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, a nd fine solids with a low energy hydrophobic surface were examined usi ng 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 precip itates, fine silica, and alumina powders, suggesting weak or absence o f interactions between these species and paraffin wax. At pH < 9, the presence of amine reduced the contact angle, but no pH dependence on c ontact angle was observed for a given amine concentration. A sharp dec rease 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 si gnificant, compared with the reduction by amine precipitates. At high pHs, adding lauric acid did not show any effect on the measured contac t angles. The significant effect of fine solids on contact angle was o bserved only when the solids were made hydrophobic by adsorbed surfact ants. The present study further demonstrated that both the thermodynam ic criteria and the interactions among substrate/solids/surfactants/me tal ions must be considered in identifying the effect of different fac tors on the wettability of low energy hydrophobic surfaces. (C) 1998 A cademic Press.