Microencapsulation of citronella oil for mosquito-repellent application: formulation and in vitro permeation studies

Citronella oil (CO) has been reported to possess a mosquito-repellent action. However, its application in topical preparations is limited due to its rapid volatility. The objective of this study was therefore to reduce the rate of evaporation of the oil via microencapsulation. Microcapsules (MCs) were prepared using gelatin simple coacervation method and sodium sulfate (20%) as a coacervating agent. The MCs were hardened with a cross-linking agent, formaldehyde (37%). The effects of three variables, stirring rate, oil loading and the amount of cross-linking agent, on encapsulation efficiency (EE, %) were studied. Response surface methodology was employed to optimize the EE (%), and a polynomial regression model equation was generated. The effect of the amount of cross-linker was insignificant on EE (%). The response surface plot constructed for the polynomial equation provided an optimum area. The MCs under the optimized conditions provided EE of 60%. The optimized MCs were observed to have a sustained in vitro release profile (70% of the content was released at the 10th hour of the study) with minimum initial burst effect. Topical formulations of the microencapsulated oil and non-microencapsulated oil were prepared with different bases, white petrolatum, wool wax alcohol, hydrophilic ointment (USP) and PEG ointment (USP). In vitro membrane permeation of CO from the ointments was evaluated in Franz diffusion cells using cellulose acetate membrane at 32 °C, with the receptor compartment containing a water-ethanol solution (50:50). The receptor phase samples were analyzed with GC/MS, using citronellal as a reference standard. The results showed that microencapsulation decreased membrane permeation of the CO by at least 50%. The amount of CO permeated was dependent on the type of ointment base used; PEG base exhibited the highest degree of release. Therefore, microencapsulation reduces membrane permeation of CO while maintaining a constant supply of the oil.