Surface phase transition of C12E1 at the air/water interface: A study by dynamic surface tension, external RA FT-IR, and 2D IR correlation methods

S. Azizian, K. Shibata, T. Matsuda, Takanori Takiue, Hiroki Matsubara, Makoto Aratono

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Abstract

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C12E1) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C12E1, and it was observed that a constant surface tension region appears at ∼38.5 mN m -1 in a dynamic surface tension profile at concentrations higher than 11 μmol kg-1. This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 μmol kg-1) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m ≤ 16 μmol kg -1, which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH2 stretching decreases gradually from ∼2923 cm-1 (for 10 and 11 μmol kg-1) to ∼2918 cm-1 (for m ≥ 16 μmol kg-1) with increasing concentration. The wavenumbers of 2923 and 2918 cm-1 were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m ≤ 16 μmol kg-1 were correlated to the surface phase transition (LE → LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 μmol kg-1 also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C12E 1 monolayer. The synchronous correlation maps show two strong autopeaks at ∼2922 and ∼2851 cm-1 and also show a strong correlation (cross-peaks) between antisymmetric CH2 stretching (va) and symmetric CEk stretching (vs). The asynchronous correlation maps show that both observed bands of va and vs in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m ≤ 16 μmol kg-1. The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Original languageEnglish
Pages (from-to)17034-17042
Number of pages9
JournalJournal of Physical Chemistry B
Volume110
Issue number34
DOIs
Publication statusPublished - Aug 31 2006

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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