Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. I. Nature of the noncritical adsorption profiles

Abstract : Neutron reflection has been applied to probe the nature and extent of adsorption from a mixture of (1-x)n-hexane+xperfluoro-n-hexane against silicon substrates modified with alkylsilane (fluorophobic) or fluoroalkylsilane (fluorophilic) coupled layers. For an equimolar mixture (x=0.5, 60.7 vol %) in the one-phase region at T=30 °C—removed both in temperature and composition from the upper critical point at 22.65 °C and x=0.36—the structure was resolved at both fluorophobic and fluorophilic surfaces. Liquid mixtures with three different refractive index contrasts were used to reduce model ambiguity in the ensuing analysis. For both surfaces the composition profiles of the adsorbed liquids could be represented using two-layer slab models which included interlayer Gaussian roughness. For the fluorophobic surface, the thickness of the layer closest to the substrate is ∼20 Å and composed of ∼83 vol % n-hexane, and the second, more dilute layer has a composition profile which decays smoothly into the bulk over a range of ∼100 Å. A similar result is found for the fluorophilic surface, but in this case the layer closest to the substrate is ∼15 Å thick and composed of ∼95 vol % perfluoro-n-hexane. Qualitatively similar behavior is found for adsorption from a mixture with x=0.7 against a fluorophobic substrate and for a mixture with x=0.2 against a fluorophilic substrate.