Molecular dynamics study of the structure of an oppositely charged polyelectrolyte and an ionic surfactant at the air/water interface

The adsorption of polyelectrolyte–surfactant mixture of sodium poly(acrylic acid) (NaPAA) and dodecyl trimethyl ammonium bromide (C12TAB) at the air/water interface was studied using molecular dynamics simulation. In our simulations, the transition from monolayer adsorption of a polymer–surfactant complex to a multilayer structure was observed with increasing surfactant concentration at the interface. For the multilayer structure, the two polyelectrolyte chains were linked by two layers of surfactant molecules which adopted a tail-to-tail arrangement. The results indicated that the electrostatic interaction was the main driving force for the binding of surfactants to the polyelectrolyte, meanwhile the hydrophobic interaction between surfactant tails induced a layer-by-layer packing arrangement at high surfactant concentration. The dynamic properties of inorganic ions implied that the complex of polyelectrolyte and surfactant was an ion-exchange process. Our conclusions are in accordance with experimental results on polyelectrolytes and ionic surfactants.