Plasma modification of mica; forces between fluorocarbon surfaces in water and a nonpolar liquid

• Mica has been exposed to low-temperature water vapor plasma. A combination of reaction and sputtering in the plasma environment alters the surface properties of mica and activates the surface toward with chlorosilanes. The plasma treatment technique causes minimal alteration to the surface smoothness and oscillatory solvation forces were measured in octamethylcyclotetrasiloxane (OMCTS). Reaction of the plasma-treated mica with (tridecaflouro-1,1,2,2-tetrahydrooctyl)-1-dimethylchlorosilane renders the surface hydrophobic. The oscillatory solvation force is removed by surface roughness, due to a low surface density of reacted silane ( one molecule per 5 nm2 as estimated by ESCA) and the forces between thesesurfaces in OMCTS were attractive and in agreement with the force expected from van der Waals theory. The force between two silanated surfaces in water is in good agreement with the force calculated from DLVO theory at separations greater than 50 nm but an additional exponential attraction is observed at smaller separations. The magnitude and decay length of this attraction are in close agreement with those found previously between mica surfaces hydrophobed by Langmuir-Blodgett deposition of surfactant monolayers. Consequently, the hydrophobic interaction observed between surfactant-coated surfaces does not result from instabilities in the surfactant layer.

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