THERMODYNAMIC AND KINETIC EFFECT OF ORGANIC SOLVENT ON THE NUCLEATION OF NESQUEHONITE

Nesquehonite crystallization experiments are conducted in H₂O and binary solvent of H₂O-dimethylformamide (DMF) to examine solvent effect on the mineral’s nucleation process. Results show a reduction in the induction time of crystallization upon the introduction of DMF and an inverse relation between the nucleation rate and the concentration of DMF. Data analysis in the context of the classic nucleation theory reveals an unexpected increase in the surface energy of nesquehonite accompanying the solvent change. Whereas the energy increase may be understood in light of the poorer salvation of surface species associated with the presence of aprotic DMF, it implies that kinetic controls, rather than surface energetics, is the driving force behind the resultant faster nucleation. Further analysis using the Smoluchowski’s coagulation theory indicates that the rate of formation of stable clusters in the crystallization system is independent of the sizes of the precursors but increases linearly with DMF concentration, suggesting nesquehonite nucleation may be controlled by a cluster aggregation mechanism. We propose that increased surface energy leads to the formation of more sub-critical molecular clusters. The low stability (hence high reactivity) of these clusters ensues a rapid coagulation amongst them and ultimately an elevated nucleation rate.