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

Marinho, Ricardo R. T.; Walz, Marie-Madeleine; Ekholm, Victor; Öhrwall, Gunnar LU ; Björneholm, Olle LU and de Brito, Arnaldo Naves (2017) In Journal of Physical Chemistry B 121(33). p.7916-7923
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 [... (More)

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.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Journal of Physical Chemistry B
volume
121
issue
33
pages
8 pages
publisher
The American Chemical Society
external identifiers
  • scopus:85028348758
ISSN
1520-6106
DOI
10.1021/acs.jpcb.7b02382
language
English
LU publication?
yes
id
ba30da82-7ee9-4f62-8fc1-6a342b3c5dfb
date added to LUP
2017-09-06 13:54:33
date last changed
2017-09-06 13:54:33
@article{ba30da82-7ee9-4f62-8fc1-6a342b3c5dfb,
  abstract     = {<p>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-[CH<sub>3</sub>-CH<sub>2</sub>OH]) 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-[CH<sub>3</sub>-CH<sub>2</sub>OH] 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.</p>},
  author       = {Marinho, Ricardo R. T. and Walz, Marie-Madeleine and Ekholm, Victor and Öhrwall, Gunnar and Björneholm, Olle and de Brito, Arnaldo Naves},
  issn         = {1520-6106},
  language     = {eng},
  month        = {08},
  number       = {33},
  pages        = {7916--7923},
  publisher    = {The American Chemical Society},
  series       = {Journal of Physical Chemistry B},
  title        = {Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy},
  url          = {http://dx.doi.org/10.1021/acs.jpcb.7b02382},
  volume       = {121},
  year         = {2017},
}