van der Waals interaction, surface free energies, and contact angles: Dispersive polymers and liquids

The Lifshitz theory of van der Waals interaction has been employed to calculate the contact angles of diiodomethane, 1-bromonaphthalene, 1-methylnaphthalene, bromobenzene, 1-tert-butylnaphthalene, liquid paraffin, and hexadecane on poly(dimethylsiloxane), poly(4-methyl-1-pentene), polyethylene, natural rubber, and polystyrene surfaces. The theoretical treatment is based on the equation cos theta = [(2A(PVI)/A(LVL))(H-oLVI/H-oPVL)(2)] - 1, where theta is the contact angle and A(PVL) and A(LVL) are the non-retarded Hamaker constants for the heterointeraction between polymer and liquid across vacuum and the homo interaction of the liquid across vacuum, respectively. Nonretarded Hamaker constants have been determined from the dielectric properties of the materials and application of the Lifshitz theory. H-oLVL and H-oPVL are the equilibrium ''contact'' surface separations associated with the Liquid-liquid homointeraction and polymer-liquid heterointeraction across vacuum, respectively. H-oLVL values, and the analogous H-oPVP values associated with polymer homointeraction, have been estimated from the surface free energies of the dispersive liquids and polymers. Four different approaches, each with a different assumption regarding the heterointeraction between polymer and liquid, have been used to obtain H-oPVL values: (i) H-oPVL = H-oLVL, (ii) H-oPVL = H-oPVP, (iii) a geometric mean relationship, H-oPVL = (HoPVPHoLVL)(1/2), and (iv) an arithmetic mean relationship, H-oPVL = (H-oPVP + H-oLVL)/2. Theoretical contact angles obtained with the four approaches have been compared with experimental contact angles. In general, the approaches which employ the combining rules, whether geometric or arithmetic, provide the best agreement between theory and experiment. Previous work that has dealt with the contact angles of n-alkanes on poly(tetrafluoroethylene) has also been reexamined. For the perfluorocarbon-hydrocarbon system none of the approaches are able to reconcile theory and experiment.