Adsorption of anionic—nonionic and cationic—nonionic surfactant mixtures on kaolinite

Abstract

Adsorption on Na—kaolinite from surfactant mixtures of nonionic octaethylene glycol mono- n-dodecyl ether (C 12EO 8) with the anionic sodium dodecyl sulfate (C 12SO 4Na) and the cationic dodecyltrimethylammonium chloride [C 12(CH 3) 3NCl was studied in the surfactant concentration range varying from below CMC to above. It was found that in both systems, the adsorption of the ionic surfactant was enhanced by the presence of the nonionic surfactant and vice versa in the pre-CMC region. Chain—chain interaction between the adjacent ionic and nonionic surfactants on the particles has been proposed to be responsible for the enhanced adsorption. Such chain—chain interaction leads to the formation of surfactant clusters at the interface which abstract the hydrocarbon chains of the highly surface active C 12EO 8 into them. Under these conditions hydrophobicity of the particles as measured by skin flotation was found to be lower due to the exposed ethylene oxide groups. Decrease in the adsorption level of C 12SO 4Na with increase in C 12EO 8 in the region above CMC has been attributed to the decrease of C 12SO 4Na monomer concentration due to mixed micellization with C 12EO 8. The adsorption density of C 12EO 8 in plateau region passes through a maximum at equimolar ratio of the two surfactants. This has been suggested to be due to the most compact arrangement of surfactants in the cluster formed at the interface at the equimolar composition of the surfactants. Similar adsorption behavior from anionic—nonionic and cationic—nonionic surfactant mixtures suggests that the synergism arises mainly from the chain—chain interaction between the two surfactants in the adsorbed state.