Adsorption isotherm and orientation of alcohols on hydrophilic SiO 2 under ambient conditions

Anna L. Barnette, David B. Asay, Michael J. Janik, Seong H. Kim

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The adsorption isotherm and orientation of small alcohol molecules (n-propanol and n-pentanol) on clean, hydrophilic silicon oxide surfaces under ambient conditions were studied with attenuated total reflectance infrared (ATR-IR) spectroscopy and density functional theory (DFT). The ATR-IR study reveals that as the alcohol partial pressure relative to its saturation vapor pressure (P/P Sat) increases from 0 to ̃ 10%, the isotherm thickness of n-propanol and n-pentanol increases rapidly to ̃0.3 nm and ̃0.6 nm, respectively. Upon further increase in P/P sat, the isotherm thickness increases only slightly until the condensation occurs at the near saturation vapor condition. The alkyl chains of the alcohol molecules adsorbed at 5% P/P Sat appear to be tilted toward the surface, while the hydroxyl groups are oriented toward the surface normal direction. As P/P sat increases further, the molecular orientation of adsorbed molecules drastically changes to a structure that is either random or oriented 40-50° from the surface normal. The DFT calculations for n-pentanol adsorbed on SiO 2 support the molecular structure determined from the ATR-IR experiment. The similar transition in molecular orientation with increase of the adsorbed thickness was observed for longer chain alcohols (n-decanol and n-octadecanol). The alkyl chain packing into a self-assembly like structure was observed only when the chain length is long enough and the substrate is heated. These observations are discussed in terms of the adsórbate-substrate interactions as well as intermolecular interactions.

Original languageEnglish (US)
Pages (from-to)10632-10641
Number of pages10
JournalJournal of Physical Chemistry C
Issue number24
StatePublished - Jun 18 2009

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • General Energy
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films


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