SPREADING OF THIN-FILMS OF ORDERED NONIONIC SURFACTANTS - ORIGIN OF THE STEPPED SHAPE OF THE SPREADING PRECURSOR

We report on the phenomenon of surface-induced self-assembly, observed as nonionic siloxane poly-(ethylene oxide) surfactants spread at the solid-air interface. Microdroplets of pure surfactants are put on high- and low-energy solid surfaces and the shape evolution is monitored by ellipsometry. Spreading, if any, occurs through an autophobic thin precursor film growing at the foot of the nonwetting main drop. Strikingly different shapes of the precursors are observed, depending on the interfacial properties of the substrate. No specific structure appears as the surfactants spread over high-energy surfaces. On low-energy surfaces, however, these molecules assemble into a densely packed bilayer with long-range intrinsic order. Stratified or ''stepped'' precursors are seen on medium-energy surfaces. The profile of the precursor is the outcome of the relative strengths of the substrate-surfactant and surfactant-surfactant interaction potentials as well as the molecular dynamic picture within the layer precursor. The area covered by the spreading film always increases linearly with time, yielding apparent diffusion coefficients that are sensitive to surfactant composition, surface chemistry, and perhaps most importantly to the atmospheric humidity. The ability of the surfactants to form dense bilayers is related to the geometry of the molecule, while the sensitivity to humidity is caused by specific interactions between the ethylene oxide part of the surfactants and water. Spreading on low-energy surfaces is observed for surfactants with trisiloxane hydrophobes and relatively short ethylene oxide chains, but not for corresponding hydrocarbon surfactants. This is argued to be the consequence of the relative bulkiness of the trisiloxane hydrophobe allowing space for the hydrophilic ethylene oxide chain and the high density of external hydrophobic methyl groups on the siloxane backbone. The conjunction of these properties may explain the ''superspreading'' ability of these trisiloxane surfactants.