Head-group-induced structural micellar transitions in mixed cationic surfactants with identical hydrophobic tails

Abstract

The mixed micelle formation by hexadecyltrimethylammonium bromide (HTAB), hexadecylpyridinium bromide (HPyBr), and hexadecylpyridinium chloride (HPyCl) with benzylhexadecyldimethylammonium chloride (BHDACl) was studied with the help of conductivity, κ, and viscometric measurements. Each κ curve for HTAB, HPyBr, and HPyCl showed a single break, whereas double breaks were observed in the case of BHDACl and its binary mixtures with HTAB, HPyBr, and HPyCl. The first break, c1,M, was due to mixed-micelle formation, whereas the second one, c2,M, was due to the structural micellar transitions in the mixed micelles formed during the first break. The quantitative analysis of the mixed-micelle formation corresponding to the first break was carried out with the help of regular solution and Motomura's approximations. The BHDACl plus HTAB and BHDACl plus HPyBr mixtures showed weak synergistic interactions, whereas the BHDACl plus HPyCl mixture was close to ideal. A variation in the degree of dissociation corresponding to the first and second breaks demonstrated a similar variation over the whole mixing range and this was attributed to the identical nature of synergism in both kinds of micellar aggregates available at c1,M, and c2,M. They were rich in BHDACl over most of the mole fraction range. Unlike conductivity measurements, the relative viscosity demonstrated that the micellization of HTAB, HPyBr, and HPyCl proceeded through a minimum, whereas that for BHDACl and its mixtures showed a strong maximum in each case, being stronger in the case of the BHDACl plus HTAB and BHDACl plus HPyCl mixtures.