The present paper reports on an investigation of the effect of the interface layers in enhancing thermal conductivity of Cu-Ar nanofluids. The approach is based on linear response theory combined with equilibrium molecular dynamics simulations. For a wettability parameter of 1.4 and volume fraction of 5.8%, simulation results show enhancements in thermal conductivity as high as 50%. Among others, the most salient result concerns the contribution of the vibration modes of liquid Ar atoms around Cu nanoparticles (NPs) in enhancing thermal conductivity of the nanofluid. Our findings reveal that these vibration modes coincide on a large domain of frequencies (10-50ps^{-1}) with those of Cu atoms of the NPs. The enhancement of the thermal conductivity was explained by the increase of vibrational mean-free paths.