Transient hydrate formation during disintegration of film-coated pharmaceutical tablets

The transformation of anhydrous tablet excipients into their hydrated forms occurs simultaneously with film coating dissolution, yet the kinetics of this process remain poorly understood. Water molecules diffuse through the immediate release film coating and form hydrogen bonds with the anhydrous components within the tablet core, creating a thermodynamic driving force that competes with the hydration of the coating polymer. This study investigates the influence of film coating properties, including the base polymer (polyvinyl alcohol versus hydroxypropyl methylcellulose), coating thickness, and coating density, alongside tablet core properties such as porosity and excipient type. Using polymer-coated tablets containing anhydrous lactose or magnesium sulphate, we demonstrate that terahertz pulsed imaging (TPI) captures this diffusion process non-destructively. We characterise the anhydrous-to-hydrate transformation based on subtle refractive index changes at the interface. We identify distinct hydration mechanisms for channel hydrates versus ion-coordinated hydrates, providing further evidence that the core formulation actively regulates water ingress prior to coating failure. These findings offer critical insights for the predictive modelling of film-coated tablet disintegration and dissolution.