Interfacial Water Structure of Binary Liquid Mixtures Reflects Nonideal Behavior

Abstract

The evaporation of molecules from water–organic solute binary mixtures is key for both atmospheric and industrial processes such as aerosol formation and distillation. Deviations from ideal evaporation energetics can be assigned to intermolecular interactions in solution, yet evaporation occurs from the interface, and the poorly understood interfacial, rather than the bulk, structure of binary mixtures affects evaporation kinetics. Here we determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid, by combining surface-specific vibrational spectroscopy with molecular dynamics simulations. We find that the free, dangling OH groups at the interfaces of these differently behaving nonideal mixtures are essentially indistinguishable. In contrast, the ordering of hydrogen-bonded interfacial water molecules differs substantially at these three interfaces. Specifically, the interfacial water molecules become more disordered (ordered) in mixtures with methanol and ethanol (formic acid), showing higher (lower) vapor pressure than that predicted by Raoult’s law.

Dual-Beam Photothermal Spectroscopy Employing a Mach–Zehnder Interferometer and an External Cavity Quantum Cascade Laser for Detection of Water Traces in Organic Solvents

“…Water exhibits a strong and broad absorption peak at about 1660 cm –1 . The slight blue shift when compared to pure water (peak maximum at ∼1645 cm –1 ) of the H–O–H bending mode in the spectra of water–ethanol mixture is due to intermolecular interactions. , This effect has recently been reported for water in ethanol by Yu et al as well as in other alcohol mixtures. , Several studies have been carried out concerning the hydrogen bonding in water–ethanol binary solutions …”

Mid-IR photothermal sensing of liquids for trace analysis

“…Indeed, H-O-H bending mode of water-ethanol mixture has been reported to be affected by a small blue shift (1660 cm -1 ) and a broader band-shape in comparison to the bending mode of pure water. [7][8][9] In water-chloroform mixtures, the H-O-H bending mode has been reported to be red shifted and to have significantly narrower band-shape as the hydrogen bonds are disrupted in this solvent. 10 Both these effects are seen in our PTS spectra as well as in the FTIR reference measurements.…”

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

“…Surface plasmon-enhanced vibrational spectroscopies such as IR (SEIR), Raman scattering (SERS), and sum frequency generation (SESFG) have been demonstrated to be powerful diagnostic techniques that can enhance vibrational signals of molecules by several orders of magnitude when the molecules are located at or close to the surface of noble metal nanostructures. − Sum frequency generation vibrational spectroscopy (SFG-VS) is an inherently second-order nonlinear coherent spectroscopy whose signals rely on a tensor product of the IR transition dipole moment and the Raman polarizability tensor. , It allows one to selectively identify the molecular structures and dynamics on a surface or at an interface. − Because SFG is a sensitive probe of surfaces and has a high sensitivity to molecular configuration, it has been found that SFG can detect a few percent of a monolayer on nanoparticle surfaces. ,, Therefore, the coupling of SFG-VS with surface plasmon is expected to be able to largely enhance the SFG signals and shows the potential to watch the reaction and actions of the individual molecule at the surface in real time. Currently, several surface-enhanced SFG phenomena have been reported.…”