Electrolyte crystal growth mechanisms

Abstract

When very soluble electrolytes crystallize from aqueous solution they usually grow by a linear rate law - the rate being proportional to the difference between the concentration and the solubility. The rate is controlled by the transport of the ions in the bulk solution or by the adsorption process. Sparingly soluble electrolytes mostly grow by a parabolic or exponential rate law at small or moderate supersaturations, and may change to transport control at larger concentrations. Both with parabolic and with exponential rate laws the rates can be accounted for in terms of the classical theories of surface spirals caused by screw dislocation, and surface nucleation, respectively, when it is assumed that the rate-determining molecular mechanism is the integration of the cations into crystal lattice positions at kinks in the surface steps (whether caused by dislocations or surface nucleation). The rate of integration at a kink is of the order of one thousandth of the rate of dehydration of the cation. The factor 10 -3 is interpreted as due to the activation energy of diffusion, as an ion, in order to integrate, must dehydrate and make a diffusional jump at the same time. The theory is confirmed by application to 20 electrolytes following the parabolic rate law, and two electrolytes following the exponential law.