Interaction of the chloropentaamminecobalt(III) cation with dihexadecylphosphate vesicles in the absence and the presence of poly(propylene glycol). A kinetic study

The electrostatic interaction between the cationic chloropentaamminecobalt( III) complex and the exterior anionic surface of sodium dihexadecylphosphat e (DHP) vesicles in the absence and the presence of varying amounts of the polymer PPG-425 [poly(propylene glycol) with M-w = 425] has been studied at 25.0 degrees C by examining the vesicular retarding effects on the kinetic s of the alkaline hydrolysis of the cobalt(III) complex. The kinetic data i ndicate that two reaction paths contribute to the overall hydrolysis rate. In the absence of polymer the interactions involving the cobalt complex, as deduced from the estimated binding constant (K-A), are markedly larger tha n those found previously with anionic micelles of sodium dodecyl sulfate (S DS). Moreover, the rate constant (k(v)") for the pathway involving the hydr oxide ion in the outer aqueous vesicular compartment and the complex solubi lized in the exterior vesicle surface is smaller than the corresponding rat e constant obtained with SDS micelles. These results suggest the existence of greater charge density at the DHP vesicular surface. In the presence of polymer both K-A and k(v)" parameters remain substantially unchanged up to the polymer content of 0.30%, but the binding constant undergoes a decrease and the rate constant k(v)" an increase at higher polymer concentration. T hese findings are primarily attributed to the decrease in the charge densit y of the external vesicular surface induced by the polymer penetration.