the technique of measuring the difference of Ch concentra-The true thermodynamic activity (A T ) of cholesterol (Ch) in tion in a solution to estimate the amount of Ch in the silicone aqueous solutions containing taurocholate (TC) -Ch was deterpolymer. The relationship between A T and Ch concentration mined by employing a direct assay of a 1 1 2-cm silicone polymer in the tauroursodeoxycholate (TUDC) -Ch, TC-Ch, and film with 0.025 cm thickness. Using the A T data, information on taurochenodeoxycholate (TCDC) -Ch systems has been rethe nature of micellar species present in the TC-Ch system, and ported to be linear up to unit activity (i.e., A T Å 1), indicatemploying a binding-site model previously developed for tauroursoing that Henry's law is obeyed (8-10). Based on the A T deoxycholate (TUDC) -Ch and taurochenodeoxycholate (TCDC)data, Liu et al. presented a quantitative model to interpret Ch systems, it appeared that the Ch-binding affinity for simple bilethe nature of Ch binding by simple micelles (12). This salt micelles corresponds precisely with the order of hydrophobicity, TUDC õ TC õ TCDC. Further, although simple TC micelles and model showed that simple TUDC micelles have only one simple TCDC micelles have similar binding capacities, the first Ch binding site, whereas simple TCDC micelles exhibit a twobinding to a simple TC micelle may not significantly facilitate the site binding capacity. second Ch binding, as occurs in simple TCDC micelles. For TUDC-
Although the difference method is a good start for de-Ch, TC-Ch, and TCDC-Ch systems, the concentration of bound termining the A T , a large amount of the polymer needs to simple micelles increased with increasing A T values, whereas the be used to increase sensitivity. Actually, the amount of Ch unbound simple micelle concentration decreased proportionally. in the silicone polymer phase (i.e., A T ) can be determined These results provide insights into the possible influence of bile-salt directly by ethanol extraction (10). In addition, since the species on Ch-binding to simple micelles in bile-salt-Ch solutions. rate of transfer of Ch from solution into the silicone polymer