The Interfacial Physics Group

Abstract

Surfactant solutions exhibit a wide variety of wetting and dewetting behaviors on high energy surfaces. These behaviors are driven by surfactant self-assemblies at the moving contact line. To probe these self-assemblies, we have undertaken a study of surfactant structure at the three interfaces near a receding contact line. We immerse a hydrophilic silica surface in aqueous solutions of polyethyleneglycol monododecyl ether (C12En, 1≤n≤8) below the critical micelle concentration. The substrate is withdrawn from solution at a speed, U<Ucrit, the critical velocity for pulling a macroscopic film on the solid surface, so that a receding contact line moves across the surface. We determine the area per molecule adsorbed at the solid-liquid and liquid-vapor interfaces, and the structural details of the monolayer deposited to the solid-vapor interface at the receding contact line. We also describe in detail a new technique which we have developed for objectively interpreting data from x-ray reflectivity measurements, our primary tool for probing structure at the solid-vapor interface. We find that the adsorbed amount at the solid-liquid interface is a small-to-negligible contribution to the monolayer deposited at the solid-vapor interface for all n. The primary source of the deposited surfactant is the self-assembled layer at the liquid-vapor interface. The density of the deposited monolayer is substantially less than the density at the liquid-vapor interface. Conservation of mass demands a dividing streamline in the bulk, along which surfactant from the liquid-vapor interface is returned to solution. We note a transition at n=6 from reversible to partially irreversible adsorption, suggesting the ethylene oxide (EO) head groups begin to behave like PEO polymer for n≥6. At the liquid-vapor interface the area per molecule increases monotonically with n, suggesting increasing disorder in the head group region. The deposited monolayer at the solid-vapor interface shows a more complicated, non-monotonic dependence on n. Substantial rearrangement of molecules takes place as surfactant is deposited from the liquid-vapor to the solid-vapor interface. Processes at the receding contact line and the structure of the deposited monolayer show marked transitions at n=3, indicating a significant interaction between head group and substrate for n>3.