Ethanol in Aqueous Solution Studied by Microjet Photoelectron Spectroscopy and Theory

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: By combining results and analysis from cylindrical microjet photoelectron spectroscopy (cMJ-PES) and theoretical simulations, we unravel the microscopic properties of ethanol−water solutions with respect to structure and intermolecular bonding patterns following the full concentration scale from 0 to 100% ethanol content.In particular, we highlight the salient differences between bulk and surface. Like for the pure water and alcohol constituents, alcohol−water mixtures have attracted much interest in applications of X-ray spectroscopies owing to their potential of combining electronic and geometric structure probing. The water mixtures of the two simplest alcohols, methanol and ethanol, have generated particular attention due to their delicate hydrogen bonding networks that underlie their structural and thermodynamic properties. Macroscopically ethanol−water seems to mix very well, however microscopically this is not true. The aberrant thermodynamics of water−alcohol mixtures have been suggested to be caused by energy differences of hydrogen bonding between water−water, alcohol− alcohol and alcohol−water molecules. These networks may perturb the local character of the interaction between X-rays and matter, calling for analysis that go beyond the normally applied local selection and building block rules and that can combine the effects of light−matter, intra-and intermolecular interactions. However, despite decades of ongoing research there are still controversies of the precise nature of hydrogen bonding networks that underlie the mixing of these simple molecules. Our combined analysis indicates that at low concentration ethanol molecules form a film at the surface since ethanol at the surface can expose its hydrophobic part to the vacuum retaining its two (or three) possible hydrogen bonds, while water at the surface cannot retain all its four possible hydrogen bonds. Thus, ethanol at the surface becomes energetically favorable. Ethanol molecules show a tilting angle variation of the C−C axis with respect to the surface normal as large as 60°at very low concentration. In bulk, around ca. ten %, the ethanol oxygen atoms tend to make a third acceptor hydrogen bond to water molecules. At ca. 20 %, there is a U-shaped change in the CH 3 to CH 2 OH binding energy (BE) shift indicating the presence of ring-like agglomerates called clathrate structures. At the surface, between 5 and 25%, ethanol forms a closely packed layer with the smallest C−C tilting angle variation down to ∼20°. Above 25% and below the azeotrope at the surface, ethanol shows an increase in the tilting angle variation, while at very high ethanol concentrations water tends to move to the surface so giving a microscopic explanation of the azeotrope effect. This migration is connected to the presence of longer (shorter) ethanol chains in the bulk (surface). A brief comparison with discussions and predictions from other spectroscopic techniques is also given. We emphasize the execution of an integrated a...

Vibrational Sum Frequency Generation Spectra of Water‐Vapor Interfaces Covered by Alcohols: Effects of Surface Coverage and Coupling between Oscillators

“…This sharp and narrow distribution is a result of strong hydrophobic stacking interactions between the alcohol chains. Similar kind of changes in the tilting angle have also been observed in recent experiments using cylindrical microjet photoelectron spectroscopy and VSFG spectroscopy [61–63] . In order to have a better comparison with the experimental results, we have also calculated the VSFG response from alcoholic O−H groups for all the systems studied here.…”

“…Similar kind of changes in the tilting angle have also been observed in recent experiments using cylindrical microjet photoelectron spectroscopy and VSFG spectroscopy. [61][62][63] In order to have a better comparison with the experimental results, we have also calculated the VSFG response from alcoholic OÀ H groups for all the systems studied here. The qualitative trend of the the intensity change of the VSFG response of alcohol OÀ H groups with alcohol concentration (Sec.…”

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“…Also, the hydrophobic interaction of the ethyl groups in ethanol molecules enhanced hydrogen bond structures in water (Nagasaka et al, 2022). Ågren et al (2022) showed that, at low concentrations, ethanol molecules form a film at the surface since ethanol at the surface can expose its hydrophobic part, while water at the surface cannot retain all its four possible hydrogen bonds. Thus, ethanol at the surface becomes energetically favorable.…”