Oiling Out, Agglomeration Aggregation

The most important property of a compound from a crystallization point of view may be its inherent growth characteristics. The critical question is will it grow A method to evaluate 

A substantial-sized subgroup of those compounds which do not exhibit typical crystal growth, in which a repeated lattice grouping or crystal structure, is so difficult to achieve that the compound resembles a Uquid as it emerges from solution. This is the phenomenon of oiling out, which is often accompanied by the additional compheation of agglomeration/ aggregation. 

It is perhaps helpful to begin this discussion with the consideration of oiling out since it may be the first event in the pathway of CrystaUization or, in extreme cases, the operation may end with an oil, gel, or intractable gum or tar (Bonnett et al. 2002). 

Oihng out can be a critical factor in CrystaUization by any of the methods of creating supersaturation and becomes increasingly possible under several conditions, including 

A mechanism for oiling out can be postulated as follows When supersaturation is achieved rapidly such that the concentration is beyond the upper metastable limit—as can often be the case in a nucleation-based process—the substrate is forced to separate into a second phase by the creation of the resulting high solution concentration. However, crystallization is delayed by a slow crystallization rate. This combination may result in the creation of a nonstructured oil or possibly an amoiphous solid. The rates of phase separation and nucleation are relative to each other such that slow nucleation implies only that nucleation was not fast enough to create discrete particles before oil separation. 

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