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

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.