Examination of Surface Adsorption of Soluble Surfactants by Surface Potential Measurement at the Air/Solution Interface

Surface potential (ΔV) of the air/surfactant-solution interface was measured by using an ionizing 241Am electrode method at 298.2 K, where the effect of the head groups on ΔV was also examined. Hexadecyltrimethylammonium bromide (CTAB) and its homologous head group tetramethylammonium bromide (TAB) were used for examination of a cationic surfactant, sodium dodecyl sulfate (SDS), sodium methylsulfate (CH3-SO4Na), and sodium hydrogensulfate (NaHSO4) for an anionic surfactant, and octaethylene glycol mono-ntetradecyl ether (C14E8) and octaethylene glycol (E8) for a nonionic surfactant. For the cationic and nonionic systems, the surface potential of the homologous solutions gradually changed with concentration, whereas that of the corresponding surfactant solutions steeply increased up to 420 mV for CTAB at a concentration far below the critical micelle concentration (CMC), CMC/9, and up to 480 mV for C14E8 at CMC/18. For the anionic system, the surface potential traced a more complex variation with concentration. The above results indicate that  the molecular arrangement of CTAB and C14E8 near the interfacial region becomes established at a concentration far below the CMC. The molecular dipole moment at the surface, derived from the surface potential, was also discussed. Change of surface tension with concentration was discussed with change in the surface potential and with preceding observations depending upon the concentrations. Finally, the Brewster angle microscopy (BAM) images of the surfactant solutions below and above the CMC and that of pure water are quite the same in darkness, which strongly indicates no adsorption of the surfactants at the air/solution interface contrary to an insoluble monolayer at the interface. This fact is substantiated by the distinct change of the BAM images in darkness for a small change in refractive index increment of the substrate.