Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy

Research output: Contribution to journalArticle

Abstract

Because of the amphiphilic properties of alcohols, hydrophobic hydration is important in the alcohol-water system. In the present paper we employ X-ray photoelectron spectroscopy (XPS) to investigate the bulk and surface molecular structure of ethanol-water mixtures from 0.2 to 95 mol %. The observed XPS binding energy splitting between the methyl C 1s and hydroxymethyl C 1s groups (BES-[CH3-CH2OH]) as a function of the ethanol molar percentage can be divided into different regions: one below 35 mol % with higher values (about 1.53 eV) and one starting at 60 mol % up to 95 mol % with 1.49 eV as an average value. The chemical shifts agree with previous quantum mechanics/molecular mechanics (QM/MM) calculations [ Löytynoja, T.; et al. J. Phys. Chem. B 2014, 118, 13217 ]. According to these calculations, the BES-[CH3-CH2OH] is related to the number of hydrogen bonds between the ethanol and the surrounding molecules. As the ethanol concentration increases, the average number of hydrogen bonds decreases from 2.5 for water-rich mixtures to 2 for pure ethanol. We give an interpretation for this behavior based on how the hydrogen bonds are distributed according to the mixing ratio. Since our experimental data are surface sensitive, we propose that this effect may also be manifested at the interface. From the ratio between the XPS C 1s core lines intensities we infer that below 20 mol % the ethanol molecules have their hydroxyl groups more hydrated and possibly facing the solution's bulk. Between 0.1 and 14 mol %, we show the formation of an ethanol monolayer at approximately 2 mol %. Several parameters are derived for the surface region at monolayer coverage.

Details

Authors
  • Ricardo R. T. Marinho
  • Marie-Madeleine Walz
  • Victor Ekholm
  • Gunnar Öhrwall
  • Olle Björneholm
  • Arnaldo Naves de Brito
Organisations
External organisations
  • Federal University of Bahia
  • Uppsala University
  • University of Campinas
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Physical Chemistry
Original languageEnglish
Pages (from-to)7916-7923
Number of pages8
JournalJournal of Physical Chemistry B
Volume121
Issue number33
Publication statusPublished - 2017 Aug 24
Publication categoryResearch
Peer-reviewedYes