Coadsorption of n-propanol and water on SiO 2: Study of thickness, composition, and structure of binary adsorbate layer using attenuated total reflection infrared (ATR-IR) and sum frequency generation (SFG) vibration spectroscopy

Anna L. Barnette, Seong H. Kim

Research output: Contribution to journalArticle

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Abstract

The thickness and structure of the binary adsorbate layer of n-propanol and water molecules formed on fused silica at near equilibrium vapor pressure at room temperature were studied using attenuated total reflectance infrared spectroscopy (ATR-IR) and sum frequency generation (SFG) vibration spectroscopy. The thickness of the binary adsorbate layer on silica is kept relatively constant at ∼0.9 nm when the n-propanol vapor fraction (y propanol) is between 0.6 and 1 and then gradually increases up to ∼6.5 nm as y propanol decreases from 0.6 to 0 (y water increasing from 0.4 to 1). The composition of the binary adsorbate layer as well as the n-propyl group at the adsorbate/air interface shows a drastic change at the azeotrope composition of the vapor mixture (y propanol = 0.36). The binary mixture is propanol-rich at y propanol > 0.36 and water-rich at y propanol < 0.36, which is consistent with the vapor-liquid equilibrium. However, the vapor composition dependence of the adsorbate/air interface structure appears drastically different from that of the liquid/air interface. The n-propanol SFG signal at the adsorbate/air interface gradually decreases as y propanol decreases from 1 and suddenly drops at y propanol = 0.36, while the n-propanol SFG signal increases to a maximum value at y propanol = 0.36 for the liquid/air interface. Comparison of the ATR-IR and SFG results suggests that the binary adsorbate layer of n-propanol and water assumes a layered structure in which n-propanol is at the adsorbate/vapor interface and water is inside the adsorbate layer, and unlike the liquid/vapor interface, the propanol molecules do not form a paired dimer-like structure at the adsorbate/vapor interface.

Original languageEnglish (US)
Pages (from-to)9909-9916
Number of pages8
JournalJournal of Physical Chemistry C
Volume116
Issue number18
DOIs
StatePublished - May 10 2012

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

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