PRECIPITATION MECHANISM OF POLYMORPHOUS CLATHRATE CRYSTALS OF NI-COMPLEX AT HIGH-CONCENTRATIONS OF 1-METHYLNAPHTHALENE

In methylcellosolve solutions containing Ni(C6H7N)4(SCN)2 (Ni-complex) and 1-methylnaphthalene (1-MN), the polymorphous clathrate crystals of γ and γ' (monoclinic) precipitate at concentrations of 1-MN higher than 0.4 mol/1. The γ' crystal clathrates only a 1-MN molecules with the molar ratio of 2.0 to Ni-complex. The precipitation fields of γ and γ' are separated into three parts of 1-MN concentration, connected with the three parts of the β and γ system in the lower 1-MN concentration. Solubility curves of Ni-complex host forming γ and γ' structures cross at the 1-MN concentration of 0.624 mol/1, which is the transition point with respect to the 1-MN concentration. It was cleared that the relative amounts of the polymorphs in the precipitates are determined by their nucleation process and are hardly influenced by the growth process. Their relative nucleation rates are controlled by their relative supersaturation ratios and both polymorphs can nucleate competitively at a very narrow range (0.97-1.05). The solution- mediated transformation from γ to γ', which accompanies the exchange of the guest molecules, was observed at 1-MN concentrations higher than the transition point, and the reverse solution-mediated transformation from γ' to γ was observed at lower 1-MN concentrations. © 1990.