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The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 angstroms (1 Å = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers (1-3). However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the angstrom scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength (4-6). Here we report a combined experimental and theoretical study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the 'free OD' transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase., The vibrational spectroscopy of aqueous interfaces has progressed significantly in recent years with the development of surface-selective spectroscopic techniques (7-12), and with the ability to model the dynamic H-bond network [...]

The frequency-domain sum frequency generation (SFG) spectroscopy and the time-domain SFG-free induction decay technique are combined for characterization of more complex molecular surfaces under ambient conditions. Relative amplitudes change and additional transitions emerge at lower surface pressure indicating changes in molecular orientation and onset of disorder.

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314.

The effect of lipid surfactants on the solvation dynamics at air/water interface is examined using time-resolved second harmonic generation spectroscopy for this purpose. It is found that the carboxylate and sulfate surfactants induce similar dynamics, suggesting that the solation is not sensitive to the chemical composition of the headgroup but rather to the total negative surface charge density.

A study was conducted to investigate the effect of the negatively charged carboxylate group of the stearic acid surfactant CH(sub 3)(CH(sub 2))(sub 16)COO(super -) on the solvation at the surfactant air/water interface. It was found that the static solvation energetics appears to be relatively insensitive to the ionization state of the stearic acid surfactant.

The effect of a Langmuir monolayer of fatty acid on static solvation properties as well as femtosecond solvation dynamics at aplanar air-water interface is investigated using second harmonic generation as a surface-specific probe. The results demonstrate that the stearic acid monolayer induces significant changes in both static and dynamic solvation at the air-water interface.

Wepresent a general response function formalism describing thecontribution of orientational dynamics of molecules at interfacesto spectroscopic line shapes in vibrational sum frequency generation(SFG). When reorientation occurs on the time scale comparable to vibrationaldephasing, its dynamics can be extracted from polarization-selectedSFG spectral line shapes. Unique features of orientational motionat interfaces are (1) the anisotropic case-specific equilibrium orientationaldistribution and (2) possible dynamic anisotropy (e.g., differentin-plane versus out-of-plane relaxation rates), both of which mustbe taken into account. Within the small-step rotational diffusionmodel, we present solutions for two cases, the weak-confinement model,applicable when the deviations from the isotropic case are not severe,and the wobbling-in-a-cone model, which considers a hard-wall orientingpotential. SFG line shapes are calculated for a rod-like chromophoreas a function of the rotational diffusion rate. For certain equilibriumorientational distributions, orientational dynamics may result inanomalous bi-Lorentzian line shapes (two Lorentzians of differentwidths centered and the same frequency). [ABSTRACT FROM AUTHOR]