Adsorption of n-alkylpolyethylene glycol non-ionic surfactants from aqueous solution on to silica

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Abstract
Adsorption isotherms are reported for three nonionic surfactants (C8E4, C12E5, C12E8) on silica immersed in water. Three distinct stages of adsorption for C12E5 were recognised. Initially C12E5 was adsorbed parallel to the silica surface but at higher concentrations the C12 chains were lifted from the surface and E5 chains were arranged in a looped configuration with a proportion of the oxygen atoms in each molecule interacting with surface silanol groups via hydrogen bonds. C12E5 was adsorbed in bilayers at concentrations above the critical micelle concentration (c.m.c.). Contact angle and X-ray diffraction data supported the conclusions. Similar results were obtained for C8E4 although the distinctions between the successive stages of adsorption were less clear. The adoption of the parallel orientation at low coverages by C12E8 was superseded by limiting adsorption above the c.m.c. which did not correspond to the plateau adsorption for C8E4 and C12E5. Two possible explanations are offered for this different behaviour.

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