Adsorption behaviors of cationic surfactants and wettability in polytetrafluoroethylene-solution-air systems

Measurements of the advancing contact angle (θ) and adsorption properties were carried out for aqueous solutions of four cationic surfactants, hexadecanol glycidyl ether ammonium chloride (C₁₆PC), Guerbet alcohol hexadecyl glycidyl ether ammonium chloride (C₁₆GPC), hexadecanol polyoxyethylene(3) glycidyl ether ammonium chloride(C₁₆(EO) ₃PC), and Guerbet alcohol hexadecyl polyoxyethylene(3) glycidyl ether ammonium chloride (C₁₆G(EO)₃PC), on the polytetrafluoroethylene (PTFE) surface using the sessile drop analysis. The obtained results indicate that the contact angle decreases to a minimum with the increasing concentration for all cationic surfactants. Surfactants with branched chain show lower θ values. Moreover, an increase of adhensional tension on the PTFE-water interface has been observed for the four cationic surfactants, and the branched ones have larger increases of adhensional tension. It is very interesting that the sharp decrease of θ appears mainly after critical micelle concentration (cmc) for C₁₆GPC, C ₁₆(EO)₃PC, and C₁₆G(EO)₃PC, which is quite different from traditional cationic surfactants reported in the literature. Especially for C₁₆G(EO)₃PC, there are two saturated adsorption stages on PTFE surface after cmc (which means the saturated adsorption film at air-solution interface has been formed). In the first saturated stage, the C₁₆G(EO)₃PC molecules are oriented parallel to the PTFE surface with saturated monolayer formed through hydrophobic interaction and hydrogen bond. In the second saturated stage, the hemimicelle has been formed on the PTFE surface, which can be supported by the QCM-D and SPR measurements.