Characterisation of instantaneous water absorption properties of pharmaceutical excipients.

Powders absorb water by both capillary imbibition and swelling. The capillary process is almost instantaneous but swelling occurs over a period of time. An isothermal transient ionic current technique was used in this study to characterise the instantaneous absorption properties (rate and capacity) of a few selected pharmaceutical excipients. The results indicate that the instantaneous and long term water absorption properties of pharmaceutical powders can differ considerably. The rate of instantaneous water absorption appears to correlate with the total surface area while the absorption capacity correlates more with the porosity of the powder.