Association of adsorbed aromatic solutes
Adsorption of a number of mono-ionic aromatic solutes, mainly dyes—both anionic and cationic—on a variety of finely divided solids, e.g., specific samples of alumina, silica, titania, graphite, etc., has been measured from isotherms determined at room temperature. By comparing the adsorptions with the specific surfaces of the solids determined by other methods most of the solutes are clearly adsorbed to a far greater extent than necessary to cover the surface as a monolayer of mono-disperse aromatic ions. The factor by which the adsorption exceeds the theoretical value for such a monolayer—termed the coverage factor(CF)—can be calculated for different assumed orientations of the adsorbed aromatic ions; that based on flat orientation is statistically the most highly significant and reveals that the coverage factor rises with the cube of the ionic weight of the aromatic ion.
Aggregation numbers for some of the dyes in aqueous (buffer) solutions are available and agree with the CF values for adsorption from the same respective solutions. It is suggested that in all cases the solutes are adsorbed as aromatic ionic micelles, in which the ions have face-to-face packing. The CF for each solute is considered a maximum value. In practice, any lower value down to zero may be obtained if some or all of the surface of the solid has low reactivity by virtue of its history.