Surfactant-polymer aggregates of mixed cationic micelles and anionic polyelectrolytes: a surfactant head group contribution

Abstract

Single and mixed-micelle formation by benzyldimethylhexad-ecylammonium chloride (BHDACl), hexadecyltrimethylammonium bromide (HTAB), and their mixtures in aqueous anionic polyelectyrolytes, viz. carboxymethylcellulose sodium salt, dextran sulfate sodium salt, polystyrene sulfonate sodium salt, and polyacrylic acid sodium salt, were studied with the help of conductivity, κ, viscosity, η, turbidity, τ, and NMR studies. κ showed a single aggregation process for pure and mixed surfactants and the concentration at which it took place was slightly higher than the actual critical micelle concentration in pure water. Hence, it was termed the critical aggregation concentration (cac). Application of the Clint equation revealed that the attractive interactions between BHDACl and HTAB even in the presence of polyelectrolytes were responsible for the large deviation from ideal behavior. Both η_r and τ did not indicate the presence of a cac but instead they showed another aggregation process at much lower concentration than the cac, which was termed the pre-cac. They also demonstrated strong electrostriction effects in the case of BHDACl—polyelectrolyte systems, whereas weak electrostriction effects were observed for HTAB—polyelectrolyte. The former was explained on the basis of weak electrostatic interactions due to the steric hindrances created by an aromatic ring in the BHDACl head group. NMR results showed that the proton resonance of head groups and hydrophobic tails of both surfactants underwent deshielding and shielding effects, respectively. The former was more significant in the case of BHDACl, whereas the latter one was more significant in the case of HTAB. Both factors indicated the formation of compact mixed-micelle-polyelectrolyte aggregates.