Effects of bulk aggregation on PEI-SDS monolayers at the dynamic air-liquid interface: depletion due to precipitation versus enrichment by a convection/spreading mechanism

The dynamic interfacial adsorption from mixtures of high molecular weight poly(ethylene imine) (PEI) and sodium dodecyl sulfate (SDS) has been studied by neutron reflectometry and ellipsometry at the expanding air-water interface created by an overflowing cylinder. Aggregation of PEI and SDS in the bulk solution has very different consequences on the dynamic interfacial properties depending on the polyelectrolyte charge density. At high pH, where PEI has low charge density, bulk aggregation depletes the solution of surface-active monomers and small complexes; the aggregates do not adsorb to the dynamic liquid interface as a result of their slow diffusion, and the surface excess is depleted. At low pH, where PEI has a high charge density, bulk aggregates reach the surface and reorganise to spread thin transient patches of material across the interface, enriching rather than depleting the surface excess. This study highlights that in order to optimise the performance of polymer-surfactant mixtures under conditions relevant to processing and applications, further work is required to understand their behaviour under non-equilibrium conditions.