Interactions in coloured powders and tablet formulations: a theoretical approach based on solubility parameters

Abstract

Based on a simple but crude model for calculating the relative intensity of the adhesive and cohesive interactions in two-component systems using partial solubility parameters, it has been possible to predict the properties of binary mixes of a number of components commonly used in tablet formulation with the insoluble colouring agents red iron oxide and carbon black. Three general cases have been identified, one where the adhesive interaction within the system is lower than either of the cohesive interactions, one where the adhesive interaction within the system is lower than the cohesive interaction within the colour but higher than that within the other component and one where the adhesive interaction within the system is higher than the cohesive interaction within the colour but lower than that within the other component. While the first will result in a stable uniformly coloured mix the others will result in enrobement of either the colour by the added component or the added component by the colour, leading to mottling in the powder mix. These trends have been confirmed using real systems.