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

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.

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“…Hence, we have preliminarily acquired the SFG spectra of the air–pristine water interface before analyzing the spectral signatures of samples (Figure , black line). The spectral assignments of pristine water have been well documented and elaborated on in detail in the literature. ,− ,, The lower-wavenumber region centered at ∼3225 cm –1 represents the characteristic of strongly hydrogen-bonded (H-bonded) water molecules, whereas the higher-wavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm –1 , respectively. ,,, The prominent peak located at 3705 cm –1 represents the free (dangling) OH groups protruding toward the air. ,,, The lower- and the higher-wavenumber regions mainly depict the inter- and intramolecular H-bonding among the water molecules. ,, The details of the fitting parameters are given in Table S1. The SFG spectra at the air–aqueous interface of the buffer system used for this study were also acquired (Figure S4).…”

“…23,[27][28][29]41,47 The lower-wavenumber region centered at ∼3225 cm −1 represents the characteristic of strongly hydrogenbonded (H-bonded) water molecules, whereas the higherwavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm −1 , respectively. 27,29,41,47 The prominent peak located at 3705 cm −1 represents the free (dangling) OH groups protruding toward the air. 27,29,41,47 The lower-and the higher-wavenumber regions mainly depict the inter-and intramolecular Hbonding among the water molecules.…”

“…27,29,41,47 The prominent peak located at 3705 cm −1 represents the free (dangling) OH groups protruding toward the air. 27,29,41,47 The lower-and the higher-wavenumber regions mainly depict the inter-and intramolecular Hbonding among the water molecules. 41,47,48 The details of the fitting parameters are given in Table S1.…”

“…Hence, we have preliminarily acquired the SFG spectra of the air−pristine water interface before analyzing the spectral signatures of samples (Figure3, black line). The spectral assignments of pristine water have been well documented and elaborated on in detail in the literature 23,[27][28][29]41,47. The lower-wavenumber region centered at ∼3225 cm −1 represents the characteristic of strongly hydrogenbonded (H-bonded) water molecules, whereas the higherwavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm −1 , respectively 27,29,41,47.…”

“…The spectral assignments of pristine water have been well documented and elaborated on in detail in the literature 23,[27][28][29]41,47. The lower-wavenumber region centered at ∼3225 cm −1 represents the characteristic of strongly hydrogenbonded (H-bonded) water molecules, whereas the higherwavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm −1 , respectively 27,29,41,47. The prominent peak located at 3705 cm −1 represents the free (dangling) OH groups protruding toward the air 27,29,41,47.…”