Role of Surfactant-Adsorbent Acidity and Solvent Polarity in Adsorption-Desorption of Surfactants from Nonaqueous Media

Abstract

Adsorption of surfactants at the solid -liquid interface is governed in general by the properties of the surfactant, solid, and solvent. However, very little is known about the role of these properties in the case of adsorption from organic media. In this study the effect of adsorbent and surfactant acidities and solvent polarity on the adsorption of surfactants on oxide minerals in nonaqueous solvents has been investigated. The studies conducted using anionic (Aerosol OT) and cationic (dimethyldodecylamine) surfactants on the basic and acidic oxide minerals (alumina and silica) reveal that for a given surfactant the polarity differential between the adsorbent and the solvent is responsible for the partitioning of the solute to the solid -liquid interface. Following this, acid-base-type interactions between the solute and the adsorbent can take place to enhance the attachment of the former to the particles. The acid-base interactions depend on the nature of the surfactant and the substrate, with an acidic surfactant interacting strongly with a basic adsorbent and vice versa. By using the solvent dielectric constant as an indicator of solvent polarity, it was found that while polar interactions control the adsorption from solvents of low polarity, hydrocarbon chain interactions with the surface playa major role in determining adsorption from solvents of higher polarity.