Forces between non-ionic surfactant layers

The nature of the repulsive forces acting between non-ionic hydrophilic surfaces has been investigated by the surface-force technique. For that purpose the interactions between head-groups of several non-ionic surfactants adsorbed on mica or hydrophobed mica were measured. In addition, the swelling pressure between monoglyceride layers in both the lamellar and the gel phases was determined using a water vapour pressure method. Repulsive short-range forces with an effective range of 1–3 nm were observed. They are believed to result from a combination of hydration and steric effects. The temperature dependence of the short-range force depends on the type of non-ionic group. When ethylene oxide or dimethylphosphine oxide groups are exposed towards the aqueous phase, an increased temperature results in a thicker layer, a steeper repulsive force, and a stronger attractive minimum. On the other hand, the layer thickness and the force acting between surfaces with exposed dimethylamine oxide groups are largely temperature independent, whereas in the case of monopalmitin, the force is more repulsive at higher temperatures. This behaviour is discussed with respect to the phase relations in the corresponding surfactant–water phase diagrams.