Microencapsulation of Thyme Oil by Coacervation: Production, Characterization and Release Evaluation

• Main Author: Martins, Isabel Maria Duque
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2012
• Subjects: Chemical engineering
• ISBN: 9781073953844; 107395384X

In this work polylactide (PLA) microcapsules have been produced by coacervation having in view the encapsulation of Thymus vulgaris L. (thyme oil), an antioxidant and antimicrobial active agent. Biodegradable microcapsules of PLA have received extensive attention as drug delivery systems since they can be hydrolysed in the body, and its degradation products easily resorbed or eliminated. The core material, thyme oil, is extracted from an aromatic and medicinal plant of increasing economic importance in North America, Europe and North Africa.The novelty of the developed process consists on dissolving PLA in dimethylformamide (DMF) which is a good solvent for PLA but in addition has high solubility in water. Upon contact with water the PLA dissolved in the DMF solution precipitate covering the oily droplets. As so, an easy and executable method of coacervation was put in practice allowing the encapsulation of an oily active principle by simply preparing an o/w emulsion.Several nonionic surfactants with different hydrophilic-lipophilic balance (HLB) values were evaluated focusing the encapsulation efficiency of polar and apolar compounds of oil. Thus, Tween 20, Tween 80, Tergitol™ 15-5-9 and a combination of Tergitol™ 15-5-9 with Span® 85 have been used covering the range between 11 and 16.5. For all the studied cases, microcapsules have shown a spherical shape and the obtained particle size distribution in volume was bimodal, with a mean size comprised between 30 and 40 μm. The amount of encapsulated thyme oil reaches a maximum of 65% when using Tergitol™ 15-5-9, a polyglycol ether surfactant with a HLB value of 13.3. The study confirmed the encapsulation efficiency dependence on the surfactant HLB, putting also in evidence a preferential encapsulation of apolar compounds of thyme oil in detriment of polar ones.The release behaviour of the thyme oil itself and of its individual components, through the PLA microcapsules wall, was evaluated by using the microcapsules solution during the first day period after production and using GC-FID to discriminate and quantify individual components. The developed diffusion model was applied to single-layer microcapsule systems resulting that the release of the polar compounds of thyme oil was faster than the apolar ones. The diffusion coefficient in first hour of release was 1.39x105 m2/s for thymol and 5.21x10" m³/s for p-cymene. However, the diffusion was slower if considering a 5 days period thus obtaining diffusion coefficients of 3.81x10 m²/s for thymol and 1.43x10m²/s for p-cymene.Complementary studies considering the production and characterization of vanillin, thymol and p-cymene, used as model core materials, have been also performed. The obtained microcapsules presented similar morphology as the thyme oil ones, i.e., spherical shape, but with a somewhat smaller mean particle size (21 μm for vanillin, 25 μm for thymol and 37 µm for p-cymene). The vanillin release has been monitored along with time, but no amount was detected in the outside solution of microcapsules pointed out that the vanillin stayed entrapped in the produced microcapsules. However, the results show that the release of thymol and p-cymene is faster in the first hour keeping almost constant in next days. The diffusion coefficient in first hour of release was 1.99x10 m³/s for thymol and 4.34x10 m³/s for p-cymene. However, the diffusion is slower for a period of 5 days with the diffusion coefficients of 3.34x10 m2/s for thymol and 3.45x10¹ m³/s for p-cymene. The release rate for thymol was slower when used as model core material, since it was observed that only 40% of the encapsulated oil was released during the first day.