Electrokinetic Studies on Emulsions Stabilized by Ionic Surfactants: The Electroacoustophoretic Behavior and Estimation of Davies’ HLB Increments

Model oil-in-water emulsions were made. To stabilize these emulsions ionic surfactants were added. Their electrokinetic behavior was studied applying the electroacoustophoresis technique. The so-called electrokinetic sonic amplitude (ESA) was determined as a function of ionic strength, pH, and concentration and type of surfactant at ambient temperature and pressure. More than 30 different types of surfactant, mainly cationics, were included. Ionic strength reduced the ESA. The pH influenced the ESA strongly when the surfactant associated with acid or base. In the case of a fatty acid-based emulsion the ESA increased with increasing pH. In the case of fatty amines and derivatives the reverse trend was observed. At extreme pH levels the ESA always decreased with further increases or decreases in the pH. Surfactant concentration initially gave rise to an increase in the ESA. At higher concentrations the ESA leveled off or even decreased with further increases in the surfactant concentration. The type of surfactant also influenced the ESA. The longer the alkyl chain, the stronger the ESA became. The surfactant headgroup determined the sign of the ESA. More hydrophilic headgroups gave rise to lower ESA than did less hydrophilic ones. These observations were interpreted in terms of effects induced by creating an electric double layer around the emulsion droplets. Ionic strength reduces the double layer thickness, while the pH can generate more charge. Curves of ESA vs surfactant concentration were analyzed by first correcting them for the effect of ionic strength. Subsequent modeling including a hyperbolic, Langmuir adsorption-like equation allowed us to distinguish between surfactant effectivity and efficiency and to analyze the relation of the electrokinetic behavior to surfactant molecule structure. A relation of effectivity to efficiency has been indicated. It was determined that the ESA depends on surfactant molecular structure following the well-known Traube rule. Further elaboration was done by relating the derived dependence to Davies’ HLB system. HLB increments were derived by applying the carboxylate data as yardstick. Typical increment values are 20–25 for both cationic and anionic headgroups.