Phase behavior, microstructure transition, and antiradical activity of sucrose laurate/propylene glycol/the essential oil of Melaleuca alternifolia/water microemulsions

Abstract

Nonionic sucrose ester microemulsions composed of sucrose laurate (SL), propylene glycol (PG) and water were prepared with the essential oil of Melaleuca alternifolia, commonly known as tea tree oil (TTO), as oil phase to investigate the phase behavior, microstructure, and antiradical activity. The pseudo-ternary phase diagrams were constructed to elucidate the phase behavior of the microemulsion formations at different weight ratios of surfactant and cosurfactant (Sm = SL/PG) of 1:1, 2:1, and 3:1. The extension of the microemulsion zone was found to be strongly dependent on the Sm ratios. The single phase microemulsion domain, especially o/w microemulsion region increased when Sm ratio is increased from 1:1 to 3:1 and no liquid crystalline structure was observed for all formulations studied. Microstructural aspects were studied by electrical conductivity and pulsed gradient spin echo (PGSE) NMR measurements along water titration line L28 (Ro = 2:8). The results from these combined techniques were in good agreement in regard to the microstructure transition points. The microstructural inversion of w/o to bicontinuous microemulsions occurred at ∼30 wt.% water while the transition from bicontinuous to o/w structure occurred at ∼55 wt.% water. The physical stability on storage temperature and time was examined by dynamic light scattering after the centrifuge test and freeze-thaw cycles. The droplet size was kept almost the same without any phase separation, providing less temperature-sensitivity up to 70 °C and good stability for 3 months at room temperature. The chemical profile and radical scavenging activity of TTO in o/w microemulsions was evaluated by means of gas chromatography-mass spectroscopy (GC-MS) and 2,2′-diphenyl-1-picrylhydrazyl free radical (DPPH{radical dot}) scavenging method, respectively. The major abundant constituents of crude TTO, monoterpene alcohols (terpinen-4-ol (41.65%), α-terpineol (3.18%)) and hydrocarbons (γ-terpinene (22.95%), α-terpinene (10.16%)) were identified and the composition percentage of each constituent was calculated form the GC peak areas by normalization method. The DPPH{radical dot} scavenging activity of TTO microemulsion was lower than pure TTO because the SL surfactant may obstruct the interaction between the TTO and DPPH{radical dot}, reducing the number of effective collisions. Crown Copyright © 2009.