Wetting and dewetting of Si/SiO ₂-wafers by free and lipidmonolayer covered aqueous solutions under controlled humidity

(Received 6 August 1993, received in final form 22 November 1993, accepted 10 December 1993)

Abstract
The spreading and stabilization of free and lipid monolayer covered films of aqueous solutions (of NaCl, glucose, polyethylene glycol and polylysine) on Si/SiO ₂ were studied between 60 and 95 % relative humidity. A film balance, equipped with an ellipsometer and a closed chamber for humidity control, was used. The time evolution of the film thickness and its lateral variation was monitored by imaging ellipsometry, while high precision thickness measurements were performed by null ellipsometry. The film thickness of NaCl and glucose solutions could be reversibly varied between about 10 Å and 1 000 Å for relative humidities between 60 and 95 %. Film thicknesses were about a factor of 4 to 10 larger than reported previously. The thickness- vs.-humidity curves are explained quantitatively by the Brunauer-Emett-Teller theory (BET) of multilayer adsorption, as well as by a modified disjoining-pressure-model by accounting for the chemical potential equilibrium of water between the atmosphere and the film and with respect to Raoult's law of vapour pressure lowering. Solutions of polyethylene glycol thinned to a thickness of one adsorbed macromolecular monolayer and the thickness could be reversibly varied between 8 and 14 Å for humidities between 60 and 95 %. Non soluble fatty acid monolayers spread spontaneously on the pre-stabilized fluid films by viscous fingering, driven by the Marangoni Effect. At repulsive disjoining pressures, the composite films remained stable and could be reversibly swollen and deswollen. Dewetting occurred at attractive interactions or at negative spreading parameters either by collapse from the edges or by local hole formation. 40 mM glucose solution films covered by an arachidic acid monolayer decomposed laterally (and in a reversible way) at high humidities ( > 94.8 %) into domains varying in water layer thickness by about 1 000 Å.