The Influence of Additives and Impurities on Crystallization

Christiane Schmidt, Matthew J. Jones, and Joachim Uirich 

Crystallization is governed by the usual thermodynamic variables of temperature, composition, and pressure. It is common to describe the thermodynamics in terms of dominant chemical species present, vhich in the majority of crystallization processes are the material to be crystallized and a small number of solvents. For all real systems, a further influence has to be taken into account, that is, of impurities. These are present in every system, in varying amounts. These impurities can have an effect on the solubility or melting point of the material to be crystallized, if small. The presence of additional components in the solution is often more noticeable in their effect upon the kinetics of crystallization and more specifically on growth rates of crystals. For the sake of clarity, additive is used as a collective noun for any minor component in a given system, whether this additive is in fact an impurity stemming from the raw materials employed or a by-product from the reaction stages required to manufacture the final product, or a true additive supplied to the system to achieve a specific effect. It is noted that the word impurity is even more widely used, as solvent or solvent mixture can also act as additives. 

The discussion on characteristic crystallization processes center first on the properties of the material to be crystallized (in the case of melt crystallization) or on those of the solution from which a given component is to be crystallized. However, any real system is not entirely pure and does contain any number of impurities at concentrations that may vary from barely detectable traces to significant concentrations. 

As the solubility of a given substance in a given solvent depends upon the composition of the system, the solubility will change with the presence of additional 

Crystallization Basic Concepts and Indu rial Applications, First Edition. Edited by Wol ang Bedanann. 

---

Unintentional and perhaps unrecognized incorporation of impurities may occur during crystal preparation. The addition of a controlled quantity of a selected additive is termed doping, and specific systems of this type are discussed in later chapters. Many studies of the influences of additives on decompositions have been reported. Fewer investigations have been concerned with the influences of compositional variations over extensive ranges on the chemical reactivities of solid solutions. 

As far as crystal growth is concerned, studies of the influence of tailor-made impurity additives on differential face development has led to important insights [37]. An unexpected bonus has been an independent confirmation of the correctness of absolute configurations determined by anomalous dispersion methods [38]. Tailor-made additives can also be used to inhibit formation of the thermodynamically stable polymorph and thus lead to formation of a metastable one [39]. 

There are literally thousands of reports in the scientific literature concerning the effects of impurities on the growth of specific crystals, and it would be superfluous to attempt a summary here. General reviews on the influence of additives in the control of crystal morphology have been made by Kem (1965), Boistelle (1976), Davey (1979), Botsaris (1982), Nancollas and Zawacki (1984), van Rosmalen, Witkamp and de Vreugd (1989), Davey et al. (1991) and Pfefer and Boistelle (1996). 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

With respect to the preparation of die surface, a measurement of a surface property is obviously only as good as the preparation of the surface on which the measurement is made. Ideally one would desire to have a surface which was atomically flat on a crystallographically perfect and chemically pure crystal. After reliable information had been obtained chi such ideal surfaces, it would then be necessary to determine the influence of surface roughness, of impurities, and of crystal imperfections of various kinds on the oxidation process. Most of the measurements to be described in this paper, however, have been made on surfaces which were prepared by the best methods available at this time. A convenient method of determining the important faces of a metal for detailed study involves the initial use of the specimen in the form of a sphere exposing all possible crystal faces such methods have been previously described, it should be emphasized that much additional work... 

Results obtained from thermal studies can often be compared with observations obtained in parallel investigations of the radiolysis of azides [3], Additional information is also often available from measurements of the conductivities and photoconductivities of the solids, including the influences of various impurities in the crystals. Preirradiation generally increases the rate of subsequent thermal decomposition of azides and such studies can provide information on decomposition... 

The various influences of impurities and additives on crystal growth phenomena can be best understood in terms of their influence on habit. Following, the need arises for a tool to model these influences on a molecular level. This molecular model can then be used to predict the effect of additives, which in the end results in tailoring additives to facilitate the goals of a crystallization process. 

The main objectives of this chapter are to clarify the roles of the hydrophobic emulsifier additives added in the oil phase of O/W emulsions how they modify fat crystallization and where they interact within the emulsion droplets. One may ask why the hydrophobic emulsifiers accelerate the nucleation process. The answer may not be straightforward, because their influences on fat crystallization are controlled by their physical and chemical properties and the nature of the interactions with the fat molecules occurring in the oil phase and at the oil/water interfaces. However, the results we have obtained so far indicate that the addition of hydrophobic emulsifiers in the oil phase has remarkable effects on crystallization. Fat crystals typically form a number of polymorphs, whose crystallization properties are influenced by many factors, such as temperature, rate of crystallization, time evolution for transformation, and impurity effects, as is commonly revealed in various examples [27,28], It is reasonable to expect that these polymorphic properties of fats may interfere with the clarification of the essential properties of the interface heterogeneous nucleation that occurs in O/W emulsions. 

---