Measurements of the Force between Fluorocarbon Monolayer Surfaces in Air and Water

Abstract

We have measured forces between fluorocarbon surfaces prepared by three different methods using heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethoxysilane (FTE): LB deposition (LB), adsorption from diluted FTE solution with chloroform (FTE/CHCl3), and adsorption from undiluted FTE liquid (FTE/neat). In dry air, a van der Waals attraction was observed between the surfaces except for the FTE/neat surfaces. The Hamaker constant was found to be 7×10-20 J, which is comparable to that of the fused silica substrate. In saturated water vapor, a capillary bridging force was observed with FTE/CHCl3 and FTE/neat owing to the adsorption of water between the substrate and the fluorocarbon layer while a van der Waals attraction identical to that observed in dry air was detected between the LB surfaces. Only the LB surfaces were stable enough to effectively partition the adsorbed water layer beneath the fluorocarbon layer and air. In water, long-range forces from 120 to 150 nm were observed with FTE/neat while a rather shorter-range attraction within 40 nm was observed with LB and FTE∥CHCl3. The change in force profile with time could be explained by a change in surface properties occurring on immersion in water. It is suggested that the greater the number of mobile molecules, the longer-range the attraction observed between the surfaces. On the LB surfaces, after unbound molecules were removed, the force profile did not change for at least 5 days and a strong attractive force at separations below 12-13 nm was observed in water. The range of this force is in agreement with other studies showing only a comparatively short-range attraction between stable and homogeneous hydrophobic surfaces. The magnitude of the attraction is close to that of the short-range force measured previously between mica surfaces coated with fluorocarbon surfactant monolayers. This suggests that the strong, short-range attraction measured between many LB surfaces is related to the force between stable hydrophobic surfaces but that the very long-range exponentially decaying force has a different origin.