Manuscript received on 26 October 2015, in final form 23 September 2016, accepted 24 September 2016. Corresponding author: D.A. Mansour. DOI: 10.1109/TDEI.2016.005697 3364 D.-E. A. Mansour et al.: The Role of Interfacial Zone in Dielectric Properties of Transformer Oil-based Nanofluids