Efficient Encapsulation and Controlled Release of N,N-Diethyl-3-methylbenzamide (DEET) from Oil-in-Water Emulsions Stabilized by Cationic Nanocellulose and Silica Nanoparticles

Oil-in-water emulsions containing N,N-diethyl-3-methylbenzamide (DEET) were developed aiming to extend the sustained release of the active compound, by using two oppositely charged nanomaterials, namely silica nanoparticles (SiNP) and cationic cellulose nanofibrils (CCNF), as stabilizers, and a mixture of food-grade nonionic surfactants to avoid precipitation by electrostatic aggregation. The formulations were stable for more than four months at room temperature, and strongly resistant to destabilization upon centrifugal and thermal stress. The results were correlated with the effect of SiNP on strengthening the network formation of CCNF in the aqueous phase by electrostatic interactions, which increased the viscosity of the external phase and, hence, emulsion stability. There was a significant size reduction of the internal oil phase containing DEET in the presence of CCNF, which was attributed to the increased viscosity in the external aqueous phase, as well as to interfacial stabilization. The combined action of CCNF and unmodified SiNP in the stabilization of the DEET-containing oil phase significantly decreased the release rate of the active compound, compared to non-emulsified DEET. Moreover, in the emulsions containing the CCNF/SiNP mixture there was a more sustained release for the period of 6 h, demonstrating the potential of these formulations for extended protection.