Nanofluids have become of great interest due to the excellent properties and reduced size of nanoparticles making them useful for different applications. The agglomeration of nanoparticles depends on the interparticle interactions. To determine the degree of agglomeration of particles in silica nanofluids the rheological and viscoelastic behavior was used. Nanofluids at different volume fractions (phi=0.002-0.132) and pH values (2,7 and 10) were prepared. Final packing of particles at each pH condition was obtained from the modeling of viscosity data to the Quemada equation using effective volume fractions for the first time. Similar equations were proposed by the authors to model the elastic and viscous moduli and the same packing fractions were obtained. The densest packing is achieved at the isoelectric point where particles are uncharged (phi(m)=0.33). Higher surface charges generate looser packing. Packings are always lower than for hard spheres due to the cohesive forces. (C) 2012 Elsevier Ltd. All rights reserved.