Polymer-Induced incompatibility in the mixed micelle formation of cationic surfactants with bulky polar head groups

The conductances of tetradecyltrimethylammonium bromide (TTAB)+tetradecyl triphenylphosphonium bromide (TTPB) mixtures over the entire mole fraction range were measured in aqueous poly(vinyl pyrrolidone) (PVP) containing 1 to 10 wt% of PVP at 30°C. Each conductivity (κ) curve showed a single break corresponding to the mixed micelle formation in the presence of PVP over the whole mole fraction range. From the conductivity data, various micellar parameters in the presence of PVP were computed and discussed in terms of micelle-polymer interactions. The mixing behavior of TTAB+TTPB in pure water was close to ideal, whereas in the presence of PVP a significant degree of nonideality was sobserved that was evaluated by computing the mole fraction of TTAB in the mixed micelle state (x ₁) and the interanction parameter (β) by using the regular solution approach. The former is less than the value in the ideal state, and the latter is much greater, indicating the enhancement in the antagonistic mixing behavior of TTAB+TTPB in the presence of PVP. The results have been explained on the basis of an increase in the incompatibility between the monomers of TTAB and TTPB in the mixed state due to the steric hindrances created by the PVP upon adsorption at the micelle-solution interface. The results have also been further supported by comparing with those of structurally similar cationic binary combinations in the presence of PVP.