Simultaneous surfactant aggregation in aqueous solution and at the solid–liquid interface. Coadsorption of decanoyl-N-methylglucamide and phenol on silica gel

The adsorption isotherm of the nonionic surfactant decanoyl-N-methylglucamide (MEGA-10) on silica gel exhibits two steps, the latter of which arising from the formation of micelle-like surface aggregates, as is evident from coadsorption experiments with phenol. Deviating from general adsorption characteristics of surfactants on hydrophilic substrates, the onset of the second step coincides with the critical micelle concentration (CMC). Hence, the hemimicelle concept is extended to the limiting case of a simultaneous self-assembly of dissolved and adsorbed surfactant molecules, admitting a minor increase in the content of non-associated monomers in the micellar region. At a given phenol concentration, the composition of the surface aggregates is nearly independent of the adsorbed amount of surfactant and resembles the respective composition of the coexisting bulk micelles.