Binary mixed micelles of polysorbates (Tween 20 and Tween 60) and bile salts (Na-hyodeoxycholate and Na-cholate): Regular solution theory and change of pKa values of micellar bile acid – a novel approach to estimate of the stability of the mixed micelles

• In the mixed micelles of bile salts (HD, C) and Tween 20 or 60 ΔpKa characterizes the stabilization.
• Mixed micelles of HD and Tween 20 are more stable than the system HD and Tween 60.
• Mixed micelles of C and Tween 60 are more stable than the system of C and Tween 20.

The study examines binary mixed micelles of sodium-hyodeoxycholate (HD) and polysorbates: Tween 20 (T-20), Tween 60 (T-60) from α = 0.1 to α = 0.5 molar fraction of polysorbates in the mixture of surfactants at 298.15 K. At every value of α there are synergistic reactions between different building units of mixed micelle. In the mixed micelle HD–T-20 the thermodynamic stability of the real mixed micelle compared to the ideal binary mixed micelle is greater than in the mixed micelle HD–T-60. The pKa values of the micellarly bound bile acids are higher than the pKa values of non-micellar bile acids (ΔpKa). At constant ionic strength this means that the change in Gibbs energy of protolysis of the micellar bile acid has excess Gibbs energy determined by the energy of stability of bile acid in micellar phase. The ΔpKa value of the micellar bile acid from the mixed micelle of bile salts and homologous polysorbates, similar to the interaction coefficient (βij) from regular solution theory, characterizes the thermodynamic stability of the mixed micelle and also enables its fast evaluation.

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