Raman spectroscopy in combination with multivariate curve resolution (Raman-MCR) is used to explore the interaction between water and various kosmotropic and chaotropic anions. Raman-MCR of aqueous Na-salt (NaI, NaBr, NaNO3, Na2SO4, and Na3PO4) solutions provides solute-correlated Raman spectra (SC-spectra) of water. The SC-spectra predominantly bear the vibrational characteristics of water in the hydration shell of anions, because Na(+)-cation has negligible effect on the OH stretch band of water. The SC-spectra for the chaotropic I(-), Br(-), and NO3(-) anions and even for the kosmotropic SO4(2-) anion resemble the Raman spectrum of isotopically diluted water (H2O/D2O = 1/19; v/v) whose OH stretch band is largely comprised by the response of vibrationally decoupled OH oscillators. On the other hand, the SC-spectrum for the kosmotropic PO4(3-) anion is quite similar to the Raman spectrum of H2O (bulk). Comparison of the peak positions of SC-spectra and the Raman spectrum of isotopically diluted water suggests that the hydrogen bond strength of water in the hydration shell of SO4(2-) is comparable to that of the isotopically diluted water, but that in the hydration shell of I(-), Br(-), and NO3(-) anions is weaker than that of the latter. Analysis of integrated area of component bands of the SC-spectra reveals ∼80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I(-), Br(-), NO3(-), and SO4(2-) anions. In the case of trivalent PO4(3-), the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at ∼3250 cm(-1) is compensated by the increased signal of strongly hydrogen bonded (but decoupled) water species in the hydration shell. The peak area-averaged wavenumber of the SC-spectrum increases as PO4(3-) < SO4(2-) < NO3(-) < Br(-) < I(-) and indeed suggests strong hydrogen bonding of water in the hydration shell of PO4(3-) anion.

The hydration energy of an ion largely resides within the first few layers of water molecules in its hydration shell. Hence, it is important to understand the transformation of water properties, such as hydrogen-bonding, intermolecular vibrational coupling, and librational freedom in the hydration shell of ions. We investigated these properties in the hydration shell of mono- (Cl(-) and I(-)) and bivalent (SO4(2-) and CO3(2-)) anions by using Raman multivariate curve resolution (Raman-MCR) spectroscopy in the OH stretch, HOH bend, and [bend+librational] combination bands of water. Raman-MCR of aqueous Na-salt (NaCl, NaI, Na2SO4, and Na2CO3) solutions provides ion-correlated spectra (IC-spectrum) which predominantly bear the vibrational characteristics of water in the hydration shell of respective anions. Comparison of these IC-spectra with the Raman spectrum of bulk water in different spectral regions reveals that the water is vibrationally decoupled with its neighbors in the hydration shell. Hydrogen-bond strength and librational freedom also vary with the nature of anion: hydrogen-bond strength, for example, decreases as CO3(2-) > SO4(2-) > bulk water ≈ Cl(-) > I(-); and the librational freedom increases as CO3(2-) ≈ SO4(2-) < bulk water < Cl(-) < I(-). It is believed that these structural perturbations influence the dynamics of coherent energy transfer and librational reorientation of water in the hydration shell of anions.

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.

Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) results in spectra that, as a function of the probe delay time, yield information about the dynamics of the coherently excited vibrational modes. A change of the shape of the exciting laser pulses has a dramatic influence on the spectral response. A feedback-controlled optimization of specific modes making use of phase and amplitude modulation of the Stokes laser pulse is applied to selectively influence the anti-Stokes signal spectrum. The role of phase and amplitude changes of the frequency components of the ultrashort pulse is analyzed. It can be demonstrated that the optimization process is clearly dominated by the effect of timing of the dispersed pulse segments. The modulation of the spectral amplitudes has only a small influence on the mode ratios. We conclude that mode focusing in time domain CARS spectroscopy can be achieved only by correctly setting the phases of the spectral pulse components (here demonstrated for the Stokes laser).