The adsorption of fluorinated dopants at the surface of 5CB: a neutron reflection study

The adsorption of dopants at the surface of 5CB has been studied using neutron reflection. The dopants were versions of 11OCB with partly fluorinated chains, and the 5CB was interfaced with air or with silica treated with fluorocarbon or hydrocarbon coatings. At the air interface, the F17-11OCB adsorbed homeotropically, and the amount increased on cooling into the nematic phase of 5CB. At low temperatures, the adsorption from the nematic phase appeared to saturate at a bilayer. At the solid interfaces, there was some evidence for a thin planar layer at the surface, but most of the adsorbed dopant was in the form of diffuse layers. In both cases, an increase in dopant concentration caused higher adsorbed amounts. For F17-11OCB, the amount absorbed was much greater for a fluorinated coating as opposed to a hydrocarbon-coated surface. Also no adsorption of the F3-11OCB could be detected, suggesting that the adsorption process is driven by the oleophobic effect. The results demonstrate that it is possible to manipulate surface properties of liquid crystals using surfactant-like dopants.