Adsorption of extended surfactants at the water-PTFE interface: The effect of PO number

Abstract

The main purpose of this work is to investigate the effect of PO groups on the wetting properties of surfactant solutions on polytetrafluoroethylene (PTFE) surfaces. The surface tension and contact angle at the PTFE surface were measured for the extended surfactants dodecyl (polyoxyisopropyl)4 sulphate (L-C12PO4S) and dodecyl (polyoxyisopropyl)11 sulphate (L-C12PO11S). The results show that PO groups can significantly reduce the critical micelle concentration (CMC) for the surfactants. The contact angles for L-C12PO4S and L-C12PO11S remain almost constant below the CMC and decrease significantly above the CMC. L-C12PO4S and L-C12PO11S show a linear relationship between the surface tension and adhesion tension. It is noteworthy that the solid–liquid interfacial tension of the surfactant decreases more significantly above the CMC. This implies that the adsorption behaviour of surfactants is different below and above the CMC. Below the CMC, surfactants are adsorbed at the PTFE surface through hydrophobic chains and form a loose film. Compared to the anionic surfactant SDS, the introduction of the PO group leads to unique adsorption behaviour for the surfactant. The extended surfactant molecules form a saturated adsorption film at the air-liquid interface but continue to adsorb and form semi-micelles at the solid–liquid interface above the CMC.