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New nonionic surfactants, N-alkyl-[N,N-bis(3-lactobionylamido)propyl] amine (N-alkyl: &z.sbnd;C12H25, &z.sbnd;C16H33, &z.sbnd;C18H38) and a gemini surfactant containing cetyl chain were investigated as stabilizers for n-tetradecane/ethanol (0.5 or 1 M) emulsions. These are nontoxic substances and could be applied for pharmaceutical or dairy emulsion preparation. The dynamic light scattering technique was applied for determination of the average effective diameter and zeta potential of the droplets as a function of time up to two days. These parameters were determined subsequently on the same sample without any mixing of it. Depending on the surfactant concentration (10−6, 10−5 and 10−4 M) and the emulsion pH, an increase or decrease in the droplet size was observed relative to the emulsion in the alcohol solution alone. It seems that the dynamic light scattering technique is a useful one for tracking changes in droplet size distribution in the emulsion systems. Some of the emulsions were more stable in the surfactant presence. However, in some other cases the surfactants destabilized emulsion. The zeta potentials in 10−5 and 10−4 M surfactant solutions were positive at pH natural (5–8) and acidic (pH 4) while at pH 11 they were negative. The isoelectric point of the emulsion droplets occurred in pH range 8–10, depending on the kind and concentration of the surfactant. It indicated that H+ and OH− ions were potential determining via adsorbed on the oil surface the surfactant molecules. In surfactant-free emulsion, the zeta potentials of n-tetradecane droplets were negative in pH range 4–11. It was concluded that hydrogen bondings between surfactant/alcohol and alcohol/water polar groups played an essential role in the emulsion stabilization (the extended DLVO theory). [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]