Interfacial interaction between sodium dodecyl sulfate and hydrophobicallymodified ethyl(hydroxyethyl)cellulose. A surface force study

The effect of addition of sodium dodecyl sulfate (SDS) to a layer of hydrop hobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC) preadsorbed on hy drophobized mica has been studied with an interferometric surface force app aratus (SFA). The force between the surfaces was always repulsive on both c ompression and separation. The range of this force, which is 1500 Angstrom for the HM-EHEC layers, undergoes a non-monotonic change on addition of SDS . Initially, at low SDS concentrations the layer expand, while at higher su rfactant concentrations a contraction of the adsorbed layer is observed. At 1.5 mM SDS the range of the force is at maximum, while at 6 mM SDS the lay er is significantly thinner than before addition of SDS. The results are di scussed in terms of polymer-surfactant interfacial association and competit ive adsorption. The data obtained by surface force measurements agree quali tatively with ellipsometry findings on the same system.