Adsorption, 2D-condensation, and redox reactions of bile acids on the hanging mercury drop electrode

Bile acids (BAs) are natural surfactants thanks to the facial amphiphilic character of the steroid skeleton. For the first time, the electrochemical and adsorption behaviour of selected bile acids differing in the number of hydroxyl groups on the steroid skeleton - lithocholic acid (3 alpha-OH), deoxycholic acid (3 alpha-and 12 alpha-OH), and cholic acid (3 alpha-, 7 alpha-and 12 alpha-OH) - was investigated on hanging mercury drop electrode (HMDE) in mixed Britton - Robinson buffer: methanol (9:1) media. In cyclic voltammetry, all BAs yielded a negative signal at the potential of ca-1.4 V with intensity depending on pH of the solution. Further, sharp tensammetric signals confirming adsorption/desorption or re-organization processes of the adsorbed molecules were observed. AC voltammetric measurements revealed two-dimensional (2D) condensation of lithocholic acid in the pH range 12.0-7.0 visualized by the formation of a capacitance pit. Adsorption of the other studied BAs was indicated by the AC current density decrease. Measurements of differential capacitance dependences -potential (C-E curves) revealed this behaviour to be concentration and temperature dependent. A study on effect of halides (Cl-, Br-, I-) on the adsorption of bile acids showed involvement of primarily hydrophobic interactions in the BA interaction with the HMDE surface.