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).

JOURNAL OF COSMETIC SCIENCE 250 did not obviously decrease the droplet size of emulsion. These results agree with other studies that discuss the relationship between surfactant concentration and droplet size of emulsion (12,15). THE EFFECT OF HLB VALUES The emulsion was prepared by using the mixed surfactant of sorbitane trioleate and poly- oxyethylene (10) oleyl ether. The So ratios were adjusted to satisfy the proper HLB values for optimum emulsifi cation condition. Emulsions with 10.0 wt% D -limonene and So ra- tio at 0.4 were prepared at different HLB values. The relationship between the droplet size of emulsions and the HLB values is shown in Table I. The results showed that droplet size of emulsions can be affected by the HLB values. The droplet size of emulsion was 332 nm at the HLB value of 2 and with an increase in the HLB value to 12, the droplet size decreased to 23 nm. The proper HLB values of mixed surfactants were key factor for the formation of emulsion droplets. During the formation of O/W emulsion, the lipophilic surfactants have more affi nity to dispersed droplet of emulsion than the hydrophilic sur- factant. A proper HLB value is needed to maintain the oil phase and water phase equilib- rium, and with optimum HLB values, it can stabilize and narrow down the newly formed droplets during the emulsifi cation. The appearance of emulsion is milk-like at HLB val- ues of 2–10, and transparent or translucent to the naked eye at HLB value of 12. TEM OBSERVATION To observe the physical properties of the nanostructured droplet in the nanoemulsion system, TEM analysis was carried out with negatively stained samples. The results are shown in Figure 4, where phosphotungstic acid–stained D -limonene droplets are clearly Table I The Effect of HLB Values on the Emulsion Droplet Size and Appearance of D -Limonene in Water Emulsion System No. HLB values Droplet size (nm) Appearance 1 2 332 ± 27.3 Milk-like 2 3 335 ± 22.12 Milk-like 3 4 287 ± 14.21 Milk-like 4 5 226 ± 13.52 Milk-like 5 6 207 ± 7.33 Milk-like 6 7 175 ± 2.58 Milk-like 7 8 149 ± 3.14 Milk-like 8 9 102 ± 2.11 Milk-like 9 10 73 ± 1.29 Milk-like 10 11 54 ± 0.67 Milk-like 11 12 23 ± 0.86 Translucent So ratio = 0.4 applied power = 18 W ultrasonic time = 120 s.