Glass transition effects on the molecular transport of caffeine from condensed k-carrageenan/polydextrose systems

Abstract

We investigate the effect of added KCl on the glass transition of potassium-κ-carrageenan/polydextrose and molecular transport of caffeine. Samples were prepared with 2% (w/w) κ-carrageenan, 82% (w/w) polydextrose and 1% (w/w) caffeine to a total solids level of 85% (w/w). KCl was added at 0, 50, 100 and 200 mM to induce a dependence of structural properties on the potassium ion concentration. High-resolution spectral data over a wide spectral range for the tertiary system were collected using Fourier transform infrared spectroscopy (FTIR), which were complemented by tangible evidence of its morphology with scanning electron microscopy (SEM). Work then focused on the estimation of the mechanical (Tgm) and calorimetric (Tgc) glass transition temperature using in-shear dynamic oscillation and modulated differential scanning calorimetry (MDSC). Molecular transport of caffeine in the condensed polysaccharide/co-solute system was followed with UV–vis spectroscopy over a wide temperature range (−20 to 30 °C) and modelled with extended diffusion theory. Time-dependent mass transport of the diffusant was correlated to the structural relaxation of the polymeric matrix seen in increasing values of the mechanical glass transition temperature with higher additions of potassium counterions to κ-carrageenan. It was concluded that the mechanical glass transition temperature has a prominent effect on the release kinetics of caffeine in these materials, as compared to the calorimetric glass transition temperature.