Effect of ionic strength on electrokinetic properties of oil/water emulsions with dipalmitoylphosphatidylcholine

Extensive investigations of electrokinetic properties of emulsions and suspensions deal among other things with the application of natural stabilizers. Dipalmitoylphosphatidylcholine (DPPC) is present in many biochemical processes including living organisms. In this paper the dynamic properties of n-tetradecane/water emulsions were studied in the presence of DPPC which changes the interfacial free energy and the zeta potential. The zeta potential of the emulsion was determined as a function of pH and electrolyte concentration. H+/OH− ions were found to determine the potential, and also a clearly visible effect of the Na+, Ca2+ and La3+ ions appeared. The decrease in absolute value of the zeta potential was usually accompanied by a growth in effective diameter of the emulsion droplets. To better describe the stabilizing mechanism of DPPC, the electric charge at the shear plane was calculated using the zeta potentials and diameters of the droplets, assuming spherical particles. In n-tetradecane/electrolyte emulsion stabilized by DPPC the zeta potential may be also related to acid-base interactions. From the results, however, repulsion of double layers was concluded to be the main stabilizing mechanism. Possible stabilizing effect of DPPC liposomes and/or other aggregates with adsorbed ions needs further investigations.