In this work, we present the dielectric behavior of water with a novel flexible model, FAB/ ϵ , that improved three-site water models. Different concentrations of the ionic liquid 1-butyl-3-methylimidazolium bmim and bis(trifluoromethanesulfonyl)imide Tf 2 N with water have been investigated. The study was performed by molecular dynamics simulations using three water models, being two non-polarizable 3-site SPC/E and SPC/e and a flexible 3-site FAB/ ϵ model. Systematic thermodynamics and dynamical and dielectric properties were investigated, such as density, self-diffusion coefficient, heat of vaporization Δ H vap , and surface tension at 300 K and 1 bar. Extrapolating the experimental molar fraction of the mixtures, a pattern change for all properties was observed, evidencing the phase separation previously reported by experimental data. The results also show the dielectric effect in the system, which is important in ionic systems and fundamental for understanding why this ionic liquid behaves hydrophobic.