The adsorption behavior of four bile salt species on graphite in water—evaluation of effective hydrophobicity of bile acids

Abstract

The adsorption behavior on graphite was examined for four kinds of sodium salts of bile acids; deoxycholate (NaDC), chenodeoxycholate (NaCDC), ursodeoxycholate (NaUDC), and cholate (NaC) in tetraboratecarbonate buffer solution (at pH 10.0) mainly at 30°C. All the salts seemed to exhibit the Langmuir type isotherm (plots of adsorption amount vs. equilibrium concentration), but when examined in more detail a small step was observed in each isotherm, suggesting that a certain phase transition or adsorption onto the first monolayer might take place. All the isotherms obtained enabled us to construct both the Langmuir plot and the BET plot. From both plots the maximum amounts of monolayer adsorption, N m, the constants of adsorption equilibrium, K, and the mean surface areas, A m, occupied by a bile salt molecule were determined; N m decreases in the order NaDC, NaCDC, NaC and NaUDC. These data were compared with the results of previous work on the adsorption behavior of these four bile salts at the cholesterol (Ch) crystal/water interface (the order of N m on Ch crystals is NaDC > NaCDC > NaUDC > NaC). The adsorption behavior of each bile salt was discussed in terms of the hydrophobic indexes (HI), the limiting surface area, A 0, determined from the previous monolayer study, the solubilizing power (Sp) for cholesterol, and so on.