Surface free energy components of silica gel determined by the thin layer wicking method for different layer thicknesses of gel

Abstract

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 studied. 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, α-bromonaphthalene and two polar liquids: water and formamide were made for this purpose. From the obtained relationships x2=f(t) the suitability of Washburn’s equation in the whole penetration range (9 cm) was confirmed for all thicknesses of porous layers. However, the penetration rate of probe liquids changed with the thickness of the deposited layer, it was mainly dependent on an effective (apparent) radius of the interparticle pores. Using these results and the appropriate form of Washburn’s equation surface free energy components of silica gel 60 (for four thicknesses of layer) were calculated. It was found that values of apolar Lifshitz–van der Waals (γLWs=41.7±0.9 mJ m-2) and polar acid–base (γABs=11.5±0.5 mJ m-2): electron donor (γ-s=50.8±0.9 mJ m-2) and electron acceptor (γ+s≡0.7±0.1 mJ m-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 layers are prepared in laboratory conditions, i.e. from suspensions by water evaporating, and the deposited layer is of less controlled thickness. When diiodomethane and α-bromonaphthalene are considered as weakly polar liquids, the value of Lifshitz–van der Waals component of silica gel is very close to γLWs determined from n-alkanes.