USE OF THE WASHBURN EQUATION FOR SURFACE FREE-ENERGY DETERMINATION

The thin-layer wicking technique was used to determine the solid surface free energy components for thin-layer chromatography silica (Merck) by the Washburn equation. It has been found that wick-wetting is a reproducible process and the results of the wetting rates expressed as the relationship of the penetrated distances squared, x2, are a linear function of time, t. However, in some systems an inflection appeared at a distance 3 less-than-or-equal-to x less-than-or-equal-to 5 cm, giving thus two straight-line sections with somewhat different slopes. n-Alkanes, diiodomethane, water, and formamide were used as the penetrating liquids. Van Oss et al.'s approach to the surface free energy was applied. Thus, apolar, gamma(s)LW, and polar electron donor, gamma(s)-, and electron acceptor, gamma(s)+, components were determined for both the laboratory-made glass plates with a deposited SiO2 thin layer as well as commercial ones. Practically the same value of gamma(s)LW was obtained for both kinds of plates. However, the commercial plates showed higher values of the gamma(s)- component. It is probably due to the presence of some adhesive additives. The polar gamma(s)+ component is small for both plates. It is believed that this technique and the approach to the surface free energy components are very useful for testing the wetting properties of solids by liquids.