New Perspective on the Cliff Edge Peak in the Surface Tension of Oppositely Charged Polyelectrolyte/Surfactant Mixtures

We present how dramatically the nonequilibrium nature of an oppositely charged polyelectrolyte/surfactant mixture can affect the interfacial properties. We show for the first time that the cliff edge peak in the surface tension of the poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate system is produced as a direct result of depletion of surface-active material from the bulk solution due to a slow precipitation process in the phase separation region. Simple illustrations are given of how to control the production of the peak, to eliminate the feature for equivalent aged solutions through the use of different sample handling methods, and even to change its characteristics at short surface ages. The potential to tune nonequilibrium, steady-state interfacial properties for such strongly associating systems is clearly demonstrated. We propose that our findings in general may be applicable to a broad range of mixtures containing surfactants and oppositely charged macromolecules such as polymers, proteins, and DNA.