Investiguation of crystallization phenomena and characterization of crystals using using microcalorimetric principles

Studies on crystallization kinetics and surface characterization of crystals are associated with experimental difficulties and theoretical short comings. It was the objective of this study to explore the potential of microcalorimetry in these important areas of research. It was also the objective of this study to develop the necessary mathematical models for a quantitative treatment of the calorimetric data.; In this study a simple model system involving the crystallization of dl-pseudoephedrine from aqueous solutions was utilized to investigate crystallization (thermodynamics and kinetics) by microcalorimetric means. A good agreement was observed between the magnitude of heat of crystallization determined by direct calorimetry and the heat of solution obtained from the solubility-temperature data. The use of calorimetrically obtained desupersaturation profile in deriving the growth parameters was exemplified. The studies were extended to crystal dissolution using a heat conduction microcalorimeter. The dissolution of salicylic acid into a nearly saturated solution was monitored calorimetrically. The study demonstrated that microcalorimetry can be utilized to (indirectly) monitor extremely small changes in concentration which may result from very slow dissolution (or crystallization) of a solid. Very few experimental methods are capable of measuring such small changes in concentration continuously. The method has potential application in the determination of dissolution rate constants and hence, in the characterization of crystal modifications.; A mathematical model analogous to the widely used BET equation was derived to quantitatively describe the calorimetric adsorption isotherms in the low pressure region. The equation satisfactorily described the shape of calorimetric adsorption isotherms and provided a measure of the "monolayer" capacity (V{dollar}rmsb{lcub}m{rcub}){dollar} of the solid for the adsorbate. Application of the modified BET model to the calorimetric data available in the literature provided values of V{dollar}rmsb{lcub}m{rcub}{dollar} that are in general agreement with those obtained by other means. The model was utilized to differentiate the adsorption characteristics of sodium benzoate samples produced under different crystallization conditions. It was concluded that microcalorimetry is valuable in the study of crystallization (and related) phenomena and characterization of crystals. With the help of the models developed in this study the high sensitivity and versatility of the modern calorimeters can be better utilized.