Abstract
A quantitative kinetics analysis of the solution-mediated phase transformations in aqueous-ethanol between the enantiotropically related alpha and beta polymorphs of p-aminobenzoic acid (PABA) is presented. A new temperature-controlled transmission flow-cell design for use with a laboratory X-ray source which can be utilized for diffraction or scattering experiments is described. This transmission cell in combination with in situ slurry X-ray diffraction is utilized to provide a quantitative analysis of the percentage by mass of respective solid phases in flowing suspensions, coupled with solution concentration as measured with UV–vis spectroscopy. The separate phases are identified through individual fingerprint peaks, the (1 0 3) reflection at 2θ = 15.6° relating to the alpha phase and the (0 2 2) reflection at 2θ = 25.3° for the beta phase. Examination of the conversion of a slurry of beta PABA in a 70:30 water/ethanol solvent mixture to the alpha polymorph, at a temperature range of 24–30 °C reveals t...
Highlights
Polymorphism is a property exhibited by many organic molecules whereby a single chemical entity can pack in crystallographically different forms; these forms are the same compound but may have very different physicochemical properties.[1,2]
Transformations between polymorphic forms are often solution mediated,[6] and the thermodynamic and kinetic factors governing such transformations are important if a process is to be, fully understood and characterized, and if it is to be controlled in a reproducible manner
This study focuses on the phase transformation of the two polymorphic forms of p-aminobenzoic Acid (PABA) which has two enantiotropically related polymorphs, alpha and beta
Summary
Polymorphism is a property exhibited by many organic molecules whereby a single chemical entity can pack in crystallographically different forms; these forms are the same compound but may have very different physicochemical properties.[1,2] Polymorphism is of specific interest to the pharmaceutical sector due to the sometimes catastrophic effects small changes in molecular assembly have on the physical or chemical properties of a material. A step toward fast, online methodologies for monitoring crystallization parameters in situ which can be transferable to various scale sizes is needed in the industrial sector as a means of cost reduction and in order to reduce sampling errors.[9]. This desire for online monitoring of crystallization processes in an attempt to control unwanted phase transitions has led to an increase of research into this area as well as the development of numerous online monitoring techniques. A number of studies have implemented the use of in situ Raman spectroscopy probes to monitor the polymorphic phase interconversion of organic crystal systems, along with various techniques to quantify solution concentration such as UV−vis spectroscopy and FTIR spectroscopy.[10−15]
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