Micellization and adsorption behaviour of bile salt systems

The interfacial and bulk phase properties of sodium cholate (NaC), sodium deoxycholate (NaDC), sodium taurocholate (NaTC), sodium taurodeoxycholate (NaTDC) and their equimolar binary and ternary combinations in aqueous solution have been investigated using surface tensiometry, conductometry, microcalorimetry, and fluorescence probing. The obtained experimental results are utilized to evaluate critical micellar concentration (cmc), counterion binding (f), surface excess, minimum area per molecule (A(min)), thermodynamics of adsorption and micellization, and microenvironment of the bile-salt micelles. The magnitude of counterion binding and its temperature dependence was found to be low. Binary and ternary bile salt mixtures show higher and lower A(min) values, respectively, compared with the (ideal)A(min) at the experimental temperatures. The low I-1/I-3 values of single and mixed bile salts suggest the microenvironment of the micellar entities to be nonpolar as hydrocarbons. The enthalpy of micellization values obtained following the methods of van't Hoff and microcalorimetry have been compared. The composition of the mixed micelles have been estimated on the basis of regular solution, Clint, Rubingh and Rubingh-Holland theories along with the activity coefficients and interaction parameters at different temperatures.