Investigation of the effect of substrata on the surface free energy components of silica gel determined by thin layer wicking method

The components of surface free energy of silica gel deposited on glass plates, aluminium or plastic sheets (commercial Merck's plates for TLC) were determined by applying two theoretical approaches: the Lifshitz-van der Waals acid-base approach (LWAB), the geometric mean of apolar and polar interactions approach (GM). The thin layer wicking method was applied. Besides, from the measured contact angles of probe liquid (diiodomethane, alpha -bromonaphthalene, water, formamide, glycerol and ethylene glycol) surface free energy components were calculated for the substrata on which the silica gel was deposited. In this case the equation of state (ES) was additional by applied. The study has shown that both the kind of substratum and that of the chambers used for measuring the penetration rate of the liquid do not influence the calculated values of the surface free energy components of silica gel. The components calculated from the two models of interfacial interactions: gamma (LW)(s) and gamma (AB)(s) or gamma (d)(s) and gamma (p)(s) appeared to be practically the same if diiodomethane and alpha -bromonaphthalene were considered as weakly polar liquids, i.e. donor-acceptor interactions in the surface tension were taken into account. However, when gamma (+)(l) and gamma (-)(l) of these liquids are neglected, considerable differences occur, particularly in the determined value of Lifshitz-van der Waals component. The values of the total surface free energy of substrata calculated by applying three thermodynamic approaches are very similar, but if diiodomethane and alpha -bromonaphthalene are considered as weakly polar liquids this similarity is more distinct. (C) 2000 Kluwer Academic Publishers.