Preferential orientation and expulsion/accumulation of trimethylamine N-oxide (TMAO; a protecting osmolyte) and tert-butyl alcohol (TBA; a denaturant) have been investigated at the hydrophobic air–water interface by phase-sensitive heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. The imaginary χ(2) spectrum (Imχ(2); χ(2) is the second order electric susceptibility), which is directly obtainable from the HD-VSFG measurement, provides the accurate absorption characteristics of interfacial molecules, and the sign of Imχ(2) reveals the net orientation of these molecules at the interface. For the aqueous TMAO and TBA solutions, the Imχ(2) spectra in the CH-stretch region show a negative sign, which demonstrates that both TMAO and TBA orient in the same manner at the air–water interface, by pointing their methyls away from the aqueous phase (“methyl-up” orientation). Nevertheless, they affect the interfacial water quite differently: TMAO increases the H-bond strength and preferential H-down orientation of interfacial water, while the dangling OH remains almost unperturbed. TBA, on the other hand, does not affect the H-bond strength and preferential orientation of interfacial water, but reduces the propensity of the dangling OH at the air–water interface. The preferential orientation of TMAO and TBA and their distinct effect on the interfacial water have been correlated with their hydration characteristics in bulk water by retrieving the vibrational spectrum of water in their respective hydration shells, using Raman multivariate curve resolution (Raman-MCR) spectroscopy. The MCR-retrieved hydration water spectra clearly show that the water around TBA has strong water–water interaction (hydrophobic hydration) and that around TMAO has a hydrophobic hydration around the N-methyl ((CH3)3N+−) group and a hydrophilic hydration around the N-oxide group (strong H-bonding of water with the N-oxide group). The different hydration characteristic of the N-methyl and N-oxide groups orients the TMAO molecules as “methyl-up” at the air–water interface. Moreover, the strong hydration of the N-oxide group leads to a depletion of TMAO from the hydrophobic water surface, such that the preferentially oriented TMAO molecules are located beneath the topmost water layer at the air–water interface. As a result, the topmost water molecules are largely unaware of the presence of TMAO at the interface, even at very high bulk concentration of TMAO (5.0 mol dm–3). In the case of TBA, the hydrophobic hydration leads to an accumulation of TBA at the water surface, mainly affecting the topmost water molecules.

Although the hydrophobic size of an amphiphile plays a key role in various chemical, biological, and atmospheric processes, its effect at macroscopic aqueous interfaces (e.g., air-water, oil-water, cell membrane-water, etc.), which are ubiquitous in nature, is not well understood. Here we report the hydrophobic alkyl chain length dependent structural and orientational transformations of water at alcohol (CHOH, n = 1-12)-water interfaces using interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman multivariate curve resolution (Raman-MCR) spectroscopic techniques. The HD-VSFG results reveal that short-chain alcohols (CHOH, n < 4, i.e., up to 1-propanol) do not affect the structure (H-bonding) and orientation of water at the air-water interface; the OH stretch band maximum appears at ∼3470 cm, and the water H atoms are pointed toward the bulk water, that is, "H-down" oriented. In contrast, long-chain alcohols (CHOH, n > 4, i.e., beyond 1-butanol) make the interfacial water more strongly H-bonded and reversely orientated; the OH stretch band maximum appears at ∼3200 cm, and the H atoms are pointed away from the bulk water, that is, "H-up" oriented. Interestingly, for the alcohol of intermediate chain length (CHOH, n = 4, i.e, 1-butanol), the interface is quite unstable even after hours of its formation and the time-averaged result is qualitatively similar to that of the long-chain alcohols, indicating a structural/orientational crossover of interfacial water at the 1-butanol-water interface. pH-dependent HD-VSFG measurements (with HO as well as isotopically diluted water, HOD) suggest that the structural/orientational transformation of water at the long-chain alcohol-water interface is associated with the adsorption of OH anion at the interface. Vibrational mapping of the water structure in the hydration shell of OH anion (obtained by Raman-MCR spectroscopy of NaOH in HOD) clearly shows that the water becomes strongly H-bonded (OH stretch max. ≈ 3200 cm) while hydrating the OH anion. Altogether, it is conceivable that alcohols of different hydrophobic chain lengths that are present in the troposphere will differently affect the interfacial electrostatics and associated chemical processes of aerosol droplets, which are critical for cloud formation, global radiation budget, and climate change.

In the present work, six novel high energy materials (HEMs) were investigated by femtosecond and nanosecond LIBS techniques with an intention to comprehend the influence of different functional groups as well as their positions on the LIBS emission.

Iodine plays a key role in tropospheric ozone destruction, atmospheric new particle formation, as well as growth. Air–water interface happens to be an important reaction site pertaining to such phenomena. However, except iodide (I–), the behavior of other iodine species, for example, triiodide (I3 –) and iodate (IO3 –, the most abundant iodine species in seawater) at the aqueous interface and their effect on the interfacial water are largely unknown. Using interface-specific vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation), we recorded the imaginary-χ(2) spectra (Imχ(2); χ(2) is the second-order electric susceptibility in OH stretch region) of the air–water interface in the presence of IO3 –, I3 –, and I– (≤0.3 M) in the aqueous subphase. The Imχ(2) spectra reveal that the chaotropic I3 – is the most surface-active anion among the iodine species studied and decreases the vibrational coupling and hydrogen-bonding of interfacial water. Interestingly, the IO3 –, even being a kosmotrope, is quite prevalent in the interfacial region and preferentially orients the interfacial water as “H-down” (i.e., water dipole moment is pointed toward the bulk water). Mapping of the OH stretch response of ion-affected water at interface (i.e., ΔImχ(2) = Imχ(2) air–water−iodine salt – Imχ(2) air–water) with that in the hydration shell of the respective ion (hydration shell water response is obtained by Raman multivariate curve resolution spectroscopy) reveals a correlative link between the ion’s influence on the interfacial water and their hydration shell structure. The distinct water structure of stronger as well as weaker H-bonding in the hydration shell of the polyatomic IO3 – anion promotes the anion to stay at the interfacial region. Thus, the surface prevalence of the iodine species and their effect on the interfacial water are perceived to be crucial for the transfer of iodine from seawater to the atmosphere across the marine boundary layer and the chemistry of iodine at aqueous aerosol surface.