Phase structural transitions of polyelectrolyte-surfactant complexes between poly(vinylamine hydrochloride) and oppositely charged sodium alkyl sulfate

Synchrotron small-angle X-ray scattering was used to investigate the nanostructures of polyelectrolyte-surfactant complexes between linear poly(vinylamine hydrochloride) chains and oppositely charged surfactants of sodium alkyl sulfate (SCnS) at room temperature (similar to 23 degreesC). Both the pH variance of complexation and the surfactant tail length change could induce phase structural transitions. A novel phase structure of a 2D distorted hexagonal packing of deformed cylinders or ribbons with a = b and an enclosed angle a larger than 120 degrees was, for the first time, observed in polyelectrolyte-surfactant complexes at n = 11-12 and certain pH values. In sequence with increasing pH values of complexation between the PVAC1 and SC11S solutions from pH = 5.9 to pH = 12.0, the structures of 2D distorted hexagonal, bilayer lamellar, and 2D hexagonal close packing of cylinders were determined, When the pH values for complexation were fixed at 5.9 and 8.9, the decrease in carbon atom numbers of surfactant alkyl chains from n = 12 to n = 9 induced the same structural transition. This structural transition was caused by the increase in the ratio of the area of polar groups to the volume of alkyl chains of surfactants arranged inside the complexes, thus increasing the mean curvature at the polar/apolar interfaces of surfactant ions. The increase in surfactant tail length could also induce a lamellar structural transition. When the lamellar structure in complexes was formed by SCnS with n = 10 and 11, the alkyl chains of surfactant were fully extended and perpendicular to the lamellar surface, while when the lamellar structure in complexes was formed by SCnS with long tails of n = 14 and 16, the alkyl chains of surfactant were partially overlapped or tilted to the lamellar surface in the bilayer arrangement due to the stronger hydrophobic interactions.