The zeta potential of solid asphaltene in aqueous solutions and in 50:50 water + ethylene glycol (v/v) mixtures containing ionic surfactants

Abstract The purpose of this research work was to investigate the effect of adding a polar organic co-solvent, namely ethylene glycol, as well as anionic and cationic surfactants on the zeta potential of solid asphaltene particles dispersed in aqueous 1.0 mM NaNO 3 solutions. In the absence of ethylene glycol and ionic surfactant, the zeta potential of asphaltene is negative in the pH interval from 3.5 to 10.5, and its magnitude increases slightly as the pH increases. These results suggest that the surfaces of asphaltene contain pH-dependent ionizable functional groups that can undergo dissociation and protonation. In 50:50 water + ethylene glycol (v/v) mixtures, the zeta potential of asphaltene is more negative in the pH range from 4.3 to 10.5, and the isoelectric point shifts to pH 4.2. This electrokinetic behavior indicates that ethylene glycol molecules adsorb in the Stern layer and affect the electric characteristics of the interface. The addition of an aqueous solution of long-chain anionic or cationic surfactant also affects the zeta potential of asphaltene in 50:50 water + ethylene glycol (v/v) mixtures. 1.0 mM sodium dodecyl sulfate (an anionic surfactant salt) makes the already negative zeta potential of asphaltene significantly more negative in the pH range investigated. At low pH, adsorption of the dodecyl sulfate anion onto electropositive asphaltene particles may involve an electrostatic mechanism whereas at pH values at which the zeta potential of asphaltene is negative without surfactant, a hydrophobic mechanism must predominate. In solutions 1.0 mM of cetylpyridinium chloride and dodecylamine hydrochloride (both cationic surfactants) the zeta potential of asphaltene is positive in all the pH range investigated. Under these conditions, cationic surfactant adsorption onto positively charged asphaltene surfaces may involve a hydrophobic mechanism whereas an electrostatic contribution may predominate when the zeta potential of asphaltene is negative.