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Assessing the stability of Tween 80-based O/W emulsions with cyclic voltammetry and electrical impedance spectroscopy A Physicochemical and engineering aspects

Roldan-Cruz, C., Vernon-Carter, E.J., Alvarez-Ramirez, J.
Colloids and surfaces 2016 v.511 pp. 145-152
capacitance, centrifugation, creaming, dielectric spectroscopy, droplets, emulsions, nonionic surfactants, oils, polysorbates, zeta potential
The stability of oil-in-water (O/W) emulsions made with the nonionic surfactant Tween-80 was explored using cyclic voltammetry and electrical impedance spectroscopy (EIS). Emulsions were prepared with a disperse phase volume fraction of 0.25 and Tween-80 concentrations ranging from 0.25 to 4% v/v with respect to the aqueous phase. As a preliminary step, the emulsions were characterized in terms of mean droplet diameter, creaming rate from analytical centrifugation and ζ-potential. The results showed that the mean droplet diameter and the creaming rate decreased as the concentration of Tween-80 was increased. In contrast, the ζ-potential showed a decreasing behavior probably caused by the displacement of ionic species from the oil/water interface. In a second step, cyclic voltammetry and EIS measurements were performed. The former indicated the increase of the system capacitance with the incorporation of surfactant molecules. A Randles equivalent circuit was used to fit EIS measurements, indicating that double-layer resistance decreased and the double-layer capacitance increased as the amount of Tween-80 increased up to about 2% v/v. For higher Tween-80 concentrations, the parameters achieved a nearly constant value, which was interpreted as the threshold point where the O/W emulsion droplets became saturated by surfactant molecules. Thus, it was concluded that cyclic voltammetry and EIS are suitable techniques for characterizing the stability of O/W emulsions made with nonionic surfactants, and for pinpointing optimal surfactant concentration for achieving full emulsion droplets surface coverage.