Zeta potential and droplet size of n-tetradecane/ethanol (protein) emulsions

Abstract

Zeta potential, average diameter and multimodal size distribution were studied for n-tetradecane emulsions in aqueous solution of ethanol (0.5 and 1.0 M) in which bovine serum albumin (BSA), α-lactalbumin or β-casein (1, 2, or 5 mg/100 ml) was also dissolved. The emulsion pH was natural (7.3) or regulated to 4 or 11. The parameters were determined as a function of time, i.e. after 5, 15, 30, 60, 120 min, 1, and 2 days, and 1 week, since the emulsion preparation. The emulsions were prepared by dissolving 0.1 ml of the n-alkane in a proper amount of ethanol and then water or protein solution was added to obtain 100 ml of the emulsion in which total concentration of ethanol was 0.5 or 1.0 M. Next, the emulsion was sonicated for 15 min and the measuring polyacrylic cells of the apparatus were filled with the emulsion. The isoelectric point (i.e.p.) of the droplets in the presence of investigated proteins (2 mg/100 ml) and in 1 M ethanol occurred at pH 4.9, 4.7 and 2.2, for BSA, β-casein and α-lactalbumin, respectively. In pH range 5.5–10, the zeta potentials of freshly prepared emulsions in 1 M ethanol were negative and relatively large, from −45 to −60 mV. In β-casein presence, the n-alkane droplets were larger and negative zeta potentials higher than in the presence of two other investigated proteins. However, in the presence of each of the investigated proteins the droplet size increased slightly relative to that in ethanol solution alone. Nevertheless, the emulsions were relatively stable. In 0.5 M ethanol, the protein presence stabilized the emulsions. On time scale, the changes of negative zeta potential did not correlate in a straight way with changes in the droplet size (the emulsion stability). Experiments showed that without ethanol presence in the emulsion, β-casein alone can be used as an emulsifier already at its concentration of 1 mg/100 ml for 0.1 ml of n-tetradecane content both at natural and alkaline environment. However, no stable emulsion could be obtained using BSA or α-lactalbumin. Multimodal size distribution analysis showed that the droplet sizes in the studied emulsions could be grouped in one or two comparable populations only.