Deviation of dissolution behaviour of benzoic acid from theoretical predictions with lowering of temperature: limitations as a model dissolution substance

Benzoic acid dissolution was found to be non-ideal on lowering the dissolution temperature and serious deviation from classical diffusion was observed. Experimental dissolution rates, measured using a rotating disc apparatus, differed from the values predicted using the Levich equation, the deviations becoming more pronounced and negative as the temperature was reduced. Measurement of dissolution rates at varying rotational velocities (ω) of the disc enabled application of a test for diffusional behaviour: plots of dissolution rate against ω ^{1 2} were linear only at 37°C but were curved at 25°C and below. Since the deviation was negative at high ω-values, it could not have been due to turbulence, the rate-limiting process evidently being an interfacial reaction. In further tests, the addition of a complexing agent, caffeine, to increase the dissolution rate eliminated the curvature at 25°C when the caffeine concentration was high but not at low concentrations, thus confirming the postulation of an interfacial process. Additional evidence was provided by an Arrhenius plot of the experimental data which deviated at low temperatures whereas at 25-37°C, the slope was in accord with an activation energy value suited to transport control.