Structure and hydration of poly(ethylene oxide) surfactants at the air/liquid interface. A vibrational sum frequency spectroscopy study
2007 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, no 31, 11642-11652 p.Article in journal (Refereed) Published
Adsorption layers of penta(ethylene oxide) n-dodecyl ether (C12E5) at the air/liquid interface have been studied using the surface-sensitive technique vibrational sum frequency spectroscopy (VSFS). The CH and COC stretching vibrations of the surfactant molecule, as well as the OH stretching vibrations of the surface water molecules, have been targeted to obtain a comprehensive picture of the adsorption process. The concentration range studied comprises different adsorption regimes, starting from the neat water surface until attaining the saturated liquid expanded monolayer when approaching the critical micellar concentration (cmc). The surfactant molecules were found to first adsorb to the air-liquid interface with their hydrocarbon tails preferentially orientated close to the surface plane, surrounded by patches of unperturbed surface water. These patches were only seen to disappear at areas per molecule close to 65 A(2), coinciding with a sudden change in the orientation of the surfactant alkyl chains, which adopted a more upright configuration. Nonetheless, gauche defects 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 water molecules 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-based headgroup. 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 temperature dependence of ethylene oxide-based surfactants and polymers in water.
Place, publisher, year, edition, pages
2007. Vol. 111, no 31, 11642-11652 p.
air-water-interface, monododecyl pentaethylene glycol, self-assembled-monolayers, liquid-gas interface, ch stretching modes, air/water interface, molecular-orientation, generation spectrum, neutron reflection, phase-separation
IdentifiersURN: urn:nbn:se:kth:diva-16834DOI: 10.1021/jp070246rISI: 000248482300020ScopusID: 2-s2.0-34548047466OAI: oai:DiVA.org:kth-16834DiVA: diva2:334877
QC 201005252010-08-052010-08-052011-02-15Bibliographically approved