Stability of oil/water (ethanol, lysozyme or lysine) emulsions

Emulsions of n-tetradecane in water (0.1%v/v) homogenized by ultrasounds (1 5 min) were stabilized by 0.5 or 1.0 M ethanol and in the presence of lysozyme (4 mg 100 ml(-1)) or 1 mM lysine monohydrochloride (14.6 mg 100 ml(-1)). The zeta potentials and multimodal size distributions of the droplets after 5, 15, 30, 60, 120 min, and 1 and 2 days were determined by dynamic light scattering technique using ZetaPlus apparatus (Brookhaven Instr., USA). Both parameters were determined on the same sample subsequently without any mixing. The effect of pH [4, 6.8 (natural), and 11] was also investigated. The most stable emulsions in 1 M ethanol solutions alone were at pH 6.8 and 11 (the effective diameter D-eff and 350 nm, respectively), while in 0.5 M at pH 4 (D-eff nm). The most stable emulsions with lysozyme were obtained at pH 4 and 1 M ethanol (D-eff nm), and with lysine at pH 6.8 and 0.5 M ethanol (D-eff nm). Except for the emulsions with lysozyme at pH 4 and 6.8, in the rest systems the zeta potentials were negative and ranged between - 5 and - 85 mV as a function of time and pH. The changes of zeta potential indicate that H+ ions are not much potential determining, while OH- ions increase the negative zeta potentials. However, H+ ions affect functional groups of lysozyme molecules adsorbed on the alkane droplet, what appears in essential changes of zeta potential and even reversed sign of it in some systems. The results point that stability of these emulsions may also be determined by hydrogen bonding. (C) 2000 Elsevier Science B.V. All rights reserved.