Nucleation and crystal proliferation kinetics: Amorphous‐crystalline transformation of basic copper carbonates

Abstract

The transformation of amorphous basic copper carbonate precipitates into the crystalline form in aqueous solutions was experimentally studied. The unique blue-to-green color change and drastic volume reduction of the precipitates in transition made continuous observation and quantitative determination of the transformation process feasible. The transformation, which involves both nucleation and crystal growth proliferation, was autocatalytic. Experimental results are in good agreement with the rate equation formulated on the two-stage kinetics model postulated for the system. Also studied were the effects of temperature and precipitate concentration on the rate of transformation. In the temperature range between 34" and 39"C., the apparent nucleation activation energy was 19.8 kcal./g.-mole.The art and science of crystallization are of interest to a wide range of physical and biological scientists and technologists (1, 6, 11, 16, 20, 30). The vast diversity of interest and objectives calls for the study of mechanism and techniques, such as to grow large perfect crystals, to grow crystals of uniform sizes, and to prevent the formation of crystals. The fundamental forces involved in all types of crystallization, whether in polymers, in organic and inorganic salts, or in metals, are believed to be essentially identical, but various crystallizing systems may show totally different behavior in the course of crystallization under different physical situations.A great amount of work has been done on the various aspects of crystallization with crystallizing systems as diverse as insulin and synthetic zeolites. The effects of physical factors, such as degree of supersaturation, temperature, and cooling rate, on the growth rate of crystals (7, 13, 18, 25), size distribution (4, 15, 17, 29), and crystal morphology (2, 26) have been extensively investigated.It is well established that there are two processes, nucleation and crystal growth, which are instrumental in the crystallization process. Because the elementary kinetic steps are little known and are specific for different systems, only empirical kinetic treatment of individual systems has been advanced by various workers ( 1 0,17,21,23,24,27).The system reported here is a complex inorganic precipitate, basic copper carbonate, formed in an aqueous medium. The stoichiometric relation of the reaction under controlled conditions conforms to the following equations 2NazC03 + 2CuS04 + H 2 0 = CUCO~.CU(OH)Z ( 9 , l Q ) :The initial precipitates formed by mixing aqueous sodium carbonate and cupric sulfate solutions were gelatinous and noncrystalline, but were able to transform themselves into crystalline particles of pure malachite on aging ( 9 , 2 8 ) .The purpose of the present work is to study the kinetics AlChE Journal (Vol. 17, NO. 6) of nucleation and crystal growth inherent in such transformation. Incidentally, this crystallizing system is particularly desirable in that the transformation process can be visually tracked. THEORYCrystal growth can be considered to consist ...

Synthesis of Mordenite Type Zeolites

“…The activation energy for the nucleation and crystal growth are determined from the crystallization curves. Assuming that the formation of nuclei of a stable size (which do not redissolve but grow into a crystal) is an energetically activated process and since the nucleation during the induction period is the rate-determining process, the apparent activation energy for the nucleation, En, can be calculated from the expression d In (1/0) En d(l/T) R where 0 is the induction time, i.e., the point on the crystallization curve where conversion to the crystalline phase is just starting (Hsu, 1971), T is the temperature, and R is the gas constant.…”

Synthesis of mordenite type zeolite using silica from rice husk ash

“…The activation energies for the nucleation and crystal growth are determined from the crystallization curves. Assuming that the formation of nuclei of a stable size (which do not redissolve but grow into a crystal) is an energetically activated process and since the nucleation during the induction period is the rate-determining process, the apparent activation energy for the nucleation, Ea, can be calculated from the expression d In (1/0) Ea d(l/T) ~~R where 0 is the induction time, i.e., the point on the crystallization curve where conversion to the crystalline phase is just starting (Hsu, 1971), T is the temperature, and R is the gas constant.…”