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

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 The adoption of the parallel orientation at low coverages by C₁₂E₈ was superseded by limiting adsorption above the c.m.c. which did not correspond to the plateau adsorption for C₈E₄ and C₁₂E₅.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.