Abstract
The purpose was to show, using destructive/nondestructive methods, that the interplay between water, tablet structure, and composition determine the unique spatiotemporal hydration pattern of polymer-based matrices. The tablets containing a 1:1 w/w mixture of sodium alginate with salicylic acid (ALG/SA) or sodium salicylate (ALG/SNA) were studied using Karl Fischer titration, differential scanning calorimetry, X-ray microtomography, and magnetic resonance imaging. As the principal results, matrix specific features were detected, e.g., “locking” of the internal part of the matrix (ALG/SA); existence of lamellar region associated with detection of free/freezing water (ALG/SA); existence of water penetrating the matrix forming specific region preceding infiltration layer (ALG/SNA); switch in the onset temperature of endothermic water peak associated with an increase in the fraction of non-freezing water weight per dry matrix weight in the infiltration layer (ALG/SNA). The existence of complicated spatiotemporal hydration patterns influenced by matrix composition and molecular properties of constituents has been demonstrated.
Highlights
Considering the processes taking place in polymer matrix tablets during hydration, one notices their complexity, interdependence, parallelism, and multi-faceted nature
We proposed an integrated methodology to effectively evaluate multiple spatiotemporal aspects of matrix tablet hydration: (1) quantitative water distribution using Karl Fischer (KF) titration of precisely localized samples; (2) water–polymer interaction using spatially localized differential scanning calorimetry (DSC); (3) water distribution and its molecular mobility in hydrophilic polymeric matrix upon hydration with non-invasive MRI [30]
The initial water content of the unhydrated alginate tablets containing salicylic acid determined by the KF (wc0(KF) ) method was 7.1%
Summary
Considering the processes taking place in polymer matrix tablets during hydration, one notices their complexity, interdependence, parallelism, and multi-faceted nature. The simple methods of polymer matrix expansion recording by photographing in visible light, video recording or measuring the refraction have been applied [10,11,12,13]. Starting from these studies, the existence of several fronts or regions have been proposed to describe the matrix and its temporal evolution e.g., swelling, diffusion and erosion fronts [8]; water penetration, phase transition, apparent gel, dissolution fronts [14]; penetration, swelling, erosion fronts [15] etc. The studies in the proceeding decades show that the processes occurring in the hydrating matrix are much more complex [6,16]
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