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 4–2 3–2 – 3– –. 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
