Dipolar magnetism and electrostatic repulsion of colloidal interacting nanoparticle system

Abstract

Thermally activated Brownian kinetics of magnetic nanoparticles in water potentially results in particle aggregation, owing to magnetic and electrostatic interparticle interactions. To determine the significance of those competing interactions in changing the equilibrium hydrodynamic particle volume, we examined the mean distance between polydispersed particles as an important factor for initiating a secondary particle clustering among individually dispersed particles or their primary cluster structures. We particularly characterized the magnetic properties of a superparamagnetic ferrofluid in different ionic environments and at different particle (mass) concentrations. Regarding the concentration of ionic components added to dense ferrofluid samples, the spectral relaxation shift and the reduction in magnetic susceptibility were attributed to different hydrodynamic polydispersities of aggregates. However, no substantial changes in complex susceptibility with increasing ion concentration in dilute ferrofluid samples were confirmed, which indicates relatively stable dipolar-interactions. Regarding the adjustment of particle concentration, we found that the changes in the effective relaxation time constant are proportionally associated with the magnetopotential energy of a given oscillatory magnetic field.