Double-Layer Distribution of Hydronium and Hydroxide Ions in the Air–Water Interface

Pengchao Zhang, Muye Feng and Xuefei Xu*, 
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Abstract

The acid–base nature of the aqueous interface has long been controversial. Most macroscopic experiments suggest that the air–water interface is basic based on the detection of negative charges at the interface that indicates the enrichment of hydroxides (OH), whereas microscopic studies mostly support the acidic air–water interface with the observation of hydronium (H3O+) accumulation in the top layer of the interface. It is crucial to clarify the interfacial preference of OH and H3O+ ions for rationalizing the debate. In this work, we perform deep potential molecular dynamics simulations to investigate the preferential distribution of OH and H3O+ ions at the aqueous interfaces. The neural network potential energy surface is trained based on density functional theory calculations with the SCAN functional, which can accurately describe the diffusion of these two ions both in the interface and in the bulk water. In contrast to the previously reported single ion enrichment, we show that both OH and H3O+ surprisingly prefer to accumulate in interfaces but at different interfacial depths, rendering a double-layer ionic distribution within ∼1 nm near the Gibbs dividing surface. The H3O+ preferentially resides in the topmost layer of the interface, but the OH, which is enriched in the deeper interfacial layer, has a higher equilibrium concentration due to the more negative free energy of interfacial stabilization [−0.90 (OH) vs −0.56 (H3O+) kcal/mol]. The present finding of the ionic double-layer distribution may qualitatively offer a self-consistent explanation for the long-term controversy about the acid–base nature of the air–water interface.

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氢离子和氢氧根离子在空气-水界面的双层分布
水界面的酸碱性质一直存在争议。大多数宏观实验表明,空气-水界面是碱性的,因为在界面上检测到负电荷,表明氢氧化物(OH-)富集;而微观研究大多支持酸性空气-水界面,因为在界面顶层观察到氢离子(H3O+)积累。澄清 OH- 和 H3O+ 离子的界面偏好对于理顺争论至关重要。在这项工作中,我们进行了深度势能分子动力学模拟,以研究 OH- 和 H3O+ 离子在水界面的优先分布。神经网络势能面是在密度泛函理论计算的基础上利用 SCAN 函数训练出来的,它能准确地描述这两种离子在界面和水体中的扩散。与之前报道的单离子富集不同,我们发现 OH- 和 H3O+竟然都更喜欢在界面中富集,但富集深度不同,从而在吉布斯分界面附近 ∼ 1 nm 范围内形成了双层离子分布。H3O+ 优先驻留在界面的最顶层,而 OH- 则富集在较深的界面层,由于界面稳定的负自由能[-0.90 (OH-) vs -0.56 (H3O+)kcal/mol],其平衡浓度较高。目前关于离子双层分布的发现,可以从质量上为空气-水界面酸碱性质的长期争议提供一个自洽的解释。
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CiteScore
3.70
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期刊介绍: ACS Physical Chemistry Au is an open access journal which publishes original fundamental and applied research on all aspects of physical chemistry. The journal publishes new and original experimental computational and theoretical research of interest to physical chemists biophysical chemists chemical physicists physicists material scientists and engineers. An essential criterion for acceptance is that the manuscript provides new physical insight or develops new tools and methods of general interest. Some major topical areas include:Molecules Clusters and Aerosols; Biophysics Biomaterials Liquids and Soft Matter; Energy Materials and Catalysis
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