Mechanism of Superspreading
The reasons for these differences are explanined and summarized in the following schemes:
"Normal" micelle-forming surfactants are shaped like an icecream cone and therefore can only assemble to form curved aggregates, e.g. spherical micelles dispersed in water or hemimicelles adsorbed to a hydrophobic substrate. In this case, the hydrophilic head groups and water is in contact with the hydrophobic substrate, which is not at all an ideal situation considering that the interfacial tension between the hydrophobic substrate and the aqueous surfactant solution should be minimized. The spreading process itself is promoted by the surface tension gradient between the precursor film of water and the surface of the droplet saturated with surfactant. The spreading rate-determining step is the transport of surfactant molecules to the freshly created interfaces at the perimeter of the droplet.
Using simple geometric considerations, surfactants with hydrophilic and hydrophobic portions of about the same size can not form spherical micelles. They have to form bilayer aggregates to minimize the contact of their hydrophobic groups with water. Such bilayer aggregates are typical of lamellar phases, L3 phases and vesicles. Considering the situation at the interface between the hydrophobic substrate and water, surfactants capable of forming bilayers are ideal to shield the substrate from water by forming a flat surfactant monolayer. Also from the aspect of spreading kinetics, bilayer aggregates are an ideal way to transport surfactant molecules to the newly created interface at the spreading front; it can be considered as "unzipping".
