Thermodynamic study on the Langmuir adsorption of various bile salts including taurine and glycine conjugates onto graphite in water

The adsorption isotherms of various free and conjugated bile salts (BS) onto the surface of graphite (Gr) powder in aqueous berate buffer solution at pH 10 were obtained as a function of temperature ranging from 20 or 25 to 42 degrees C. Analysis by Langmuir plot was made within the concentration range where the Langmuir adsorption took place, and from it the maximum amount of monolayer adsorption (N-m) and the Langmuir constant (K-L), i.e., the ratio of rate constants of adsorption and desorption (K-L = k(a)/k(d)), were determined for each BS at different temperatures. The present study shows the K-L to be the reciprocal of equilibrium concentration (C*) at half surface coverage, as K-L = 1/C*, and in addition, K-L is equal to the adsorption equilibrium constant, K-ad, which was defined by a thermodynamic consideration. For determination of K-L or K-ad, a more reliable method was proposed by to more easily examine the accuracy of the measured points. From the van't Hoff plot of K-ad, the enthalpy changes (adsorption heats, Delta H-ad(circle minus),) were determined and the entropy changes (Delta S-ad(circle minus)) were calculated from the Gibbs energy changes (Delta G(ad)(circle minus)). These data were compared among BS species and the correlation with hydrophobicity index (HI) was discussed. The so-called entropy-enthalpy compensation phenomenon was observed throughout BS species. (The compensation temperature was determined to be 304 K). The enthalpy term was found to be overwhelmingly dominant compared to the entropy term in the adsorption of BSs onto Gr in water.