OIL-IN-WATER NANOEMULSIONS 343 zeta potential of -37.8 mV after 5 min of sonication. Physically stable O/W emulsions were prepared using ultrasonic cavitation and rotor–stator homogenizer, where G′ G″ and the emulsion displays gel-like properties. Under appropriate conditions, the emul- sions develop an elastic network that provides the emulsions with good long-term stabil- ity to coalescence. ACKNOWLEDGMENT The authors thank Universiti Putra Malaysia for the fi nancial support given for this project. REFERENCES (1) P. Somasundaran, T. H. Wines, S. C. Metha, N. Garti, and R. Farinato, Emulsions and Their Behavior in Surfactants in Personal Care Products and Decorative Cosmetics (CRC Press, Boca Raton, New York, 2007), p. 504. (2) D. L. Marshall and L. B. Bullerman, Antimicrobial Properties of Sucrose Fatty Acid Esters in Carbohydrate Polyesters as Fat Substitutes (Marcel Dekker, New York, 1994). (3) D. J. McClements, General Characteristics of Food Emulsions in Food Emulsions: Principles, Practice and Tech- niques (CRC Press, Boca Raton, New York, 1999), p. 2. (4) L. L. Schramm, In Emulsions, Foams, Suspension: Fundamental and Application (Wiley VCH Verlag GmbH & Co, Germany, 2006), pp. 1–260. Figure 9. Time-dependent apparent viscosity at 25°C ( ), 30°C ( ), 35°C ( ), 40°C ( ), 45°C ( ), and 50°C ( ) for an emulsion system prepared by rotor–stator homogenizer. Figure 10. Time-dependent apparent viscosity at 25°C ( ), 30°C ( ), 35°C ( ), 40°C ( ), 45°C ( ), and 50°C ( ) for an emulsion system prepared by ultrasonic cavitation.

JOURNAL OF COSMETIC SCIENCE 344 (5) P. S. Keng, M. Basri, M. B. Abdul Rahman, M. R. S. Zakaria, A. B. Ariff, R. N. Z. Abdul Rahman, and M. B. Salleh, Newly synthesized palm esters for cosmetic industry, J. Ind. Crops Prod., 29, 37–44 (2008). (6) E. R. Gunawan, M. Basri, M. B. Abdul Rahman, A. B. Salleh, and R. N. Z. Abdul Rahman, Study on response surface methodology (RSM) of lipase-catalyzed synthesis of palm-based wax esters, Enzyme Microb. Technol., 37, 739–744 (2005). (7) D. J. McClements, Food Emulsions: Principles, Practices and Technique, 2nd Ed. (CRC Press, Boca Raton, New York, 2005). (8) Y. F. Maa and C. C. Hsu, Liquid-liquid emulsifi cation by rotor/stator homogenization, J. Controlled Release, 38, 219–228 (1996). (9) M. K. Li and H. S. Fogler, Acoustic emulsifi cation. Part 1. The instability of the oil-water interface to form the initial droplets, J. Fluid Mech., 88 (3), 499–511 (1978). (10) M. K. Li and H. S. Fogler, Acoustic emulsifi cation. Part 2. Break-up of the larger primary oil droplets in a water medium, J. Fluid Mech., 88 (3), 513–528 (1978). (11) A. Henglein and M. Gutierrez, Sonochemistry and sonoluminescence: Effects of external pressure, J. Phys. Chem., 97, 158–162 (1993). (12) C. Sauter, M. A. Emin, H. P. Schuchmann, and S. Tavman, Infl uence of hydrostatic pressure and sound amplitude on the ultrasound induced dispersion and de-agglomeration of nano-particles, Ultrason. Sonochem., 15, 517–523 (2008). (13) H. Mirhosseini, C. P. Tan, N. S. A. Hamid, and S. Yusof, Effect of Arabic gum, xanthan gum and orange oil contents on zeta potential, conductivity, stability, size index and pH of orange beverage emulsion, Colloids Surf., A, 315, 47–56 (2008). (14) T. Tadros, P. Izquierdo, J. Esquena, and C. Solans, Formation and stability of nano-emulsions, Adv. Colloid Interface Sci., 108–109, 303–318 (2004). (15) J. R. Canselier, H. Delmas, A. M. Wilhelm, and B. Abismail, Ultrasound emulsifi cation—An overview, J. Dispersion Sci. Technol., 23, 333–349 (2002). (16) B. Abismail, J. P. Canseliar, A. M. Wilhelm, H. Delmas, and C. Gourdon, Emulsifi cation processes: On-line study by multiple light scattering measurements, Ultrason. Sonochem., 7, 187–192 (2000). (17) M. Stang, H. Schuchmann, and H. Schubert, Emulsifi cation in high-pressure homogenizers, Eng. Life Sci., 1, 151–157 (2001). (18) H. A. Leiberman, M. M. Reiger, and G. S. Banker, Pharmaceutical Dosage Forms: Disperse Systems (Marcel Dekker, New York, 1989), Vol. 2. (19) A. Grabbe and R. G. Horn, Double-layer and hydration forces measured between silica sheets subjected to various surface treatments, J. Colloid Interface Sci., 157, 375–383 (1993). (20) E. Dickinson, An Introduction of Food Colloids (Oxford University Press, Oxford, 1992). (21) J. H. Prentice, Measurements in the Rheology of Foodstuffs (Elsevier Applied Science Publishers, London, 1984). (22) R. Pal, Rheology of Emulsions Containing Polymeric Liquids: Encyclopedia of Emulsion Technology (Marcel Dekker, New York, 1996). (23) B. B. Niraula, N. S. Tiong, and M. Misran, Vesicles in fatty acid salt—fatty acid stabilized o/w emulsion—emulsion structure and rheology, Colloids Surf., A, 236, 7–22 (2004).