Saha Subhamoy, Roy Subhadip, Mathi P, Mondal Jahur A
Radiation & Photochemistry Division , Bhabha Atomic Research Centre, Homi Bhabha National Institute , Trombay, Mumbai 400085 , India.
J Phys Chem A. 2019 Apr 4;123(13):2924-2934. doi: 10.1021/acs.jpca.9b00828. Epub 2019 Mar 12.
Iodine plays a key role in tropospheric ozone destruction, atmospheric new particle formation, as well as growth. Air-water interface happens to be an important reaction site pertaining to such phenomena. However, except iodide (I), the behavior of other iodine species, for example, triiodide (I) and iodate (IO, the most abundant iodine species in seawater) at the aqueous interface and their effect on the interfacial water are largely unknown. Using interface-specific vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation), we recorded the imaginary-χ spectra (Imχ; χ is the second-order electric susceptibility in OH stretch region) of the air-water interface in the presence of IO, I, and I (≤0.3 M) in the aqueous subphase. The Imχ spectra reveal that the chaotropic I is the most surface-active anion among the iodine species studied and decreases the vibrational coupling and hydrogen-bonding of interfacial water. Interestingly, the IO, even being a kosmotrope, is quite prevalent in the interfacial region and preferentially orients the interfacial water as "H-down" (i.e., water dipole moment is pointed toward the bulk water). Mapping of the OH stretch response of ion-affected water at interface (i.e., ΔImχ = Imχ - Imχ) with that in the hydration shell of the respective ion (hydration shell water response is obtained by Raman multivariate curve resolution spectroscopy) reveals a correlative link between the ion's influence on the interfacial water and their hydration shell structure. The distinct water structure of stronger as well as weaker H-bonding in the hydration shell of the polyatomic IO anion promotes the anion to stay at the interfacial region. Thus, the surface prevalence of the iodine species and their effect on the interfacial water are perceived to be crucial for the transfer of iodine from seawater to the atmosphere across the marine boundary layer and the chemistry of iodine at aqueous aerosol surface.
碘在对流层臭氧破坏、大气新粒子形成以及生长过程中起着关键作用。气 - 水界面恰好是与这些现象相关的重要反应场所。然而，除了碘离子（I⁻）之外，其他碘物种的行为，例如三碘离子（I₃⁻）和碘酸根（IO₃⁻，海水中最丰富的碘物种）在水界面处的行为及其对界面水的影响在很大程度上尚不清楚。利用界面特异性振动光谱（外差检测振动和频产生），我们记录了在水相下层存在IO₃⁻、I⁻和I₃⁻（≤0.3 M）时气 - 水界面的虚部χ光谱（Imχ；χ是OH伸缩区域的二阶电极化率）。Imχ光谱表明，在所研究的碘物种中，离液序列高的I₃⁻是最具表面活性的阴离子，它降低了界面水的振动耦合和氢键作用。有趣的是，IO₃⁻即使是促溶剂化离子，在界面区域也相当普遍，并且优先将界面水定向为“H向下”（即水偶极矩指向本体水）。将界面处离子影响的水的OH伸缩响应（即ΔImχ = Imχ - Imχ⁰）与相应离子水合壳层中的响应（水合壳层水响应通过拉曼多元曲线分辨光谱获得）进行映射，揭示了离子对界面水的影响与其水合壳层结构之间的相关联系。多原子IO₃⁻阴离子水合壳层中较强和较弱氢键的独特水结构促使该阴离子留在界面区域。因此，碘物种在表面的普遍存在及其对界面水的影响被认为对于碘从海水通过海洋边界层转移到大气以及碘在水气溶胶表面的化学过程至关重要。
