Cholesterol monohydrate dissolution rate studies in aqueous micellar solutions of...

The influences of electrical and nonelectrical factors in the interfacially controlled dissolution of cholesterol monohydrate by mixed micelles of alpha-(nonylphenyl)-omega-hydroxydeca(oxyethylene) (polyoxyethylene [10]-nonylphenol ether; POEPE; 1) in combination with long-chain n-alkylamines or fatty acids were investigated and quantified under pH conditions where the amines and fatty acids exist in their charged and uncharged forms. The experimental findings were generally consistent with a mechanism involving micelle collision with the cholesterol surface. A significant interfacial barrier was found with neutral micelles; however, the magnitude of the barrier was smaller with uncharged mixed micelles than with the simple micelles of 1. With charged micelles, the interfacial mass transfer resistance decreased with the addition of sodium chloride to the extent that the interfacial barrier was essentially abolished and the dissolution kinetics became convective/diffusion controlled. The results are consistent with the phenomena of diffusion of charged micelles toward a charged surface, following the classical collision kinetic theory of colloids. The ease in the transfer of cholesterol monohydrate molecules from the crystal surface and into the micelles during the collision step was examined by considering factors affecting the crystal surface as well as those associated with the hydrophobic-hydrophilic structure of the micelle of 1 and the distribution of charged and uncharged amphipathic additives within the basic micelle structure of 1.