The study examines binary mixed micelles of sodium-hyodeoxycholate (HD) and polysorbates: Tween 20 (T-20), Tween 60 (T-60) from alpha = 0.1 to alpha = 0.5 molar fraction of polysorbates in the mixture of surfactants at 298.15 K. At every value of a 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 pK(a) values of the micellarly bound bile acids are higher than the pK(a) values of non-micellar bile acids (Delta pK(a)). 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 Delta pK(a) value of the micellar bile acid from the mixed micelle of bile salts and homologous polysorbates, similar to the interaction coefficient (beta(ij)) from regular solution theory, characterizes the thermodynamic stability of the mixed micelle and also enables its fast evaluation. (C) 2015 Elsevier B.V. All rights reserved.