Structure and properties of fluids composed of polyelectrolyte and ionic surfactant in organic phase: Poly(acrylic acid) and didodecyldimethylammonium bromide

The structure and properties of the oil-rich microemulsion (L2 phase) containing anionic polyelectrolyte, poly(acrylic acid) (PAA-H, M-W= 450 000) and its sodium salt PAA-Na (M-W = 30 000), were investigated. The cationic surfactant, didodecyldimethylammonium bromide (DDAB), which generates the rodlike aggregates in cyclohexane, was selected. Small-angle X-ray scattering, dynamic light scattering, electric conductivity, and viscosity measurements were performed. The rodlike structure was unchanged upon dissolving the polymers. PAA-Na is a strong polyelectrolyte, which takes a rather extended conformation in the bulk aqueous solution. However, PAA-Na adsorbed DDAB molecules by electrostatic interaction in the microemulsions. Then, the extended conformation was loosened in the microemulsion and was dissolved within the water pool of the DDAB aggregates. The characteristic size of the PAA-Na dissolved within the water pool of the DDAB aggregates was similar to that of the original DDAB aggregates. High-molecular-weight polymer PAA-H was also dissolved within the water pool, and the polymer chain connected the rodlike aggregates. However, the diameter of the rodlike constituent for the connected structure is almost the same as that of the original DDAB aggregates.