Studies on Interface-Limited Transport of Aqueous Solubilized Cholesterol -- a Direct Membrane Method.

Abstract

The purpose of this study was to develop a method useful for studying interface-controlled cholesterol transport between two aqueous phases separated by a lipid-type membrane. Interfacial transport kinetics have been studied using a nonionic surfactant, Renex-690('TM) (polyoxyethylene {10} nonyl phenol ether) as the cholesterol solubilizing agent with charged surfactant and /or electrolyte as additives. The results of these studies may be summarized as follows. (1) Membrane transport studies using 1% Renex-690('TM) solubilized cholesterol were found to exhibit membrane transport limited by more conventinal diffusion/solubility resistance. Approximately one-third the total membrane transport resistance was accounted for by diffusion/solubility considerations within the membrane (Silastic 372('TM)) matrix with the remaining fraction limited primarily by aqueous-diffusion-layer parameters. (2) Transport studies of cholesterol solubilized in mixed micelles of Renex-690('TM) and charged amphipath (benzalkonium chloride or sodium oleate) exhibited a mass transfer barrier which was more than 99% interface limited. Interfacial resistance imparted by the charged surfactant was normally abolished by adequate electrolyte levels or by pH adjustment to eliminate electrical repulsion between the charged mixed micelle and charged silicone rubber interface. Furthermore, a short-circuit technique was developed which involved eliminating a single interfacial resistance component by screening out micelle-membrane electrical repulsion. Significant membrane dumping of cholesterol was observed using this technique. (3) The absolute magnitude of electrical repulsion between the mixed micelle and silicone rubber interface was estimated by measuring cholesterol membrane transport rates in Renex-690('TM) solutions containing predetermined amounts of ionic surfactant and /or electrolyte. At low electrolyte levels the data was adequately described in a quantitative manner by a physical model employing classical Smoluchowski flocculation theory of colloidal particles coupled with Verwey and Overbeek theory of electrical double layer repulsion and dispersion attraction. (4) Interface-limited mass transfer resistance was demonstrated to be comprised of physical chemical parameters which may operate in a synergistic manner. Mechanistic studies related to cholesterol dissolution and aqueous solubilized cholesterol mass transfer may be facilitated by directing emphasis toward conditions where independent mechanisms appear to be important.