Heterogeneous Nucleation of Alkali Halide Crystals from Solution

Abstract

The epitaxial nucleation of KCl, KBr, KI, NH4I, RbCl, and RbBr from solution onto mica substrates at various temperatures has been examined. Critical supersaturations, produced by cooling the substrate thermoelectrically, were found to be sensitive to lattice matching and saturation temperature. The critical supersaturation decreased with increase in saturation temperature and increased with decrease in lattice match between substrate and deposit. Experimental results conform semiquantitatively to a modified two-dimensional nucleation equation. Apart from the lattice matching and saturation temperature, adsorption energy, surface diffusion and the lattice energy of the crystal deposit are believed to play an important part in heterogeneous nucleation. Hence surface-active sites, where ions are bound strongly, favor nucleation. Materials of high lattice energy are not readily nucleated even if matching with the substrate is perfect.