Hydrophobic surfaces

Abstract

Hydrophobic surfaces are characterized by high contact angles with water, often in the range of 40 to 110 degrees, and low heats of immersion, −6 to −90 ergs/cm 2. Current theories of the interaction across interfaces indicate that the water and the hydrophobic substance interact only through dispersion forces. Thus, the water doesn't spread because only the γ H 2O d , which is 22 ergs/cm 2 not the total 72 ergs/cm 2, and only the γ S d are responsible for the interfacial free energy. The Frenkel-Halsey-Hill (FHH) plots indicate that the interaction of nitrogen or water with hydrophobic surfaces is less in the first layer and greater than for polar surfaces in the second layer and beyond. Furthermore, the break in the FHH plots for nitrogen on graphitic surfaces at 0.4 relative pressure strongly suggests that the packing is loose, one nitrogen to each ring, at the low apparent monolayer coverage; as the relative pressure is increased this layer then fills. Hence the many reported surface areas and other surface properties of graphitic solids, such as Graphon, must be altered to conform with the 20 A 2/N 2 molecule to be used in the area estimation by the BET method. Hydrophilic sites invariable present on “real” hydrophobic surfaces are interesting both theoretically and pratically. Adsorption of water and other polar molecules on these sites is energetically weak and increases with increasing temperature. The molecules form clusters around the first-down molecules at high relative pressures; the adsorbed molecules are highly entropic and the heat of immersion rises with increasing precoverage. Such sites are responsible for the heterogeneous nucleating ability of essentially hydrophoc “cloud seeders” such as silver iodide and hydrophobic silicas.