Dielectric spectroscopic and molecular simulation evidence for aggregationof surfactant-stabilized calcium carbonate nanocolloids in organic media

Dielectric spectroscopy carried out on overbased phenate micelle particles of nominal diameter similar to 2 mm containing calcium carbonate cores in h ydrophobic liquids indicates that the micelles are strongly aggregated. Mea n cluster sizes in excess of similar to 10(3) individual micelles are typic ally found in toluene, The level of association is a little higher in dodec ane which is chemically closer to engine oil, the usual solvent for these s ystems. The mean aggregate size increases dramatically with concentration a bove an effective solids volume fraction of similar to 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, Mole cular dynamics simulations of individual micelle particles reveal them to h ave large dipole moments originating mainly from the amorphous carbonate co res. Dipoles of magnitude similar to 20D are typical for a range of differe nt surfactant types used in the model. The magnitude of the dipole depends somewhat on the chemical composition of the stabilizing surfactant shell. M onte Carlo simulations of two phenate nanocolloids taking into account all atom and charge pair interactions show these particles to have a strong sho rt-range coulombic attraction of typically -25k(B)T at T = 298 K in the fav ored "side-by-side" relative arrangement. This attraction could be responsi ble for the strong level of association inferred from the dielectric spectr oscopy experiments. (C) 1999 Academic Press.