Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

Graca, Malgorzata, Bongaerts, Jeroen H. H., Stokes, Jason R. and Granick, Steve (2007) Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces. Journal of Colloid and Interface Science, 315 2: 662-670. doi:10.1016/j.jcis.2007.06.057

Author Graca, Malgorzata
Bongaerts, Jeroen H. H.
Stokes, Jason R.
Granick, Steve
Title Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces
Journal name Journal of Colloid and Interface Science   Check publisher's open access policy
ISSN 0021-9797
Publication date 2007-01-01
Year available 2007
Sub-type Article (original research)
DOI 10.1016/j.jcis.2007.06.057
Open Access Status Not yet assessed
Volume 315
Issue 2
Start page 662
End page 670
Total pages 9
Place of publication New York
Publisher Academic Press
Language eng
Subject 09 Engineering
Abstract The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta > 110 degrees). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximate to 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu similar to 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximate to 1. 1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating proper-ties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths. (C) 2007 Elsevier Inc. All rights reserved.
Keyword Tween
Contact angle
Q-Index Code C1
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: Excellence in Research Australia (ERA) - Collection
School of Chemical Engineering Publications
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