Slower dissolution rates of sulphamerazine in aqueous sodium dodecyl sulphate solutions than in water

Abstract

The solubility of sulphamerazine in 0.1 M sodium dodecyl sulphate solution (SDS) was found to be 2.8-times that in water (at standard reporting conditions). The thermodynamics of transfer to the micelle were indicative of a spontaneous, enthalpy-driven, process. Contact angle and surface energy data revealed a favoured interaction between drug and SDS micelles, however, the dissolution rate of the drug into SDS decreased with increasing SDS concentration. The activation thermodynamics revealed a slight activation barrier to solubilisation. Apparent diffusion coefficients were calculated from initial dissolution rates from rotating disks, with different rotating speeds. The greatly reduced diffusion coefficient in the presence of micelles was taken as an explanation for the reduced rate of dissolution. The changes in solubility and diffusion rate have been considered in terms of the Noyes-Whitney equation, to show the combined effect on dissolution rate is as seen, i.e., reduced dissolution rate, despite the increased solubility. Care should be taken before assuming that an SDS solution is an appropriate medium for improving dissolution test data for poorly soluble drugs.