CHARACTERIZATION OF THE ADSORBED LAYER OF A MIXTURE OF NONIONIC AND CATIONIC HYDROCARBON SURFACTANTS ON SILICA

The adsorption of hexadecyl trimethylammonium bromide (HDTAB) and nonylphenyl polyglycol ether (NP20) from mixed aqueous solutions on silica has been investigated. The adsorption from mixed solutions is enhanced at low concentration, but is decreased at high concentration compared with those of the single surfactants. Further, measurements of fluorescence spectra of pyrene-3-carboxaldehyde and of ESR spectra of 2,2,6,6-tetramethylpiperidinyl-1-oxy in the surfactant adsorbed layer indicate that the micropolarity of the adsorbed layer is affected by feed mole fraction of HDTAB, in particular over the range of 0.3-0.7, while the microviscosity of the adsorbed layer is almost constant in the same range at a high feed concentration. This suggests that over the same range, the ratio of HDTAB/NP20 at the second layer is almost the same.