Dissolution rate studies of cholesterol monohydrate in bile acid-lecithin solutions using the rotatingdisk method

A physical model approach was used to investigate cholesterol gallstone dissolution kinetics in simulated bile. Critical experimental and theoretical investigations simulating in vivo conditions showed that, in the bile acid-lecithin solutions, there is a significant interfacial barrier for both cholesterol gallstone and cholesterol monohydrate pellet dissolution. In the present study, the rotating-disk dissolution method and the accompanying Levich theory were applied to assess the contributions of the diffusion convection mass transfer resistance and of the interfacial barrier to the overall kinetics. Cholesterol dissolution rates in bile acid solutions were about 2-20 times slower than diffusion-controlled rates depending upon the degree of agitation. As found in previous studies, these rates in the presence of sufficient concentrations of dissolution accelerators approached the theoretical diffusion-convection-controlled rates. To account for the much slower dissolution rates in bile acid-lecithin solutions, two possible kinetic interpretations were investigated. The first is based upon slow crystal-micellar solution interfacial kinetics, and the second is based upon a slow rate of cholesterol solubilization in the aqueous diffusion layer. For the latter, an analytical mathematical solution was obtained.