Structure and Hydration of Poly(ethylene oxide) Surfactants at the Air/Liquid Interface. A Vibrational Sum Frequency Spectroscopy Study

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• 作者：Eric Tyrode ; C. Magnus Johnson ; Mark W. Rutland ; Per M. Claesson
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：August 9, 2007
• 年：2007
• 卷：111
• 期：31
• 页码：11642 - 11652
• 全文大小：445K
• 年卷期：v.111,no.31(August 9, 2007)
• ISSN：1932-7455

文摘

Adsorption layers of penta(ethylene oxide) n-dodecyl ether (C₁₂E₅) at the air/liquid interface have been studiedusing the surface-sensitive technique vibrational sum frequency spectroscopy (VSFS). The CH and COCstretching vibrations of the surfactant molecule, as well as the OH stretching vibrations of the surface watermolecules, have been targeted to obtain a comprehensive picture of the adsorption process. The concentrationrange studied comprises different adsorption regimes, starting from the neat water surface until attaining thesaturated liquid expanded monolayer when approaching the critical micellar concentration (cmc). The surfactantmolecules were found to first adsorb to the air-liquid interface with their hydrocarbon tails preferentiallyorientated close to the surface plane, surrounded by patches of unperturbed surface water. These patcheswere only seen to disappear at areas per molecule close to 65 Ų, coinciding with a sudden change in theorientation of the surfactant alkyl chains, which adopted a more upright configuration. Nonetheless, gauchedefects in the hydrocarbon tails were observed along the whole concentration range, even above the cmc.Moreover, the poly(ethylene oxide) headgroup was seen to induce a significant structuring of the watermolecules in direct proximity to the surfactant monolayer, despite being themselves substantially disordered.Comparison of the hydration fingerprint region is made with another non-ionic surfactant with a sugar-basedheadgroup. The temperature effect in the VSFS spectra of C₁₂E₄ and C₁₂E₈ solutions has also been considered,and the results are discussed in terms of the different models proposed to explain the peculiar temperaturedependence of ethylene oxide-based surfactants and polymers in water.