Cholesterol monohydrate dissolution rate studies in aqueous micellar solutions of polyoxyethylene nonylphenol ether and ionic surfactants

Abstract

The influences of electrical and other factors on the dissolution kinetics of cholesterol monohydrate in micellar solutions were quantified in terms of mass transfer resistances due to interfacial processes and convective/diffusional barriers. In Renex-690 (polyoxyethylene[10]nonylphenol ether) solutions, the mass transfer resistance, R, was large and constant over a wide concentration range of sodium chloride, indicating that neutral micelles were involved in slow interfacial kinetics via a collision complex mechanism. To study the effects of micellar charge and electrolyte, predetermined amounts of ionic surfactants were added to Renex-690 solutions. At constant Renex-690/benzalkonium chloride and Renex-690/sodium dodecyl sulfate ratios, curves of R versus total counterion concentration were superimposable over a range of solubilizer concentration. Also, R decreased significantly with additions of NaCl and attained an asymptotic minimum which was expected for the convective/diffusion controlled case. These results were consistent with the collision complex mechanism and predictions based upon electrostatic interactions involving a charged sphere and a charged surface. The finding that there is a significant interfacial barrier with Renex-690 alone but not when both an ionic surfactant and sodium chloride were present was interesting. It is proposed that either the presence of the ionic surfactant in the mixed micelle may catalytically facilitate the transfer of the cholesterol molecule into the micelle or that the ionic surfactant adsorbed on the cholesterol crystal surface may be able to alter the accessibility of the cholesterol molecules on the crystal surface.