Monte Carlo Simulation of State of Dispersion of Nanoparticles in Aqueous Medium
Santosh Kumar (Indian Institute of Technology Bombay)

Monte Carlo simulation results predicting the state of dispersion (single, dimer, trimer and so on) of coated superparamagnetic iron oxide (Fe 3O4) nanoparticles in an aqueous medium are compared with our experimental data for the same. Measured values of volume percentage of particles in the dispersion, core particle diameter, coating-shell thickness, grafting density of coating agent, saturation magnetization and zeta potential for the citric acid and poly (acrylic) acid [PAA] coated particles have been used in our simulation. The simulation was performed by calculating the total interaction potential between two nanoparticles as a function of their inter-particle distance. The criterion applied for the aggregation of two particles is that the minimum depth of the secondary minima in the total interaction potential must be at least equal to kBT. Simulation results successfully predicted the experimentally observed aggregated state of citric acid coated particles and isolated state of individual PAA coated particles. We further investigated how this changes for both kind of coating agents by varying particle volume percentage, particle diameter, shell thickness and grafting density.
We find that the use of lower shell thickness and higher particle volume percentage leads to the formation of larger aggregates. Possible range of values of these four variables, which can be used experimentally to prepare a stable dispersion containing isolated particles, is recommended based on predictions from our simulation.
We have extended our model to study the dispersion of dextran coated Fe 3O4 nanoparticles in water which shows bridging flocculation, instead of steric stabilization.

Keywords: Magnetic nanoparticles, aggregation, Monte Carlo simulation, Fe3O4, citric acid, poly acrylic acid, dextran.