Mixed monolayer formation of bile acid mixtures: Chenodeoxycholic acid with ursodeoxycholic acid and cholic acid with β-muricholic acid

Abstract Monolayers formed by different bile acids and their mixtures on concentrated sodium chloride solution were investigated by measuring the surface pressure (II) and the surface potential (Δ V ) or the surface dipole moment ( μ ⊥ ) as a function of molecular surface area ( A ). From the analyses of these obtained curves of II vs. A and μ ⊥ vs. A , the additivity rule with respect to A and μ ⊥ was found to approximately hold for the mixed systems of both cholic acid (CA) with β-muricholic acid (βMCA) and chenodeoxycholic acid (CDC) with ursodeoxycholic acid (UDC), meaning that these mixed systems seem to form a monolayer of ideal mixing. It was found that the mean molecular surface area and the attitude (the gradient of the plane of the steroid skeleton) of the molecules floating on the water surface are dependent mainly on the magnitude of the hydrophilicity of the α-plane of the steroid skeleton; the surface area increases in the order βMCA UDC > CDC > CA. The order of increase in the surface area was found to correspond exactly to the order of increase in the cholesterol solubilizing power of the respective bile salts.