Head-group-modification-controlled mixing behavior of binary cationic surfactants: conductometric, viscometric, and NMR studies

The mixed-micelle formation by hexadecyltrimethylammonium bromide (HTAB) with hexadecyltriphenylphosphonium bromide (HTPB) and hexadecyltributylphosphonium bromide (HTBB), and by hexadecylpyridinium bromide (HPyBr) with HTPB and HTBB has been studied with the help of conductivity, viscometry, and NMR studies. The conductivity studies showed close-to-ideal mixing in all cases with weak synergistic interactions. The relative viscosity is more sensitive than the conductivity and demonstrated that the micellization of HTAB, HPyBr, HTPB, and HTBB proceeded through the appearance of a minimum, whereas their mixtures showed a strong maximum in each case. NMR measurements suggested that bulky head groups of HTPB and HTBB induce significant steric hindrance in the mixed state upon mixing with HTAB and HPyBr. The appearance of the maximum shown by the relative viscosity in the mixed-micellization process is due to steric compulsions, which may also lead to some structural transitions.