OIL-IN-WATER NANOEMULSIONS 337 cavitation is shown in Figure 1. The particle distribution for the two emulsions is expressed as a cumulative function. The resulting plots appear as double S-shaped curves. The particle size where the cumulative distribution is 50% is known as the median droplet diameter (dv,0.5). The emulsion prepared by ultrasonic cavitation has a very small dv,0.5 with 50% of the particles under 62.99 nm, compared to the emul- sion prepared by rotor–stator emulsifi cation, where the dv,0.5 is 126.17 nm. The relationship between emulsion particle size and emulsifi cation systems can be ex- plained in terms of energy input during emulsifi cation. Emulsion droplet size (EDS) can be reduced by increasing the amount of energy supply during emulsifi cation as long as there is suffi cient emulsifi er to cover new interface and recoalescence is prevented as much as possible (7). Conventionally, it would be expected that the amount of shear would in- crease with the applied energy and the emulsion particle size should then decrease with the increasing shear. The energy input required to produce an emulsion with a given EDS depends on the energy density of the emulsifying device. According to Canselier et al. (15), the Sauter mean diameter (d32) can be calculated by involving the power density (PV) and the residence time (tres) in the emulsifi cation zone: d32 = PV-b1 tres-b2 (2) If both exponents are quite similar, and if two parameters are treated as a single variable, this equation represents the concept of “energy density” (16). On the other hand, droplet disruption in rotor–stator systems is generally less effi cient than ultrasonic devices because, according to Stang et al. (17), the dispersing zones of rotor–stator systems usually have larger volumes. Consequently, at constant energy density and volume fl ow rate, the mean power density in rotor–stator systems is lower and the mean residence time is longer than in ultrasonic devices. So, operating forces in a rotor– stator device act longer than the minimum time needed for droplet breakup. Therefore, emulsions prepared by using ultrasonic cavitation produced smaller droplet size. ζ-POTENTIAL Figure 2 shows that the ζ-potential values of emulsions prepared by rotor–stator and ultrasonic cavitation were -33.5 mV and -37.8 mV, respectively. An absolute value, less Figure 1. A comparison of emulsion particle distributions obtained for 30% w/w palm oil esters-in-water emul- sions stabilized by 5% w/w mixed surfactants, Tween 80 and Span 80, which were prepared using rotor– stator homogenizer ( ) and ultrasonic cavitation ( ).

JOURNAL OF COSMETIC SCIENCE 338 than or greater than 25 mV is indicative of fl occulated and defl occulated emulsions, re- spectively (18). Therefore, no fl occulation was shown in both emulsions. The results in- dicated that the droplet stability in the emulsions prepared by ultrasonic cavitation was higher than the emulsions prepared by rotor–stator. This could be due to the increase in energy input during emulsifi cation. The higher the absolute value of zeta potential, the larger the electrostatic repulsive forces and the separation of the particles become easier. According to the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory, a system can be regarded as stable if the electrostatic repulsion dominates the attractive van der Waals force (19). The particles have to overcome an energy barrier of electrostatic repulsion to approach closely and form agglomerates. Smaller droplet size provides an enhanced ki- netic stability in the inner droplets to coalescence or agglomerate in addition to lowered interfacial tension. It was observed that the EDS prepared by ultrasonic cavitation were smaller than the EDS prepared by rotor–stator emulsifi cation. The very small droplet size caused a large reduction in the gravity force, and the Brownian motion may be suffi cient in overcoming gravity. Thus, no creaming or sedimentation could occur and the electro- static repulsion dominates the attractive van der Waals force. RHEOLOGICAL CHARACTERIZATION 1: VISCOSITY VERSUS SHEAR RATE PLOT Figure 3 shows a log/log plot on the effect of increasing energy input on the functional relationship between viscosity and shear rate for emulsions. A drastic decrease in the vis- cosity of these emulsions was observed. The viscosity of 109.69 Pa·s for emulsion prepared by rotor–stator emulsifi cation decreased to 70.79 Pa·s after sonication for 5 min. The interactions between the emulsion molecules will be weakened and, as a result, the vis- cosity would be decreased by the destruction of the network. It was clearly indicated that the differences in the changes in viscosity were not very large, but still signifi cant. The lin- ear portions of the plots are readily fi tted to the Power Law model shown in equation (3): η = kỳn - 1 (3) where η is the viscosity (Pa·s-1), ỳ is the strain rate (s-1), k is the consistency index of the emulsion, and n is the power (shear thinning) index (n 1). Decreasing the value of n increased the degree of shear thinning of the emulsion system. Figure 2. The zeta potential of emulsions measure for emulsion prepared by using rotor–stator homoge- nizer ( ) and ultrasonic cavitation ( ).