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    Investigation of Water Evaporation Process at Air/Water Interface using Hofmeister Ions

    Journal: J. Am. Chem. Soc.

    Authors: Rana, Bhawna; Fairhurst, David J.; Jena, Kailash C.

    Organizations: Indian Institute of Technology Ropar; Science and Engineering Research Board (SERB) , India [CRG/2018/004975]; British Council for Newton Bhabha [544025790]

    Evaporation is an interfacial phenomenon in which a water molecule breaks the intermolecular hydrogen (H???) bonds and enters the vapor phase. However, a detailed demonstration of the role of interfacial water structure in the evaporation process is still lacking. Here, we purposefully perturb the H-bonding environment at the air/ water interface by introducing kosmotropic (HPO4???2, SO4???2, and CO3???2) and chaotropic ions (NO3??? and I???) to determine their influence on the evaporation process. Using time-resolved interferometry on aqueous salt droplets, we found that kosmotropes reduce evaporation, whereas chaotropes accelerate the evaporation process, following the Hofmeister series: HPO4???2 < SO4???2 < CO3???2 < Cl???< NO3 ??? < I???. To extract deeper molecular-level insights into the observed Hofmeister trend in the evaporation rates, we investigated the air/water interface in the presence of ions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The SFG vibrational spectra reveal the significant impact of ions on the strength of the H-bonding environment and the orientation of free OH oscillators from ???36.2 to 48.4?? at the air/water interface, where both the effects follow the Hofmeister series. It is established that the slow evaporating water molecules experience a strong H-bonding environment with free OH oscillators tilted away from the surface normal in the presence of kosmotropes. In contrast, the fast evaporating water molecules experience a weak H-bonding environment with free OH oscillators tilted toward the surface normal in the presence of chaotropes at the air/water interface. Our experimental outcomes showcase the complex bonding environment of interfacial water molecules and their decisive role in the evaporation process. Superscript/Subscript Available
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