The wettability of alkyl carboxylbetaine (18C) and alkyl sulfobetaine (18S) at polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMMA) surfaces have been investigated and the different physicochemical parameters such as critical micelle concentration (CMC), surface tension, contact angle, surface excess, adhesional tension and work of adhesion have been estimated. The results show that the contact angle of 18C and 18S for both solids keep almost a constant value in a wide range of surfactant concentration, but the sharp decrease of contact angle appears after CMC of individual surfactant solution because of the continued increase in surfactant molecules adsorption at solid–liquid interface above CMC, which is quite different from traditional surfactants reported in the literature. In addition, 18C has significantly lower contact angle values on PTFE at high concentrations. For PTFE and PMMA there is a linear relationship existing between the adhesional and surface tension in a range of certain concentrations for all investigated surfactants. The values of slope suggest that adsorption of 18C and 18S at PTFE/PMMA–liquid interfaces are less than that at air–liquid, and the orientation of the surfactant molecules at PTFE–liquid and PMMA–liquid interfaces should not be the same. Moreover, the decrease of PTFE–liquid interfacial tension has been observed while the PMMA–liquid interfacial tension increases at first and decreases afterwards for 18C and 18S. Especially for PTFE, the decrease of γSL for 18C is larger than that for 18S, which indicates that 18S molecules may be vertical orientation due to steric effect while the hemimicelle has been formed for 18C at the PTFE interface. On the other hand, the addition of surfactant molecules will adsorb onto monolayer at PMMA surface again through hydrophobic interaction with hydrophilic group toward the bulk phase of solution above CMC.

Applied Surface Science,2015