INFLUENCE OF POROSITY ON THE ADSORPTION OF A NONIONIC SURFACTANT ON SILICA

The adsorption of a non-ionic surfactant (Triton X100) is studied on a set of porous silicas differing only in their pore size distributions . The Barrett, Joyner and Halenda method applied to the adsorption-des orption isotherms of nitrogen at 77 K is used to determine the mesopor e distributions whose mean radii range between 3 and 30 nm. The adsorp tion isotherms of TX100 and the corresponding enthalpies of displaceme nt of water are determined. They all show for these silicas that after the anchorage of a few molecules, adsorption occurs through the forma tion of micelle-like aggregates. Moreover, decreasing the mean pore ra dius leads to both a shift of adsorption isotherms towards low concent rations and a depression of the plateau level. These facts are interpr eted by considering three types of mesopore: (i) those with diameters lower than the surface micelle thickness (negligible adsorption); (ii) those with diameters of the same order (enhancement of adsorption pot ential for surface aggregates); (iii) larger pores where only geometri cal considerations have still to be taken into account.