Nuclear magnetic resonance study of an ethyl cellulose sustained‐release delivery system I: Effect of casting solvent on hydration properties

This study was conducted in order to explore the proton relaxation time and the structural features of films that are cast from different solvents and to be used as sustained‐release delivery systems. Proton magnetic resonance measurements were performed on ethyl cellulose (EC) films cast from ethanol or chloroform solutions in the presence of polyethyleneglycol (PEG). Spin‐lattice relaxation time (T₁) was measured with a Bruker PC‐20 Multispec, at 20 MHz and at 37 °C, on the dry films and thereafter during gradual, controlled hydration. The prolongation of the rate of relaxation time for the films cast from ethanol and chloroform solutions was found to be drastically different. Water compartmentalization was then calculated according to the Free Induction Decay model. After the addition of similar amounts of water, markedly different hydration fraction (HF) values were derived for the films cast from the different solutions as a function of the amount of embedded PEG. Scanning electron micrographs confirmed that the two types of systems have different film structures that are dictated by the casting solvent and the amount of embedded PEG. From these results it can be concluded that in the presence of PEG, EC films cast from ethanol have more water binding sites and a thicker water multilayer around them than films cast from chloroform. These properties might influence the release rate of an active agent from the sustained‐release device.