On the interaction of low molecular weight poly(diallyldimethylammonium chloride) and sodium dodecylsulphate in low surfactant-polyelectrolyte ratio, salt-free solutions

Abstract

Coacervation is widely used in formulations to induce a beneficial character to the formulation but non-equilibrium effects are often manifest. Electrophoretic (eNMR), pulsed-gradient spin-echo NMR (PGSE-NMR) and small-angle neutron scattering (SANS) have been used to quantify the interaction between low molecular cationic poly(diallyldimethylammonium chloride) (PDADMAC) and the anionic surfactant sodium dodecylsulphate (SDS) in aqueous solution as a model for the precursor state to such non-equilibrium processes. The NMR data show that within the low surfactant concentration one-phase region, an increasing surfactant concentration leads to a reduction in the charge on the polymer and a collapse of its solution conformation, attaining minimum values coincident with the macroscopic phase separation boundary. Interpretation of the scattering data reveals how the rod-like polymer changes over the same surfactant concentration window, with no discernible fingerprint of micellar type aggregates, rather the emergence of disc-like and lamellar structures. At the highest surfactant concentration, the emergence of a weak Bragg peak in both the polymer and surfactant scattering suggest these pre-cursor disc and lamellar structures evolve into paracrystalline stacks which ultimately phase separate. Addition of the non-ionic surfactant hexa(ethylene oxide) dodecyl ether (C12E6) to the system seems to have little effect on the PDADMAC/SDS interaction as determined by NMR, merely displacing the observed behaviour to lower SDS concentrations, commensurate with the total SDS present in the system. In other words, the PDADMAC causes the disruption of the mixed SDS/C12E6 micelle, leading to SDS-rich PDADAMC/surfactant complexes coexisting with C12E6 – rich micelles in solution