Interpreting contact angles of surfactant solutions on microporous hydrophobic membranes

Almost every single experimental study regarding membranes involves the measurement of contact angle (CA) to quantify the membrane wetting property. However, the interpretation of CA can sometimes be tricky. In this study, we investigate an interesting phenomenon about the CA of a surfactant solution on a microporous hydrophobic membrane. Specifically, a surfactant solution with a very low surface tension can have an unexpectedly high CA on a microporous hydrophobic membrane. In contrast, a water/ethanol mixture with the same surface tension completely wicks the membrane (i.e., zero CA). The drastic difference in CA between the two types of liquid of the same surface tension results from the rapid adsorption of surfactants at the wetting frontier which substantially reduces the local surfactant concentration and increases the local surface tension. The same theory can also be applied to explain the striking difference between the two liquids in capillary rise and liquid entry pressure. The results from this study cast significant doubt on the role of surface tension in understanding the wetting behavior of surfactant solutions when they are in contact with solid with large specific area and raise important questions regarding the utility of measuring CA for surfactant solutions on microporous hydrophobic membranes.