Effect of habit modification on dissolution rate

Abstract

The effect of habit modification on the dissolution rate of NiSO 4α6 H 2O in 60% ethanol was studied using a rotating basket method. Bipyramidal crystals were grown in a fluidized-bed crystallizer and platy crystals were recrystallized from supersaturated solutions of nickel sulfate containing small amounts of gelatin. Acicular crystals were prepared by the topotactic dehydration of acicular crystals of NiSO 4 · 7 H 2O, grown from solution at room temperature. X-ray diffraction powder patterns for the 3 habits were identical and characteristic of NiSO 4α6 H 2O. DSC thermograms of platy and acicular habits were similar but differed from the bipyramidal habit. Solubilities of all habits in the dissolution medium were identical. The observed dissolution rate constant, K' obs for the bipyramidal crystals was greater than for the platy crystals at both low and high rotation rates but the difference was less pronounced at the lower rotation rate. At the high rotation speed, dissolution was under mixed surface-transport control. K' obs for the acicular crystals was similar to the bipyranidal habit and greater than the platy habit. The difference in K' obs must be due to different values of the overall surface energy of the crystals. Intrinsic dissolution rates for the 3 habits, determined by a rotating disc method, were similar confirming that differences in K' obs were due to habit modification.