{"title":"Double-Layer Distribution of Hydronium and Hydroxide Ions in the Air–Water Interface","authors":"Pengchao Zhang, Muye Feng and Xuefei Xu*, ","doi":"10.1021/acsphyschemau.3c0007610.1021/acsphyschemau.3c00076","DOIUrl":null,"url":null,"abstract":"<p >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<sup>–</sup>), whereas microscopic studies mostly support the acidic air–water interface with the observation of hydronium (H<sub>3</sub>O<sup>+</sup>) accumulation in the top layer of the interface. It is crucial to clarify the interfacial preference of OH<sup>–</sup> and H<sub>3</sub>O<sup>+</sup> ions for rationalizing the debate. In this work, we perform deep potential molecular dynamics simulations to investigate the preferential distribution of OH<sup>–</sup> and H<sub>3</sub>O<sup>+</sup> 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<sup>–</sup> and H<sub>3</sub>O<sup>+</sup> 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 H<sub>3</sub>O<sup>+</sup> preferentially resides in the topmost layer of the interface, but the OH<sup>–</sup>, 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<sup>–</sup>) vs −0.56 (H<sub>3</sub>O<sup>+</sup>) 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.</p>","PeriodicalId":29796,"journal":{"name":"ACS Physical Chemistry Au","volume":null,"pages":null},"PeriodicalIF":3.7000,"publicationDate":"2024-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://pubs.acs.org/doi/epdf/10.1021/acsphyschemau.3c00076","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"ACS Physical Chemistry Au","FirstCategoryId":"1085","ListUrlMain":"https://pubs.acs.org/doi/10.1021/acsphyschemau.3c00076","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
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.
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