SURFACE FREE-ENERGY COMPONENTS OF SILICA-GEL DETERMINED BY THE THIN-LAYER WICKING METHOD FOR DIFFERENT LAYER THICKNESSES OF GEL

The influence of the thickness of silica gel layer on the penetration rate of selected liquids and, in consequence, on the value of surface free energy components determined by thin layer wicking method was stu died. Plates of the following gel thickness were used: 0.25, 0.5, 1.0 and 2.0 mm. Measurements of the penetration rate of apolar liquids, i. e. three alkanes: octane, nonane and decane, diiodomethane, alpha-brom onaphthalene and two polar liquids: water and formamide were made for this purpose. From the obtained relationships x(2) = f(t) the suitabil ity of Washburn's equation in the whole penetration range (9 cm) was c onfirmed for all thicknesses of porous layers. However, the penetratio n rate of probe liquids changed with the thickness of the deposited la yer, it was mainly dependent on an effective (apparent) radius of the interparticle pores. Using these results and the appropriate form of W ashburn's equation surface free energy components of silica gel 60 (fo r four thicknesses of layer) were calculated. It was found that values of apolar Lifshitz-van der Waals (gamma(s)(LW) = 41.7 +/- 0.9 mJm(-2) ) and polar acid-base (gamma(s)(AB) = 11.5 +/- 0.5 mJm(-2)): electron donor (gamma(s)(-) = 50.8 +/- 0.9 mJm(-2)) and electron acceptor (gamm a(s)(+) = 0.7 +/- 0.1 mJm(-2)) components of surface free energy were very similar for different layer thicknesses. Even in the case of a 2 mm thick layer reproducible values of both Lifshitz-van der Waals and acid-base component were obtained. It is important when the thin layer s are prepared in laboratory conditions, i. e. from suspensions by wat er evaporating, and the deposited layer is of less controlled thicknes s. When diiodomethane and alpha-bromonaphthalene are considered as wea kly polar liquids, the value of Lifshitz-van der Waals component of si lica gel is very close to gamma(s)(LW) s determined from n-alkanes.