Surface Free Energy, Adsorption and Zeta Potential in Leacril/Tannic Acid System

Surface free energy components, apolar Lifshitz−van der Waals, γ s LW, and polar electron acceptor, γ s +, and electron donor, γ s -, were determined for leacril (a copolymer of 90% acrylonitryle and 9% vinyl acetate) fibers with preadsorbed tannic acid solutions (10 - 5 −10 - 2 M). A thin layer wicking method was applied for this purpose. The components were calculated considering van Oss et al.'s ( J. Chem. Rev. 1988, 88, 927) polar components of surface tension of probe liquids (formamide and water, γ w - = γ w + = 25.5 mJ/m 2 for water) as well as those suggested very recently by Lee ( Langmuir 1996, 12, 1681), who considered for water γ w - = 19 mJ/m 2 and γ w + = 34.2 mJ/m 2 . The trend of changes of the calculated components is the same, except for the surface treated with 10 - 2 M tannic acid. However, the electron donor components of the leacril calculated with Lee's data are somewhat lower. From the calculated work of spreading of water, results indicate that the bare leacril surface is slightly hydrophobic, the surface treated with 10 - 5 M tannic acid becomes low hydrophilic, and in the range of concentrations 10 - 4 −10 - 3 M becomes increasingly hydrophobic. When equilibrated with 10 - 2 M it is again hydrophilic. The changes can be explained by reorientation of the tannic acid molecules on the leacril surface and micelle formation above cmc, which was determined to be at ca. 3 × 10 - 4 M. Adsorption measurements show that the process is very fast and physical in nature. The rate constants and empirical diffusion coefficients were evaluated at 275, 283, 293, and 303 K. Also thermodynamic functions for the adsorption process were calculated from the adsorption isotherms. It was concluded that hydrogen bonding and Lifshitz−van der Waals interactions are responsible for the adsorption, while electrostatic interactions play a rather second-order role. The leacril shows small negative zeta potentials which drop almost to zero at concentrations higher than 10 - 4 M.