by the widely used Lennard-Jones 6–12 potential model. We suggest that new measurements of SEIEs of heavy and light noble gases may represent a novel set of constraints with which to refine hydrophobic solvation theories and optimize the set of interatomic potential models used in MD simulations of water and noble gases.

更新日期：2023-11-08

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