Binary Mixed Micelles of Hexadecyltrimethylammonium Bromide–Sodium Deoxycholate and Dodecyltrimethylammonium Bromide–Sodium Deoxycholate: Thermodynamic Stabilization and Mixed Micelle Solubilization Capacity of Daidzein (Isoflavonoid)

In this paper, the thermodynamic properties of binary mixtures of the surfactants hexadecyltrimethylammonium bromide (HA)–sodium deoxycholate (DC) and dodecyltrimethylammonium bromide (DA)–sodium deoxycholate (DC) in an aqueous NaCl solution (0.3 mol kg–1) in the temperature interval T = 273.1–323.1 K are examined. In binary mixed micelles such as HA–DC and DA–DC, there are synergistic interactions between structurally different micellar building units. The dependence of the molar excess Gibbs free energy of the formation of the binary mixed micellar pseudophase on the micellar molar fractions of the surfactants (gE = f(xi)) is a symmetric function (a first-order Margules function) where the molar excess entropy is not zero, i.e., the strict theory of regular mixtures does not apply. If the phenomenon of enthalpy–entropy compensation is valid for each partial thermodynamic process during the formation of a binary mixed micellar pseudophase (at a constant temperature) and there is an interval of surfactant mole fraction from the initial binary mixture, where each ratio between the enthalpy of a certain partial thermodynamic process and the RST enthalpy is constant, then the function gE = f(xi) is a symmetric function. Both of the investigated binary micellar pseudophases in the solubilization of daidzein (D) exhibit synergistic solubilization in relation to the micellar solubilization of D with a system of monocomponent surfactants. The molar excess Gibbs free energies of D micellar solubilization with the mixed micelles HA–DC and DA–DC are negative, which means that binary mixed micellar pseudophases with the solubilizate D are more thermodynamically stable than systems with monocomponent micellar pseudophases containing solubilizate D.