Wetting of polymer surfaces by aqueous surfactant solutions

An important application of surfactants is to improve the wettability of aqueous solutions on hydrophobic surfaces, as indicated by a decrease in the contact angle of the liquid on the solid. The surfactant adsorbs at the liquid/vapor interface, reducing the surface tension and adsorbs at the solid/liquid interface, reducing the interfacial tension. In this work, the reduction of the surface tension and interfacial tension was measured as well as the advancing contact angle and surfactant adsorption as a function of surfactant concentration for three anionic and three cationic surfactants on eight different polymers with varying hydrophobicity. The Zisman equation (cosine of contact angle is a linear function of surface tension) adequately describes all systems studied as the critical micelle concentration is approached. The dependence of contact angle solely on surface tension does not mean that interfacial tension reduction due to surfactant adsorption is not important; rather, interfacial tension reduction mirrors surface tension reduction with increasing surfactant concentration; the ratio of solid/liquid interfacial tension to the liquid/vapor surface tension was shown to be independent of surfactant concentration for all 48 systems studied here. In general, interfacial tension reduction and surfactant adsorption at the solid/liquid interface are less significant as polymer hydrophobicity decreases. The critical surface tension (surface tension at which contact angle is zero as extrapolated from Zisman plots) differs for the different surfactants on a given polymer, emphasizing the limited generic applicability of the Zisman hypothesis for surfactant solutions. A mathematical analysis was developed to calculate the solid/vapor and the solid/pure water interfacial tensions which depend only on the polymer characteristics. The solid/vapor interfacial tension was found to be 33.3 mN/m and does not depend on polymer structure.