The Role of Surfactant Precipitation and Redissolution in the Adsorption of Sulfonate on Minerals

Abstract

The presence of maxima and sometimes minima in the adsorption isotherms of surfactant on minerals has been attributed to various mechanisms involving micelle exclusion, impurities, surfactant composition, mineral morphology, etc. This study on the precipitation of sulfonates with various inorganic ions (Na, K, NH4, Ca, Mg, and Al) and the subsequent redissolution when the surfactant concentration is increased shows a precipitation maximum to occur in the same surfactant concentration range where an adsorption maximum is obtained. Simultaneous abstraction, precipitation, and adsorption experiments have been conducted using chromatographically purified dodecylbenzenesulfonate and Na-kaolinite, and the results show the "abstraction vs. surfactant concentration" curves to exhibit a much more pronounced maximum than the real adsorption (abstraction minus precipitation) maximum. We suggest that the total abstraction of surfactant from the solution on contact with mineral phase, usually called "adsorption," is a summation of both the "real" adsorption and precipitation and that the precipitation/redissolution phenomenon is one of the major reasons for the adsorption maxima. Introduction of oil into the system reduces the precipitation because of partitioning of the sulfonate between the oil and aqueous phases. Precipitation and adsorption data are used to derive possible mechanisms for the precipitation/redissolution phenomena.