Theoretical Treatment of Cholesterol Monohydrate Dissolution in Renex- 690/Ionic Surfactant Systems Based on a Physical Model Approach 1

The dissolution kinetics of cholesterol monohydrate in the presence of Renex-690 and ionic surface active agents (benzalkonium chloride and sodium dodecyl sulfate) are dramatically influenced by the addition of NaCI. The mass transfer resistances decreased with increasing electrolyte concentrations and attained an asymptotic value predicted for the aqueous boundary layer-controlled situation. Electrophoretic mobility studies showed that the mixed surfactants are adsorbed on the originally negatively charged cholesterol surface to an extent that the effective surface charge is attributed to the adsorbed surface active molecules. The results suggest that electrical repulsion between charged micelles and the cholesterol surface plays a dominant role in the mechanistic interpretation of the interracial barrier to dissolution. A model embodying the classical Smoluchowski flocculation theory of colloidal particles and the DLVO potential energy theory of electrical double layer repulsion and dispersion attraction was applied to the dissolution data to obtain a quantitative description of the interfacial barrier in terms of surface charge densities, electrical double layer parameters, surface potentials, and dispersion force constants.