Dielectric Spectroscopic and Molecular Simulation Evidence for Aggregation of Surfactant-stabilized Calcium Carbonate Nanocolloids in Organic Media

Abstract

Dielectric spectroscopy carried out on overbased phenate micelle particles of nominal diameter ∼2 nm containing calcium carbonate cores in hydrophobic liquids indicates that the micelles are strongly aggregated. Mean cluster sizes in excess of ∼103individual micelles are typically found in toluene. The level of association is a little higher in dodecane which is chemically closer to engine oil, the usual solvent for these systems. The mean aggregate size increases dramatically with concentration above an effective solids volume fraction of ∼0.1 on treating the micelles as spheres. Aggregate size also depends on the level of overbasing, with lower levels of overbasing giving more micelles in the cluster. Molecular dynamics simulations of individual micelle particles reveal them to have large dipole moments originating mainly from the amorphous carbonate cores. Dipoles of magnitude ∼20D are typical for a range of different surfactant types used in the model. The magnitude of the dipole depends somewhat on the chemical composition of the stabilizing surfactant shell. Monte Carlo simulations of two phenate nanocolloids taking into account all atom and charge pair interactions show these particles to have a strong short-range coulombic attraction of typically −25kBTatT= 298 K in the favored “side-by-side” relative arrangement. This attraction could be responsible for the strong level of association inferred from the dielectric spectroscopy experiments.