J. Cosmet. Sci., 63, 333–344 (September/October 2012) 333 Preparation of emulsions by rotor–stator homogenizer and ultrasonic cavitation for the cosmeceutical industry NG SOOK HAN, MAHIRAN BASRI, MOHD. BASYARUDDIN ABD. RAHMAN, RAJA NOOR ZALIHA RAJA ABD. RAHMAN, ABU BAKAR SALLEH, and ZAHARIAH ISMAIL, Faculty of Science (N.S.H., M.B., M.B.A.R.), Institute of Bioscience (M.B., R.N.Z.R.A.R., A.B.S.), Faculty of Biotechnology and Biomolecular Sciences (R.N.Z.R.A.R., A.B.S.), Universiti Putra Malaysia, 43400 UPM Serdang, and Sime Darby Research Sdn Bhd, Carey Island, 42960 Pulau Carey (Z.I.), Selangor, Malaysia. Accepted for publication March 14, 2012. Synopsis Oil-in-water (O/W) nanoemulsions play an important key role in transporting bioactive compounds into a range of cosmeceutical products to the skin. Small droplet sizes have an inherent stability against creaming, sedimentation, fl occulation, and coalescence. O/W emulsions varying in manufacturing process were pre- pared. The preparation and characterization of O/W nanoemulsions with average diameters of as low as 62.99 nm from palm oil esters were carried out. This was achieved using rotor–stator homogenizer and ultrasonic cav itation. Ultrasonic cell was utilized for the emulsifi cation of palm oil esters and water in the presence of mixed surfactants, Tween 80 and Span 80 emulsions with a mean droplet size of 62.99 nm and zeta potential value at -37.8 mV. Results were comparable with emulsions prepared with rotor–stator homogenizer oper- ated at 6000 rpm for 5 min. The stability of the emulsions was evaluated through rheology measurement properties. This included non-Newtonian viscosity, elastic modulus G′, and loss modulus G″. A highly stable emulsion was prepared using ultrasonic cavitation comprising a very small particle size with higher zeta potential value and G′ G″ demonstrating gel-like behavior. INTRODUCTION An emulsion system consists of two immiscible liquids dispersed in one another and is thermodynamically unstable (1). An emulsion can be kinetically stable (long-term stability) with the presence of surfactant in the system by creating an energy barrier to fl occulation and coalescence (2) and existing in a metastable state (3). Most emul- sions fall into one of the two different classes: oil-in-water (O/W) and water-in-oil (W/O). The two types of emulsions are readily distinguished in principle, depending upon which kind of liquid forms the continuous phase (4). In O/W, tiny oil droplets Address all correspondence to Mahiran Basri at mahiran@science.upm.edu.my. Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

JOURNAL OF COSMETIC SCIENCE 334 are dispersed onto continuous phase, water. In W/O, water droplets are dispersed onto oil. Palm oil esters were derived from palm oil and cis-9-octadecen-1-ol through enzymatic transesterifi cation process using Lipozyme RM IM as the catalyst (5). The excellent wetting behavior of the esters without the oily feel make them have great potential in the manu- facture of cosmeceutical products (6). The preparation of such emulsions with small drop- let size is thus of particular interest. Small droplet sizes in general lead to greater emulsion stability (7). There are a number of mechanisms available for the production of such emulsions. The emulsifi cation technique can be a form of mechanical mixing of two immiscible liquids where time is required for surfactant molecules to organize at the interface of the two phases. Higher mechanical energy than what is possible from simple mixing can be achieved by rotor–stator systems. These systems are widely used to emulsify liquids of medium to high viscosity (7). The rotor–stator assembly consists of a rotor housed concentrically inside the stator with two or more blades and a stator with either vertical or slanted slots. As the rotor rotates, it generates a lower pressure to draw the liquid in and out of the assembly, resulting in circulation and emulsifi cation (8). Ultrasound delivers even higher energy and is an alternative method of producing an emulsion. In ultrasound emulsifi cation, the energy input is provided through sonotrodes (sonicator probe) containing a piezoelectric quartz crystal that can expand and contract in response to alternating electrical voltage. Ultrasonic emulsifi cation is believed to occur through two mechanisms. First, the appli- cation of an acoustic fi eld produces interfacial waves that become unstable, eventually resulting in the eruption of the oil phase into the water medium in the form of droplets (9). Second, the application of low-frequency ultrasound causes acoustic cavitation, that is, the formation and subsequent collapse of microbubbles by the pressure fl uctuations of a simple sound wave. Each bubble collapse event causes extreme levels of highly localized turbulence. The turbulent microimplosions act as a very effective method of breaking up primary droplets of dispersed oil into droplets of submicron size (10). Work by Henglein and Gutierrez (11) indicated that at low-sonication amplitudes, the effect of the cavitation threshold was dominant and both the chemical yield and sonolu- minescence arising from an acoustic fi eld decreased with increasing pressure. Conversely, at higher amplitudes, the bubble collapse effects dominated and yields increased with increasing pressure. Similarly, Sauter et al. (12) found that low overpressures improve deagglomeration of nanoparticles, whereas higher overpressures had a negative effect. Our study investigates the effect of different emulsifi cation methods on the production of an O/W emulsion to produce an optimal droplet size for the production of the emulsion. The emulsions were also characterized using measurements of rheology properties, which were subsequently used to assess the physical stability of the emulsions, directly after preparation. MATERIALS AND METHODS MATERIALS Emulsions were prepared with 15.8 wt% of palm oil esters (produced in our laboratory), 5 wt% of mixed surfactants, and 3 wt% tocotrienol (Gold Tri. E 70, purchased from Golden Hope Bioganic Sdn. Bhd, Selangor, Malaysia). Magnesium ascorbyl phosphate was purchased