Owusu-Ware, Samuel K., Cherry, Anthony J. and Antonijević, Milan D. ORCID: https://orcid.org/0000-0002-5847-7886 (2013) Pharmaceutical and biopharmaceutical applications of thermally stimulated current spectroscopy. In: 2nd Central and Eastern European Conference on Thermal Analysis and Calorimetry, 27-30 Aug 2013, Vilnius, Lithuania. (Unpublished)

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Abstract

Thermally Stimulated Current (TSC) spectroscopy is an instrumental technique used to detect slow molecular mobility in the solid state [1]. The technique has significant potential for the characterisation of low molecular weight organic materials [2] (commonly associated with the pharmaceutical, biotechnology and healthcare industries).

Aims of current study are: to investigate how variations in molecular structure, namely stereo and positional isomerism and changing carbon chain length of a homologous series of molecules, influence solid-state molecular mobility, as measured by TSDC, and secondly to assess if and how such mobility/relaxation properties relate to the thermal stability of the materials under investigations.

A global secondary relaxation of the amino acid β-alanine (β-Ala) at 45 ± 3°C, detected by TSC spectroscopy via the use of thermal windowing and relaxation map analysis, was characterized and deduced to comprise of two relaxation domains. Compensation domain A exhibits the parameters: Tc = 74°C, τc = 0.1042 s, R2 = 0.997, degree of disorder (DOD) = 45 and Tp values between 10 and 40°C. Compensation domain B displays the parameters: Tc = 23°C, τc = 1.5790x105 s, R2 = 0.997, DOD = 17 and Tp values between 42 and 56°C. Domain B was linked, via compensation analysis, to the onset of domain A suggesting that domain A is a non-cooperative molecular mobility domain and a precursor of the cooperative mobility domain B. Domain A was further linked to the onset of the primary transition and confirmed by isothermal TG studies as well as subsequent kinetic analysis to be a zero-order process attributed to sublimation.

Crystalline γ-aminobutyric acid (GABA), and its positional isomers DL-α-aminobutyric acid (AABA) and DL-β-aminobutyric acid (BABA) differ by the position of the amine group relative to the carboxyl functionality. All materials were found to undergo localised non-cooperative rotational motions at 77 ± 2˚C and 114 ± 2˚C, 104 ± 1˚C and 109 ± 1˚C for GABA, DL-BABA and DL-AABA, respectively prior to the higher temperature thermal events observed via DSC and TGA. The spatial arrangement of the amine group and thus the carbon chain length between the amine and the carboxyl moiety is found to be a key contributor to solid-state molecular mobility. Molecular relaxation frequency was found to correlate to thermal stability of the materials investigated i.e. the lower the relaxation frequency the greater the thermal stability.

Both studies demonstrate that TSC can be used to probe and characterise molecular mobility in crystalline materials. In addition, the studies reveal that molecular mobility can be related to thermal stability of materials; hence TSC has the potential to help predict solid-state stability.

Item Type:	Conference or Conference Paper (Speech)
Additional Information:	[1] This paper was presented within Oral Presentations 4, No. OP4.11 on 30 August 2013 at the 2nd Central and Eastern European Conference on Thermal Analysis and Calorimetry (CEEC-TAC2) held from 27-30 August 2013 in Vilnius, Lithuania. [2] The abstract for this oral presentation is published in: Book of abstracts of the 2nd Central and Eastern European Conference on Thermal Analysis and Calorimetry (CEEC-TAC2). 27-30 August 2013 Vilnius University, Vilnius, Lithuania. Editors: Andrei Rotaru, Daumantas Matulis.
Uncontrolled Keywords:	thermally stimulated current spectroscopy, relaxations, molecular mobility, thermal windowing
Subjects:	R Medicine > RS Pharmacy and materia medica
Faculty / School / Research Centre / Research Group:	Faculty of Engineering & Science > School of Science (SCI)
Related URLs:	Organisation Organisation
Last Modified:	17 Oct 2016 09:12
URI:	http://gala.gre.ac.uk/id/eprint/10529

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