Thermally Stimulated Current (TSC) spectroscopy studies of a pharmaceutically relevant co-crystal

Cherry, A., Leharne, S., Chowdhry, B. and Antonijevic, M. (2011) Thermally Stimulated Current (TSC) spectroscopy studies of a pharmaceutically relevant co-crystal. The AAPS Journal, 13 (S2). ISSN 1550-7416

Abstract

Purpose: Co-crystals (CCs) are of increasing contemporary importance within the pharmaceutical industry, with CC screening now added to the list of high-throughput techniques available to researcher. The development and characterisation of CCs has potential benefits including optimisation of solid-dosage forms and patentability. In line with this increased interest a study was undertaken to characterise an existing pharmaceutically relevant CC system of salicylic acid (SA) and benzamide (BA) using a novel instrumental approach in this area - Thermally Stimulated Current (TSC) spectroscopy.

Methods: TSC is used to study slow molecular mobility in organic materials and is particularly useful in ascertaining and characterising transitions and/or relaxation phenomena in the solid state. For this reason a TSCII (Setaram, France) TSC spectrometer was employed to evaluate the thermal physical/chemical properties of a co-crystal (CC) system, its constituents (SA/BA) and a physical mixture (PM) of the constituents.

Results: TSC results are presented in figure 1. TSC was able to detect a group of transitions (≈0-70°C) not realised using a range of other thermal techniques (HSM, TGA, DSC). It is clear that TSC is able to distinctly and reproducibly differentiate the four materials. Molecular displacement within the co-crystal is significantly higher than that of the physical mixture and their constituents.

Conclusions: TSC analysis detected a temperature region in which an array of previously undetected transitions reside. Many more experiments are required to characterise this temperature region of increased molecular mobility, with the employment of Thermal Windowing (TW) and Relaxation Map Analysis (RMA) likely to provide a detailed examination of this temperature region.

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