Effects of chain length and electrolyte on the adsorption of n-alkylpyridinium bromide surfactants at sand-water interfaces

The kinetic and equilibrium studies of the adsorption of four cationic surfactants (pyridinium bromide) with different chain lengths (C-16, C-14, C-12, and C-10) onto sand are presented here. The adsorption and desorption behavior in the absence and presence of different electrolytes (NaCl, CaCl2, and Na2SO4) are compared in batch and continuous column experiments. The kinetic studies show that the rates of adsorption of pyridinium bromide surfactants on sand surfaces are very high (similar to 70% of saturation adsorption occurs in 30 s) and are almost the same at low concentration (0.5 mM) for different chain lengths. The amount of surfactant adsorbed is enhanced by the presence of electrolyte because of a reduction in electrostatic repulsion among the headgroups, whereas the effect of the valence of coion does not appear to be important. Desorption studies based on column experiments show that a lower amount of surfactant is retained when eluted with electrolyte solution instead of pure water.