Spectroscopic and computational studies of aqueous ethylene glycol solution surfaces.

The combination of Monte Carlo, ab initio, and DFT computational studies of ethylene glycol (EG) and EG-water hydrogen-bonding complexes indicate that experimental vibrational spectra of EG and EG-water solution surfaces have contributions from numerous conformations of both EG and EG-water. The computed spectra, derived from harmonic vibrational frequency calculations and a theoretical Boltzmann distribution, show similarity to the experimental surface vibrational spectra of EG taken by broad-bandwidth sum frequency generation (SFG) spectroscopy. This similarity suggests that, at the EG and aqueous EG surfaces, there are numerous coexisting conformations of stable EG and EG-water complexes. A blue shift of the CH2 symmetric stretch peak in the SFG spectra was observed with an increase in the water concentration. This change indicates that EG behaves as a hydrogen-bond acceptor when solvated by additional water molecules. This also suggests that, in aqueous solutions of EG, EG-EG aggregates are unlikely to exist. The experimental blue shift is consistent with the results from the computational studies.

The air-liquid interface of benzene, toluene, m-xylene, and mesitylene: a sum frequency, Raman, and infrared spectroscopic study.

The air-liquid interface and the liquid-phase of benzene, toluene, 1,3-dimethylbenzene, and 1,3,5-trimethylbenzene are studied using broad bandwidth sum frequency generation spectroscopy, Raman and infrared spectroscopy. A vibrationally resonant sum frequency response is observed from these surfaces in spite of the small hyperpolarizabilities, in particular, the zero and near-zero hyperpolarizabilities of benzene and 1,3,5-trimethylbenzene. The orientation of the aromatic rings of these compounds at their air-liquid interfaces is tilted relative to the surface plane. Thus, on average, the plane of the aromatic ring does not lie in the interfacial plane. Comparison of the square root of the sum frequency intensity to that of the Raman multiplied bythe infrared intensity provides additional information about the molecular environment at their respective air-liquid interface.

1-Methyl Naphthalene Reorientation at the Air-Liquid Interface upon Water Saturation Studied by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy.

Vibrational broad bandwidth sum frequency generation spectroscopy was employed to investigate the surface structure of neat 1-methyl naphthalene (1-MN) and the reorientation of the 1-MN molecules upon saturation of the 1-MN liquid with water. The neat 1-MN liquid molecules have their aromatic rings aligned antiparallel to one another with their methyl groups alternating out of the surface and into the subsurface region from molecule to molecule. With the introduction of relatively few water molecules into the 1-MN liquid (1:336 water/1-MN) a rearrangement of the surface molecules is induced, leading to an increased number density of the methyl groups arranged such that more methyl groups are oriented in the same direction into the air phase at the air-liquid 1-MN interface. Surface tension measurements reveal an increase in the surface tension upon water saturation of the 1-MN liquid, indicating surface activity of the water in the 1-MN solution. It is also clear that the reorientation of the surface 1-MN molecules is reversible.