Surface properties of tetra(ethylene oxide) dodecyl amide compared with poly(ethylene oxide) surfactants. 2. Effect of the headgroup on surface forces

Surface interaction forces between layers of tetra(ethylene oxide) n-dodecyl amide, TEDAd, were compared with those of C12E5, C12E4, C10E5, and C10E4. It was found that adsorbed layers of TEDAd on a hydrophobic surface are thinner and less diffuse than layers Of C12E5. As the number of EO groups decreases, the short-range repulsion commences at shorter distances and at the same time the compressibility decreases. These results can be qualitatively explained with model mean-field lattice calculations, except for the fact that a higher attractive force was found between layers of TEDAd, whereas no adhesion was found in the case of the alcohol ethoxylates. The reason for this is proposed to be a smaller steric/protrusion force in the case of TEDAd caused by intralayer interactions between the amide groups inside the adsorbed layer. The force required to squeeze out the surfactant layers from the contact zone increased with concentration, and at the same time the adhesion decreased. Above the critical micelle concentration, no adhesion was found between the surfaces, and an outward jump from hydrophobic surface contact occurred on the repulsive side of the force curve as the compressive force was decreased. The force when this occurred was found to be highest for the largest hydrophilic headgroups, indicating that the surface pressure outside the contact region, and the pressure from surfactants remaining between the surfaces, was largest for these surfactants. In contrast to the ethoxylated surfactants, sugar surfactant layers are less compressible, show considerably larger adhesion, and are not possible to squeeze out at the compressive forces used in this study.