Micellisation of Binary Mixtures of Surfactants Na-Deoxycholate–Na-Decyl Sulfate and Na-Hyodeoxycholate–Na-Decyl Sulfate in Water Solutions: Rational Development of the Thermodynamic Model for the Excess Gibbs Energy (G^E)

Micellisation of two binary mixtures, sodium decyl sulfate (NaDS)–sodium deoxycholate (NaDC) and sodium decyl sulfate (NaDS)–sodium hyodeoxychlolate (NaHDC), is examined, in consideration with the different molar fractions of the monomers (x = 0.1–0.9). In bile acids, the OH groups attached to steroid skeletons are spatially ecraned, while the sulfate group of NaDS is not ecraned. Thus, NaDC and NaHDC in relation to NaDS have different coordination numbers in the micelle pseudophase. On the basis of stereochemical analysis, rational design shows that the excess Gibbs energy (G^E) of formation of the binary mixed micelle throughout the whole composition range can be described with a two-parameter Margules function, while the G^E function from the regular solution theories is applicable only in the narrow interval of the bile salts molar fractions (x = 0.5– 0.8). Coefficient of activity (hypothetical) of the infinitely diluted mixed micelle of NaDS in the presence of NaDC shows significant thermodynamic stabilization of decyl sulfate ions in comparison with the infinitely diluted hypothetical mixed micelle with NaHDC. This stabilization of decyl sulfate ions is probably due to the cooperative binding of NaDS by parallel OH groups of NaDC.