A Thermodynamic Study on Micelle Formation of Cationic Surfactants Derived from Different Bile Acids

The synthesis of a series of new cationic bile salt derivatives (CBADs), the hydrochloride salts of ethylenediamine monocholate (EdaC), ethylenediamine monodeoxycholate (EdaDC), ethylenediamine monochenodeoxycholate (EdaCDC) and ethylenediamine monoursodeoxycholate (EdaUDC) was carried out and aqueous solution properties such as critical micellization concentration (CMC) and degree of counterion binding (β) were determined by means of surface tension measurement (drop volume method) at different temperatures from 10°C to 50°C (at every 10°C). Surfactant properties of CBADs such as micellization, β and surface activity were noted essentially the same (except for EdaC) as those of the corresponding free bile salts, while CBADs have greater membrane permeability and biomembrane perturbation ability compared to the sodium salts of free bile acids. Examination of added salt effect on CMC indicated the degrees of counterion binding to generally be 0.25 (very low compared to alkyl chain type surfactants) not only for CBADs but free and conjugated bile acid salts as well depending little on temperature. Thermodynamic parameters subsequent to micellization (ΔG0m, ΔH0m, and ΔS0m) were estimated for the respective CBADs as a function of temperature. The entropy-enthalpy compensation relationships which have in common a compensation temperature (Tc) of 300 were found to hold for any CBAD. These parameters are discussed.