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od 2020-09-20

Title: The effect of hydrogen bonding propensity and enantiomeric composition on the dynamics of supercooled ketoprofen - dielectric, rheological and NMR studies

Source: Physical Chemistry Chemical Physics

Year : 2016

Abstract:

The aim of this work is to analyze in details the role of small hydrogen bonding (HB) structures and enantiomeric composition on the dynamics of glass-forming liquid ketoprofen. For that purpose dielectric relaxation, rheological and NMR studies were performed. Investigated samples are racemic ketoprofen, single enantiomer of ketoprofen and racemic ketoprofen methyl ester with no tendency to form dimeric hydrogen bonding aggregates. Combination of complementary experimental techniques enable us to show that the macroscopic viscosity η and structural relaxation time τα have nearly the same dependencies, whereas the relation between the rotational mobility and translational self-diffusion violates Stokes-Einstein law already at high temperature. Additionally, based on dielectric relaxation studies performed on increased pressure we were able to identify similarities and key differences in the supercooled liquid dynamics of investigated materials affected by their various tendency to form HB cyclic dimers. This includes the effect of pressure on the glass transition temperature Tg, changes in the fragility parameter m and activation volume ΔV, the role of thermal energy and density fluctuations in governing the viscous liquid dynamics (Ev/Ep ratio). Finally, we have also demonstrated that the dynamic behaviour of single enantiomer and racemic mixture of the same compound are very much alike. Nevertheless, some slight differences were observed, particurarly in the τα dependences measured in vinicity of the glass transition both at ambient and elevated pressure.

DOI: 10.1039/C6CP00578K   (Pobrane:  aktualizowanie)