ANIONIC SURFACTANT ADSORPTION ONTO SILICATE MINERALS - THE ROLE OF THE CATIONS

The adsorption isotherms of sodium tetradecyl sulfonate, sodium dodecyl sulfonate and sodium decylbenzene sulfonate onto quartz and kaolinite were determined at temperatures below the Krafft points of the surfactants. The hydrated crystal was choosen as the reference phase in order to discuss the origin of the surfactant-mineral normal bond. The undersaturation domain of the reduced isotherms depends on the nature of the cations that are present in the electrical double layer of the mineral. In the case of calcium ions alone, this domain ranges between - 4.5 and - 0.5 kT, and between - 3.5 and - 0.5 kT as soon as sodium ions are present. Adsorption isotherms of sodium lauryl sulfonate and 5-dodecyl para-ethylbenzene sulfonate, used above their Krafft point, together with electrophoretic measurements, confirm these conclusions. As adsorption proceeds at temperatures above the Krafft point, the molecular packing area of alkane sulfonate is 32 angstrom 2. Below the Krafft point, the molecular packing area is 20.5 angstrom 2. The state of the adsorbed layer is identical with the state of a reticular plane of the hydrated crystal.