Wettability of a PTFE surface by aqueous solutions of zwitterionic surfactants: Effect of molecular structure

• The contact angle decreases with increasing concentration far above CMC for zwitterions.
• Both branched chain and polyoxyethylene units will further reduce the contact angle.
• The adsorption of solid–liquid continues when the surface adsorption is completed.
• The hemimicelle may be formed for C16(EO)3PB and C16G(EO)3PB at PTFE interface.

Advancing contact angle (θ) for aqueous solutions of the zwitterionic surfactants, hexadecanol glycidyl ether glycine Betaine (C16PB), guerbet alcohol hexadecyl glycidyl ether glycine Betaine (C16GPB), hexadecanol polyoxyethylene (3), glycidyl ether glycine Betaine (C16(EO)3PB) and guerbet alcohol hexadecyl polyoxyethylene (3), glycidyl ether glycine Betaine (C16G(EO)3PB), were measured on the polytetrafluoroethylene (PTFE) surface by the sessile drop analysis. The obtained results indicate that a sharp decrease of θ appears only when the bulk concentration is higher than critical micelle concentration (CMC). Both branched chains and the introduction of polyoxyethylene (EO) units decrease the contact angle significantly. Similar, both branched chains and the EO units appear to cause a significant increase in adhesional tension on PTFE-water interface. Moreover, there are two adsorption equilibrium stages on PTFE surface. At the first equilibrium stage, the surfactant molecules mostly form a monolayer on PTFE surfaces for four zwitterionic surfactants. For C16PB and C16GPB, the adsorptions at PTFE-water interface are much smaller than at water–air interface. However, it is more easier to absorb onto solid interface for C16(EO)3PB and C16G(EO)3PB surfactants because of intermolecular hydrogen bonding by EO units. In the second saturated stage, the adsorbed molecules may become oriented more and more perpendicular to the PTFE interface with hydrophilic heads oriented toward the water for C16PB and C16GPB. In the case of C16(EO)3PB and C16G(EO)3PB, the hemimicelle formed on the PTFE surface will enhance ionic head density and result in lower PTFE-water interfacial tension.

A sharp decrease of θ appears only when the bulk concentration is higher than critical micelle concentration (CMC).Figure optionsDownload as PowerPoint slide