Compression behaviour of anhydrous and hydrate forms of sodium naproxen

The aim of the present work was to investigate the technological properties and the compression behaviour of the anhydrous and hydrate solid forms of sodium naproxen. Among the hydrates, the following forms were studied: the monohydrate (MSN), obtained by dehydrating a dihydrated form (DSN) in each turn obtained by exposing the anhydrous form at 55% RH; a dihydrated form (CSN) obtained by crystallizing sodium naproxen from water, the tetrahydrated form (TSN) obtained by exposing the anhydrous form at 75% RH. The physico-chemical (crystalline form and water content), the micromeritic (crystal morphology and particle size) and the mechanical properties (Carr's index, apparent particle density, compression behaviour, elastic recovery and strength of compact) were evaluated. We made every effort to reduce differences in crystal habit, particle size and distribution, and amount of absorbed water among the samples, so that the only factors affecting their technological behaviour would be the degree of hydration and the crystalline structure. This study demonstrates a correlation between the compression behaviour and the water molecules present in the crystalline structures. The sites where water molecules are accommodated in the crystalline structure behave like weak points where the crystalline lattice yields under compression. The crystal deformability is proportional to the number of water molecules in these sites; the higher the water content, the higher the deformability, because the densification behaviour changes from a predominantly elastic deformation to a plastic behaviour. The deformability is responsible for a higher densification tendency that favours larger interparticle bonding areas that may explain the better tabletability of TSN and CSN.