The adsorption of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) surfactants on
silica gel has been measured as a function of added molybdenum and NaCI by volumetric isotherms (from solution)
as well as Fourier-transform IR spectroscopy (FTIR-PA), in the solid state. The adsorption of both surfactants onto
molybdenum-modified silica surfaces shows two phase transitions, whereas only one phase is obtained upon the
adsorption of these surfactants onto the parent silica.
CTAB shows a significant increase upon adsorption onto molybdenum-modified silica compared with SDS especially
following the addition of NaC1 (0.5M), due to the evolution of MoO3 species following calcination, the high degree
of CTAB structure change with the addition of NaC1, and the lower degree of interference of C1- and CTA ÷ ions
than that of SD- ions.
The volumetric isotherms were shown to be a good tool for elucidating the adsorption mechanism. The IR spectra
of CTAB adsorbed onto the molybdenum-silica system in the hydroxyl group region, at the extent of the electrolyte
concentration, showed that adsorption occurs mainly via H-bonded silanols owing to steric crowding might be
obtained at increasing electrolyte concentrations, which prevent the terminal hydroxyl groups from reacting properly,
and the relatively long alkyl chain of CTAB compared with that of SDS.