(447b) Crystallization Inhibition with Additives

Experiments have been performed to measure the effect of additives on the crystallization temperature of concentrated LiBr solutions used in absorption heat pumps. The measured crystallization temperatures do not correspond to the temperatures of equilibrium solubility but to the critical temperature for heterogeneous nucleation of the hydrated LiBr salt on the glass wall of the test tube containing the sample solution. These temperatures are a few degrees centigrade below the equilibrium solubility. The influences of various additives at a concentration from 250 to 1500 ppm on the crystallization temperature have been measured. A select few soluble additives further decrease the experimental crystallization temperature by up to 22C below equilibrium solubility. Large decreases in the crystallization temperature can be correlated with large values of complexation constants of the additive for either the Li+ or the Br- ion in solution. Solution complexation, however, is not sufficient to explain the magnitude of the decrease in the crystallization temperature. The only phenomenon capable of quantitatively explaining the magnitude of the decrease in the crystallization temperature is the change in the crystal/solution interfacial energy due to adsorption of the additive on the surface of the pre-nucleation embryos. A quantitative model of the crystal/solution interfacial energy due to adsorption has been developed using both the Langmuir adsorption isotherm and Gibbs adsorption equation. This model with classical nucleation theory allows the quantitative prediction of the crystallization temperature with and without additives at various concentrations. This model has also been successful in predicting the effects of scale inhibitors on the formation BaSO4 and CaSO4 scales. There are significant conformational aspects of successful inhibitor molecules that are specific to the scale surface they are to inhibit. These aspects are clearly elucidated by molecular modeling allowing successful inhibitors to be identified from molecular conformations after docking at a crystal surface.