Adsorption and interaction forces of micellar and microemulsion solutions in ultrathin films

The interactions between surfaces across a nonionic surfactant (tetraoxyethylene dodecyl ether, C12E4) solution and its microemulsion in the dilute regime have been directly measured using the surface force apparatus. The results show that the structure of the adsorbed surfactant layer depends dramatically on the size and shape of the self-assembled structures in the bulk solution as well as the chemical nature of the substrate. On hydrophilic surfaces, the adsorption from surfactant solutions is relatively weak and the resulting long-range repulsive force is attributed to entropic confinement and elastic compression of adsorbed micelles. Microemulsion droplets adsorb much more strongly. On hydrophobic surfaces, a compact surfactant monolayer adsorbs leading, to a strong but shorter-ranged force. In contrast to the results with hydrophilic surfaces, the adsorbed surfactant layer is "destabilized" by the presence of oil from the microemulsion solution and is easily expelled upon compression. Quantitative assessments of the DLVO contribution and the entropic component of the measured force profiles are given. The entropic contribution can be explained and quantified when both the nature of the surface-surfactant interaction and the self-assembled structures in the bulk are known.