Solubility Equilibrium Isotope Effects of Noble Gases in Water: Theory and Observations
Solubility Equilibrium Isotope Effects of Noble Gases in Water: Theory and Observations
复制标题
稀有气体在水中的溶解度平衡同位素效应：理论与观察
DOI：
10.1021/acs.jpcb.3c05651
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发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Bourg, Ian C.
中科院分区：
文献类型：
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作者：
Seltzer, Alan M.;Shackleton, Sarah A.;Bourg, Ian C.
稀有气体在水中的丰度和同位素组成在地学中有着广泛的应用。近年来，新的分析技术为将稀有气体同位素的高精度测量用作地下水水文学、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(SEIES)(即稀有气体同位素的相对溶解度)及其对温度和盐度的敏感性的新测量。在这里，我们进行了一套经典的分子动力学(MD)模拟，并使用量子修正的理论方法来估计SEIES，以便与实验观测进行比较。我们发现，经典的分子动力学模拟可以在分析不确定度的范围内准确地预测Ar、Kr和Xe同位素的SEI值到0.01‰量级。然而，MD模拟总是将Ne和He的SEIES高估高达观测值的40%。我们在不同的温度、盐度和压力下进行了敏感性测试，并采用了不同的原子间势参数和水模型。对于所有稀有气体同位素，发现TIP4P水模型比SPC/E和TIP4P/ICE模型更准确地再现了观测到的SEIES。经典的MD模拟也准确地捕捉到了SEIES对重惰性气体的温度和盐度敏感性的符号和近似大小。我们发现，实验和模拟的SEIES一般遵循与质量成反比的平方质量关系，这意味着在溶剂化壳层内的稀有气体原子所经历的均方作用力对于所有稀有气体来说都是相似的。对于Ar、Kr和Xe同位素，这种平方反比质量比几乎是精确的，但他和Ne的质量依赖性略弱。我们假设，He-Ne和Ar-Kr-Xe Seies之间的明显二分法可能是由于原子大小的差异，从而较小的稀有气体更有可能自发地适合于水的空穴而不破坏水-水的氢键，从而在溶剂化壳层内的平移过程中经历更软的碰撞。我们进一步推测，对模拟的He和Ne原子的过度预测可能是由于广泛使用的Lennard-Jones 6-12势模型忽略了高阶量子修正或过于僵硬地表示了van der Waals斥力。我们认为，重惰性气体和轻惰性气体SEIES的新测量可能代表了一组新的约束条件，用于完善疏水溶剂化理论和优化用于水和惰性气体MD模拟的原子间作用势模型。
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.