Interfacial structure and rearrangement of nonionic surfactants near a moving contact line

Surfactant solutions exhibit a wide variety of wetting and dewetting behavi ors on high energy surfaces. These behaviors are driven by surfactant self- assemblies at the moving contact line. To probe these self-assemblies, we s tudy the structure Of C12En (1 less than or equal to n less than or equal t o 8) surfactants at the three interfaces near a contact line receding acros s a hydrophilic surface. We determine the area per molecule adsorbed at the solid/liquid (SL) and liquid/vapor (LV) interfaces and the structural deta ils of the monolayer deposited to the solid/vapor (SV) interface at the rec eding contact line. For all n, the monolayer deposited at the SV interface is substantially less dense than the amount delivered to it by the LV inter face, with the SL interface making a small-to-negligible contribution to th e deposited monolayer. A dividing streamline must exist in the bulk, along which surfactant from the LV interface is returned to solution. For n great er than or equal to 6 the ethylene oxide headgroups begin to behave like po ly(ethylene oxide) (PEO) polymer at the SL interface. At the LV interface t he area per molecule increases monotonically with n, suggesting increasing disorder in the headgroup region. The deposited monolayer at the SV interfa ce shows a more complicated, nonmonotonic dependence on n. Processes at the receding contact line and the structure of the deposited monolayer show ma rked transitions at n = 3, indicating a significant interaction between hea dgroup and substrate for n > 3.