Crystal Formation (Nucleation) in the Presence of Growth Restrainers
Ingo H. Leubner, Journal of Crystal Growth, 84(3): 496-502 (1987)
Growth restrainers provide the most potent mechanism to reduce the crystal size. The concepts are the basis of all nano-particle precipitations in solution; however, they are not recognized as described in this paper.
When certain compounds (restrainers) are present during nucleation in precipitations, more, and thus smaller, crystals are obtained than in their absence. Experimental evidence shows that these compounds must adsorb to the crystal surface during nucleation to show this effect. Based on the model of crystal nucleation for kinetically controlled (i.e., surface integration controlled) crystal growth, a quantitative model of the restrainer effect during nucleation was developed. It was postulated that the restrainer effect is proportional to the surface coverage by the restrainer, which proportionally inhibits the surface integration of new material on the crystal. For convenience, the Langmuir isotherm model was used for adsorption, but is not restricted to this adsorption model. The resultant model predicts that the number of stable crystals should increase in proportion to the initial restrainer concentration. This was experimentally confirmed by the effect of 1–(3-acetamidophenyl) mercaptotetrazole on the nucleation of AgCl. The present extension to the prediction of the restrainer effects supports the validity of the previously published crystal nucleation model. Further, this model should be generally applicable to describe the effect of growth restrainers on the nucleation of crystals.
The number of crystals increased from 0.035E+15 to 10.7E+15, while the size decreased from 0.734 to 0.135 µm for concentrations between zero and 500 mg/liter. At concentrations above 30 mg/liter, a significant change in morphology to multifaceted crystals was observed. This is in agreement with a change of growth rate and mechanism at the crystal surface due to adsorbed restrainer.