JOURNAL OF COSMETIC SCIENCE 248 RESULTS AND DISCUSSION THE EFFECT OF ULTRASONIC APPLIED POWER In this study, nanoemulsion droplets in D -limonene in water system were prepared by two steps. The fi rst step was to prepare a coarse emulsion with droplet size around 20 μm. Then, ultrasonic process was used to further decrease the droplet size. There are two main mechanisms of emulsion droplet formation. First, droplet disruption is controlled by the type and amount of shear force applied to droplets as well as the droplet resistance to deformation, which is determined by the surfactant (9,10). The other is droplet coales- cence. The rate of droplet coalescence is related to the droplet stability of emulsion, which is determined by the ability of the mixed surfactant to rapidly adsorb onto the surface of newly formed droplets. This pathway is governed by surfactant surface activity and con- centration. In our study, ultrasonic emulsifi cation was used as the source of shear force and the results of applied power on droplet size of emulsion are included in Figure 1. The results showed that the droplet size reached a minimum size at applied power of 18 W. A similar trend between emulsion droplet size and applied power has been observed by oth- ers for emulsions made with fl axseed oil and Tween 40 (10). Increasing the applied power of ultrasound can provide more shear force to decrease the size of emulsion droplet. THE EFFECT OF ULTRASONIC TIME The nanoemulsion droplet size under different ultrasonic time are shown in Figure 2. The result showed that the droplet size were around 200 nm during ultrasonic time of 120 s, but increase in the ultrasonic time would not obviously change in emulsion droplet size. Ultrasonic time was related to the thermodynamic equilibrium of emulsion system. Ul- trasonic time affects the rate of adsorption of surfactants to the droplet surface and the droplet size distribution of newly formed droplet. Figure 1. The effect of ultrasonic applied power on the emulsion droplet size of D -limonene in water emul- sion system (So ratio= 0.4 ultrasonic time= 120 s).

NANOEMULSION OF D-LIMONENE IN WATER SYSTEM 249 THE EFFECT OF SO RATIO In this study, the mixed surfactant (sorbitane trioleate and polyoxyethylene (10) oleyl ether) was used as the surfactant and the effect of So ratio can be seen in Figure 3. At low So ratio, there was less mixed surfactant to absorb onto the newly formed droplet, and then increasing the So ratio results in the decrease of droplet size (from So ratio 0.2 to 0.4). However, excess mixed surfactant would not decrease the droplet size of emulsion and will interfere in the stability of emulsion. The droplet size of emulsion below 50 nm was obtained with the So ratio between 0.4 and 0.6, beyond that the increase in the So ratio Figure 2. The effect of ultrasonic time on the emulsion droplet size of D -limonene in water emulsion system (So ratio= 0.4 applied power = 18 W). Figure 3. The effect of So ratio on the emulsion droplet size of D -limonene in water emulsion system (applied power = 18 W ultrasonic time = 120 s).