Interaction of Ionic Species and Fine Solids with a Low Energy Hydrophobic Surface from Contact Angle Measurement

Interactions of ionic species, (organic and inorganic) precipitates, and fine solids with a low energy hydrophobic surface were examined using a model system of paraffin wax in aqueous solutions. Contact angle measurement was used to evaluate the interactions between paraffin wax and testing variables. No changes in contact angle were observed with various types of metal and metal hydroxyl ions, metal hydroxyl precipitates, fine silica, and alumina powders, suggesting weak or absence of interactions between these species and paraffin wax. At pH <9, the presence of amine reduced the contact angle, but no pH dependence on contact angle was observed for a given amine concentration. A sharp decrease in contact angle was observed at higher pHs, where precipitates of amine molecules formed probably on wax surfaces. In the presence of lauric acid, on the other hand, contact angles reduced at a pH below 8, due to the formation of precipitates, but the reduction was less significant, compared with the reduction by amine precipitates. At high pHs, adding lauric acid did not show any effect on the measured contact angles. The significant effect of fine solids on contact angle was observed only when the solids were made hydrophobic by adsorbed surfactants. The present study further demonstrated that both the thermodynamic criteria and the interactions among substrate/solids/surfactants/metal ions must be considered in identifying the effect of different factors on the wettability of low energy hydrophobic surfaces. Copyright 1998 Academic Press.