Abstract:
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.
