Solubility and polymorphism of molecular compounds

University dissertation from Stockholm : KTH

Abstract: This thesis deals with the controlled crystallization of small organic molecules and is focused on solubility and polymorphism. The solubility was determined for phenylacetic acid, p-hydroxyphenylacetic acid, p-aminophenylacetic acid, p-hydroxybenzoic acid and ibuprofen in both water and in a range of organic solvents. Data is discussed from the standpoint of molecular aspects of solute – solvent interactions and by estimated solid phase activity. It was shown that better understanding could be acquired by making a qualitative analysis of the molecular interactions in the solution and the crystal structure of the compounds in question.Solubility predictions that are carried out by the UNIFAC method are not sufficiently accurate to serve as a basis for a reliable design of a crystallization process or selection of a suitable solvent since they deviate more than 15% from experimental values. The reason for the discrepancies are related to uncertainties in the prediction of activity coefficients by UNIFAC, as well as, difficulties in the estimation of the activity of the solid state.p-Aminobenzoic acid (PABA) has been crystallized from thirteen different solvents either by slow cooling, after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Two different polymorphs have been crystallized. The system is found to be enantiotropic with the transition temperature of 25 °C, below which the ?-form is the stable polymorph. The ?-form was obtained from all solvents by both methods. The ?-form is obtained only in carefully controlled conditions from water and ethyl acetate, well below the transition temperature. Often the ?-form appears concomitantly.It is shown in this work that sonication significantly reduces the induction time for nucleation. The ?-form crystallizes more reproducibly and at higher cooling rates when controlled sonication is used. In addition sonication is found to selectively favor the appearance of one of the polymorphs. Producing the pure ?-form was possible even above the transition temperature where other crystallization techniques were only capable of producing the stable ?-form. The ?-form structure is based on centro symmetric dimers formed by association of carboxylic acid groups. It is suggested that the preference for nucleation of the ?-polymorph is related to the formation of dimers in the supersaturated solution. Only at the condition where the formation of dimers is reduced sufficiently, (i.e. in the polar solvents or when sonication is applied) the nucleation of the ?-form is favored.

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