Adsorption of a cationic gemini surfactant from aqueous solution onto aluminosilicate powders of the MCM-41 type: effect of pore size and co-adsorption of phenol

Abstract

The adsorption of cationic gemini dodecanediyl- α, ω-bis(dodecyldimethylammonium) bromide (i.e., C 12C 12C 12) from aqueous solution onto aluminosilicate powders of the MCM-41 type (referred to as SiAl32 dx, where x is the mean pore diameter in Å) has been studied at 298 K under the conditions of free pH of the aqueous phase. Macroporous silica gel XO15M was used for comparative purposes. The discussion was based on the interpretation of experimental adsorption isotherms and differential enthalpies of displacement obtained on various solid samples. For the hydrogen-exchanged SiAl32 d28 sample (i.e., H +–SiAl32 d28), the adsorption of bromide counterions Br − at the solid–solution interface, the isotherm of the pH evolution in the equilibrated supernatant liquid, and the electrophoretic mobility of the solid particles coated with the adsorbed C 12C 12C 12 were additionally measured. The uptake of phenol (PhOH) by a surfactant–solid system from a 1.5 mmol kg −1 PhOH solution and the related thermal effect of displacement onto SiAl32 d50 were quantified with the use of the solution depletion technique combined with UV spectrophotometry and the titration calorimetry technique. Titration calorimetry was also used to determine the molar enthalpy change accompanying micellization of C 12C 12C 12 in pure deionized water and in a 1.5 mmol kg −1 PhOH solution at 298 K. The adsorption of C 12C 12C 12 occurs simultaneously on the external surface and on the pore walls and it is a strongly co-operative phenomenon. Surfactant aggregates forming at adsorption saturation are thought to be composed of the adsorbed surfactant units having their cationic head groups mostly oriented outward with respect to the solid surface. Therefore, they can provide co-adsorption sites for polarisable phenol molecules. On average, there is one phenol molecule retained for one gemini cation adsorbed. Transfer of phenol from the aqueous phase to either the bulk micelles or to the interfacial aggregates is enthalpically favourable.