The structure and composition of ethyl hexadecanoate layers spread on aqueous solutions of hexaethylene glycol monododecyl ether

J. R. Lu, T. J. Su, M. J. Lawrence, D. J. Barlow, W. Warisnoicharoen, T. Zuberi

Research output: Contribution to journalArticlepeer-review

Abstract

Specular neutron reflection combined with deuterium labeling has been used to study the structure and composition of a mixed layer of oil and surfactant at the air/water interface, formed by spreading ethyl hexadecanoate from small lenses placed on the surface of hexaethylene glycol monododecyl ether (C12E6). The area per oil molecule was found to be 128 ± 10 Å2 in the mixed monolayer and that of C12E6 to be 54 ± 3 Å2, identical to the value obtained in the absence of the oil. The thicknesses for the dodecyl chain layer and ethoxylate layer were 18.5 ± 2 Å and 20 ± 2 Å, assuming Gaussian distributions, as compared with the respective values of 16 ± 2 Å and 16.5 ± 2 Å without the oil, suggesting that oil mixing has broadened the distributions of the two fragments. The extent of penetration of the ester oil into the surfactant layer was described by the relative distances between the centers of the distributions. The use of alkyl chain deuterated and ethoxylate headgroup deuterated surfactants allowed separate determinations of the relative distances between the centers of the distributions of the dodecyl chains and of the oil (δco) and between the ethoxylate headgroups and the oil (δeo). The value of δco was found to be 0.5 ± 1 Å and that of δeo to be 10.5 ± 1 Å, showing that while the center of the ethoxylate headgroups was further into aqueous solution the ester oil layer was almost entirely overlapped with the distribution of the dodecyl chain layer. On the basis of comparisons made with the results from other systems involving the spreading of dodecane and dodecanol onto the layers of ionic surfactants and nonionic alkyl ethoxylates, we attribute the deep insertion of the ester oil into the C12E6 layer to the combined effect of the affinity of the ester groups toward water and the affinity of the oil toward the ethoxylate groups.

Original languageEnglish
Pages (from-to)4638-4648
Number of pages11
JournalJournal of Physical Chemistry B
Volume103
Issue number22
DOIs
Publication statusPublished - 3 Jun 1999

Research Beacons, Institutes and Platforms

  • Dalton Nuclear Institute

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