Process optimization and stability of D-limonene-in-water nanoemulsions prepared by ultrasonic emulsification using response surface methodology

D-limonene in water nanoemulsion was prepared by ultrasonic emulsification using mixed surfactants of sorbitane trioleate and polyoxyethylene (20) oleyl ether. Investigation using response surface methodology revealed that 10% D-limonene nanoemulsions formed at S-0 ratio (D-limonene concentration to mixed surfactant concentration) 0.6-0.7 and applied power 18 W for 120 s had droplet size below 100 nm. The zeta potential of the nanoemulsion was approximately -20 mV at original pH 6.4, closed to zero around pH 4.0, and around -30 mV at pH 12.0. The main destabilization mechanism of the systems is Ostwald ripening. The ripening rate at 25 degrees C (0.39 m(3) s(-1) x 10(29)) was lower than that at 4 degrees C (1.44 m(3) s(-1) x 10(29)), which was in agreement with the Lifshitz-Slezov-Wagner (LSW) theory. Despite of Ostwald ripening, the droplet size of D-limonene nanoemulsion remained stable after 8 weeks of storage. (C) 2011 Elsevier B.V. All rights reserved.