A new interpretation of adsorption maxima and minima

In studies of surfactant adsorption from aqueous solutions onto solid substrates, a number of investigators have reported finding maxima followed by minima. Aiken (1) found a maximum at the CMC followed by a minimum when measuring the adsorption of sodium alkyl sulfates from aqueous solution onto cotton and wool. Corrin et al. (3), Meader and Fries (19), Void and Sivaramakrishnan (30), Void and Phansalkar (29), Evans (4), Fava and Eyring (5), Flett et al. (6), and Mukerjee and Anavil (21) have all reported results similar to those found by Aiken. It is remarkable that these studies involved a variety of substrates which have no obvious common features. Carbon black, wool, cotton, polystyrene, and graphite have all been used, precluding any attempt to ascribe the observed behavior to features intrinsic to the surface. There was, however, one feature common to all of the studies. In each, the adsorbate was a long-chain amphiphilic molecule and if the adsorption maxima and minima are real in the sense that these are properties of the individual isotherms, rather than being related to the fact that it is relative adsorption which is measured, then the explanation may in some way find its origin in the structure of the micelles while being essentially independent of the substrate. The existence of an adsorption maximum may not at first thought evoke much curiosity since it is well known that, for miscible binary mixtures, the relative adsorption