The conductances of cetylpyridinium chloride (CPyCl) + tetradecyltrimethylammonium bromide (TTAB) mixtures over the entire mole fraction range of CPyCl (x(CPyCl)) were measured in aqueous binary mixtures of polyethylene glycol 1000 (PEG 1000) containing 0.5, 1, 2, 5 and 10 wt% of PEG 1000 at 30 degrees C. From the conductivity data, the critical micellar concentration (cmc), degree of counter ion association (chi) and the standard free energy of micelle formation (Delta(GM)(o)) of CPyCl and TTAB were computed. The Delta G(M)(o) value is further divided into the hydrophobic contribution of free energy of transfer of the surfactant hydrocarbon chain from the medium to the micelle (Delta G(HP)(o)), and the energy associated with the surface contributions (Delta G(S)(o)) consisting of electrostatic interactions between the head groups and counter ions. Both contributions show a linear dependence on the amount of PEG additive. These results have been explained on the basis of the medium effects of aqueous PEG. The mixed micelle formation by CPyCl+TTAB show non-ideal behavior which is quite similar in the absence, as well as in the presence, of PEG additive as evaluated by using the regular solution theory. The interaction parameter, b, is always negative and remains almost constant with respect to the amount of PEG additive. This indicates that the mixed micelle formation occurs mainly due to the synergistic interactions between the unlike surfactant monomers only.
