Zeta potential, effective diameter and multimodal size distribution in oil/water emulsion

Abstract

Zeta potentials, effective diameter and multimodal size distribution of model emulsions were studied as a function of time (5–120 min, 1 and 2 days), pH and temperature. The emulsion of n-tetradecane (0.1% v/v) in pure water or 10 −3 M KCl were obtained by mechanical mixing in a homogenizer at 10 rpm for 3 min. The emulsions in 1 M methanol, propanol and ethanol, and also in the presence of lysozyme (4 mg 100 ml −1) or lysine hydrochloride (1 mM) were obtained by dissolving tetradecane in the alcohol, then water was added and they were ultrasonicated for 15 min. The zeta potential of the emulsion depends on pH. In the presence of lysozyme, the isoelectric point occurred in pH 9.5, while in the presence of lysine hydrochloride it was negative in the pH range 3.5–9.5. The zeta potentials of the droplets in water and in 1 mM KCl were similar on the pH scale, but in case of water iep was observed in pH 3.05. In 1 M ethanol the iep occurred in pH 4.5. The changes of the zeta potential for these three emulsions were parallel in the pH range 5–9, however the negative values in 1 M ethanol were smaller. They were practically stable during 24 h. These diluted emulsions in 1 M alcohols were stable during 2 days as determined by effective diameter. The diameter was 350–450 nm for the emulsion in ethanol and propanol and 275–290 nm in methanol during the time studied. It was found that the zeta potential of the emulsion in 1 M propanol reversed it sign in natural pH, if the emulsion temperature has increased from 20 to 30 or 40°C. Calculations of the free energy of interactions were also conducted. It was found that principal interactions responsible for stability/instability of the emulsion are Lewis acid–base, i.e. hydrogen bonding between water molecules.