Understanding the mechanisms behind the interaction between the nanoparticles (NPs) and the brine/seawater is critical for successful oil recovery. In this study, the effect of ionic components of brine, including Na+, Ca2+, Mg2+, Cl-, and SO42-, at different concentrations of silica NPs on the viscosity of the fluid is investigated for the first time. A comprehensive experimental study on the mechanisms behind the variation in the viscosity of silica nanofluids is presented. The results show that the cations present in the electrolytes increase the viscosity of silica nanofluids in the order of Na > Mg > K > Ca while the electrolytes with SO42- ion have a higher viscosity than those with Cl- ion. The interaction between the ions of silica NPs and electrolytes is classified into three different regions. In region I, the measured energy-dispersive X-ray of the transmission electron microscopy image shows that an interaction between the ions of silica NPs and the electrolyte anions in the electrical double layer results in the formation of a complex compound. In regions II and III, the electrolyte forms a protective layer on the silica NPs to isolate them from one another. The average size of particles in the nanofluid at a fixed concentration of electrolytes in regions II and III decreases at concentrations of silica NPs beyond the peak value, but only region I is observed at a constant concentration of silica NPs and varying electrolyte concentrations. These findings can improve the design and implementation of NPs for an efficient oil recovery.