Adsorption and Desorption of Polymer/Surfactant Mixtures at Solid-Liquid Interfaces: Substitution Experiments

Abstract

The adsorption of mixts. of aq. solns. of cationic hydroxyethylcellulose polymer JR 400 and anionic surfactant, sodium dodecyl sulfate, using at. force microscopy (AFM) was studied. Samples with various compns. from different regions of the ternary phase diagram presented in our previous work were imaged by at. force microscopy on freshly cleaved mica, and hydrophobically modified mica and silica in soft-contact mode. A series of "washing" (subsequent injection of compns. with gradually decreasing polymer/surfactant ratio) and "scratching" (mech. agitation of the surface material with an AFM tip) expts. were performed. The morphol. of the adsorbed layer altered in a manner following the changes in morphol. in the bulk soln. These changes were evidenced in cluster formation in the layer. The results suggest that the influence of the surface was limited to the formation of the adsorbed layer where the local concns. of polymer and surfactant were higher than those in the bulk. All further modifications were driven by changes in the mixt. compn. in bulk. Force measurements upon retraction reveal the formation of network structures within the surface aggregates that will greatly slow structural reequilibration.