It is generally agreed that the most practical way to obtain the surface energetics of solids is the measurement of contact angles of well defined liquids on a solid surface. However, serious problems arise when contact angles are measured on the surfaces of porous materials, such as membranes or powders, which are of considerable technological interest. Surface roughness, heterogeneity, and the penetration of the liquid drop into the porous material may affect the measured contact angles, causing meaningless angles in terms of Young´s equation and hence in terms of a surface energetic interpretation of the contact angle data. An alternative way to obtain the solid surface tension (gsv) of rough and porous solids are capillary penetration experiments. Our strategy is based on a modified Washburn equation. However, in contrast to approaches of other authors in the literature, we determine directly the solid surface tension from the capillary penetration experiments. The determination of the penetration velocity of liquids into porous solid systems yields Kglvcosq versus glv plots, which provide gsv values for these systems; K is an unknown parameter of the constant geometry of the porous solid.
In the experiment, the porous solid systems are attached to an electrobalance (Fig. 1a) and brought into contact with several liquids. Their penetration velocities are determined by the increasing in weight as a function of time. A commercial tensiometer can be used for these gravimetric experiments. The application of this concept was demonstrated for hydrophobic polymer powders (PTFE, polypropylene, polyethylene, polystyrene), for hydrophilic (cellulose) and hydrophobic (PTFE) membranes, and for textile fiber bundles. The solid surface tension values determined by the capillary penetration technique are in good agreement with the gsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials.
Fields of research
- Determination of the solid surface tension of flat and capillary membranes used for hemodialysis, effect of different chemical modifications of the membrane material on the surface energetics
- Influence of different sizings of textile fibers or reinforcing fibres (carbon or glass fibers) on the solid surface tension using fiber bundles
- Determination of the surface energetics of pigments used in powder coating systems and of inorganic fillers used in polymer compounds
- Dr. Karina Grundke
- Heidrun Schaller