Adsorption of some quaternary onium salts on silica gel

The adsorption of quaternary onium salts such as hexadecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, benzyltrimethylammonium chloride and tetrabutylammonium bromide from aqueous solution, and of tricaprylylmethylammonium chloride (Aliquat ® 336) and hexadecyltributyl phosphonium bromide from organic solution has been studied using silica as an adsorbent. The adsorption from the aqueous phase was found to be dependent on the pH of the medium, giving a type IV isotherm in acidic and alkaline solution. The adsorption from the aqueous phase in the medium pH region and from the organic phase appears to give a Langmuir-type isotherm. The equilibrium adsorption capacity and the adsorption rates in the aqueous phase were found to be higher than those observed for adsorption from the organic phase. The concentration-time profiles generated in an agitated slurry system indicate that in the case of aqueous phase adsorption in the middle pH range, external film diffusion controls the solute uptake initially and particle diffusion controls the solute uptake in the later stages, whereas for the organic phase adsorption, both film and surface diffusion seem to control the uptake.