Impact of local inhomogeneities on the complexation between poly(diallyldimethylammoniumchloride) and sodium dodecyl sulfate

The nonequilibrium behavior of oppositely charged polyelectrolyte(PE)/surfactant(S) mixtures has been the focus of recent investigations due to its industrial relevance. The studies indicated that the initial local inhomogeneities may significantly affect the characteristics of the formed PE/S nanoassemblies. However, this phenomenon is not clearly understood and it has not been investigated systematically. In the present work, the effects of polyelectrolyte and salt concentration gradients were studied in poly(diallyldimethylammoniumchloride) (PDADMAC)/ sodium dodecyl sulfate (SDS)/ sodium chloride (NaCl) mixtures. The results revealed two distinct nonequilibrium phenomena at low ionic strengths. If the SDS concentration is significantly larger than that of the polyelectrolyte monomers (i.e. at surfactant excess), then the system can be instantly arrested in a stable colloidal dispersion state, where the size of the trapped overcharged aggregates is primarily dependent on the local PDADMAC concentration gradients. However, if the polyelectrolyte charges largely exceed the amount of surfactant molecules (i.e. at polyelectrolyte excess), then another nonequilibrium feature, e.g. a highly reproducible non-monotonic variation of the turbidity with time is observable. This is attributed to the extremely slow approach towards the equilibrium state due to the heterogeneous local distribution of the surfactant ions and their hindered repartitioning process between the PDADMAC/SDS complexes. The results of the present work may be exploited to tune the time dependent surface and bulk features of PE/S systems, which are of crucial importance for their commercial applications.