Aggregation Behavior of Tetracarboxylic Surfactants Derived from Cholic and Deoxycholic Acids and Ethylenediaminetetraacetic Acid

The reaction of 3 beta-aminoderivatives of cholic and deoxycholic acids (steroid residues) with dimethyl ester of ethylenediaminetetraacetic acid (bridge) leads to the formation of dimers carrying four carboxylic organic function,, two of them located on the side chain of each steroid residue and the other two on the bridge. As tetrasodium stilts. these new compounds behave as surfactants and have been characterized by surface tension. fluorescence intensity of pyrene (as a probe), and static and dynamic light scattering measurements. Thermodynamic parameters for micellization were obtained from the dependence of the critical micelle concentration (cmc) with temperature. For both surfactants. the fraction of bound counterions is close to 0.5. The aggregation behavior is similar to one of their bile salt residues [i.e., sodium cholate (NaC) and sodium deoxycholate (NaDC)] and can be Summarized its follows: (i) molecular areas at the interface for the new surfactants are fairly close to twice the value for a single molecule in a monolayer of natural bile salts; (ii) the environment where pyrene is solubilized is very apolar, as in natural bile salt aggregates; (iii) Gibbs free energies (per steroid residue) for micellization are not far from published values for NaC and NaDC, and the differences can e understood on the basis of less hydrophobicity of the new surfactants due to the charges in the bridge: and (iv) as for NaC and NaDC. aggregates have rather low aggregation numbers (which depend oil the amount of added inert salt, NaCl). A structure based oil the disklike model accepted for small bile salt aggregates is proposed.