SLOWER DISSOLUTION RATES OF SULFAMERAZINE IN AQUEOUS SODIUM DODECYL-SULFATE SOLUTIONS THAN IN WATER

The solubility of sulphamerazine in 0.1 M sodium dodecyl sulphate solu tion (SDS) was found to be 2.8-times that in water (at standard report ing conditions). The thermodynamics of transfer to the micelle were in dicative 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 decre ased with increasing SDS concentration. The activation thermodynamics revealed a slight activation barrier to solubilisation. Apparent diffu sion coefficients were calculated from initial dissolution rates from rotating disks, with different rotating speeds. The greatly reduced di ffusion coefficient in the presence of micelles was taken as an explan ation for the reduced rate of dissolution. The changes in solubility a nd diffusion rate have been considered in terms of the Noyes-Whitney e quation, 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.