J. Cosmet. Sci., 65, 245–252 (July/August 2014) 245 Nanoemulsion of D -limonene in water system prepared by ultrasonic emulsifi cation WEN-CHIEN LU, TING-JIE ZHANG, DA-WEI HUANG, and PO-HSIEN LI, Department of Medicinal Botanical and Health Applications, Da-Yeh University, Dacun, Changhua 51591 (W.-C.L., T.-J.Z., P.-H.L.), and Department of Food & Beverage Management, China University of Science and Technology, Nangang District, Taipei City 115 (D.-W.H.), Taiwan, Republic of China. Accepted for publication May 23, 2014. Synopsis D -Limonene is a component of essential oil extracted from citrus fruits. This component has shown chemo- preventive and therapeutic activity against a wide variety of experimental tumors, but D -limonene is unstable and lose its lemon-like fl avor under normal storage condition, and it is almost insoluble in water. Therefore, studying the formation of nanoemulsion in D -limonene in water system is probably a good method to prevent the oxidation degradation of D -limonene. For the purpose of our study, we used mixed surfactant to form D - limonene-in-water emulsion, and found the best formula for forming nanoemulsion droplets with specifi ed hydrophilic–lipophilic balance (HLB) value and droplet size. The results demonstrated that nanoemulsion droplets formed at So ratio of 0.4 and applied power of 18 W for 120 s under mixed surfactant at HLB values 12 and had droplet size of 20–50 nm. INTRODUCTION Nanoemulsions are emulsions with droplet size between 20 and 100 nm (1). Because of the small droplet size, nanoemulsions appear transparent or translucent and possess more stability against sedimentation, coalescence, fl occulation, and Ostwald ripening com- pared with convention emulsions (2,3). The physicochemical properties of nanoemulsions are interesting for practical applications because of the small droplet size and longtime stability. Nanoemulsions are used in agrochemicals for pesticide delivery (4), in cosmetics as a vehicle for personal care or skincare products (5), in fragrance as a matrix to encapsu- late the volatile compounds and controlled release which are desirable to be formulated alcohol free (6), and in beverages, to give the products an opaque appearance and suitable aroma (7). Address all correspondence to Po-Hsien Li at pohsien@mail.dyu.edu.tw.

JOURNAL OF COSMETIC SCIENCE 246 Using ultrasonic emulsifi cation to prepare nanoemulsions is a recent development phenomenon (8). Ultrasonic emulsifi cation was able to produce nanostructured drop- lets in emulsion with the advantage of less occurrence of “over processing” (9). For- mation of nanostructured droplets are controlled by the interaction between droplet disruption and droplet coalescence, the ultrasound applied excellent shear force to droplet breakup, and the rate of droplet coalescence is determined by the mixed sur- factant concentration (10). In the desired droplet size, ultrasonic emulsifi cation can reduce the surfactant concentration and energy consumption, and the emulsions were more stable compared with homogenizer or other mechanical devices (8). There are two main mechanisms during ultrasonic emulsifi cation (11). First, an acoustic fi eld produces interfacial waves to break the disperse phase into the continuous phase. Sec- ond, the formation of acoustic cavitation is used to collapse microbubbles into drop- lets of nanometric size by pressure fl uctuations. The best nanoemulsion droplets in emulsion were prepared at optimum hydrophilic– lipophilic balance (HLB) value and optimum surfactant level (12). The proper HLB values of the surfactants are important parameters for the formation of emulsion. Nanostructured droplets in emulsion are usually formulated to enhance the stability by using a mixed surfactant because of the broad chain length distribution. Paraffi n oil in water nanoemulsions have been obtained by adjusting the HLB values of the mixed surfactants Tween 80/Span 80 (2). Isohexadecane O/W nanoemulsions have been obtained in water/C12E4:C12E6/isohexadecane system at 4 and 8 wt% mixed surfactant concentration (13). Nanoemulsions containing the antioxidant astaxanthin prepared with mixed surfactant had smaller droplet size and a narrow size distribu- tion (14). The main objectives of this study were to gain a better understanding of the infl uence of mixed surfactant on the nanoemulsion droplet size by using ultrasonic emulsifi cation and also to investigate the optimum formulation for preparing D -limonene in water nano- emulsions. MATERIALS AND METHODS MATERIALS D -Limonene (RI = 1.487) was a product of Merck (Darmstadt, Germany) and used as received. Reagent grade sorbitane trioleate and polyoxyethyle ne (10) oleyl ether with an average HLB of 1.8 and 12.0 were supplied by Sigma-Aldrich (St. Louis, MO). Ethylene glycol used as a cosurfactant was obtained from Merck water was deionized and Milli-Q fi ltered. COARSE EMULSION PREPARATION Emulsions consisted of D -limonene, mixed surfactant, deionized water, and cosurfactant. All emulsions were prepared in two stages. The coarse emulsion was obtained by using Polytron (PT-MR 3000, kinematica AG, Littau, Switzerland), and then further emulsifi ed