Probing molecular structure at interfaces for comparison with bulk solution behavior: Water/2-propanol mixtures monitored by vibrational sum frequency spectroscopy

Sho Kataoka, Paul S. Cremer

Research output: Contribution to journalArticlepeer-review

53 Scopus citations

Abstract

The orientation of the isopropyl group at the liquid/vapor interface in 2-propanol/water binary mixtures was studied by vibrational sum frequency spectroscopy. The CH3 stretch modes of the two methyl groups were used to determine the molecule's orientation by employing a novel united atom approach to model the (CH3)2X moiety. For this purpose, the changes in the molecular susceptibility of the isopropyl group stretches were derived in the laboratory frame as a function of the tilt and twist angles. The results indicated that the methyl groups lay down on the surface at low alcohol mole fraction and gradually twisted with increasing mole fraction. At the azeotrope, xiso = 0.68, one of the methyl groups aligned approximately parallel to the surface normal, whereas the other was nearly parallel with the liquid/vapor interface. When the mole fraction of 2-propanol was higher than 0.68, the orientation of 2-propanol remained almost constant. The change in the alcohol's orientation with 2-propanol mole fraction closely tracked changes in its bulk activity coefficient. Such results lead to a picture in which the surface structure and bulk properties of the system are closely linked.

Original languageEnglish (US)
Pages (from-to)5516-5522
Number of pages7
JournalJournal of the American Chemical Society
Volume128
Issue number16
DOIs
StatePublished - Apr 26 2006

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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