Dilational visco-elasticity of polyelectrolyte/surfactant adsorption layers at the air/water interface: Poly(vinyl pyridinium chloride) and sodium dodecylsulfate

The dilational dynamic surface elasticity of mixed aqueous polyvinylpyridinium chloride/sodium dodecyl sulfate solutions (PVPmCl/SDS) was measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nano-scale. The concentration dependence of the real part of the dynamic surface elasticity for the mixed PVPmCl/SDS solutions in 0.1 M NaCl and without added NaCl qualitatively agreed with the corresponding results for some previously studied polyelectrolyte/surfactant solutions. This similarity indicates that strong changes of the surface visco-elasticity in a narrow concentration range can be explained by the formation of microparticles in the surface layer. The addition of 0.1 M NaCl to PVPmCl/SDS solutions led to an almost two times decrease of the surface elasticity at low concentrations and to the acceleration of the adsorption layer formation. At the same time the imaginary part of surface elasticity of PVPmCl/SDS solutions was significantly higher than the corresponding results for the earlier studied systems and the adsorption layers proved to be visco-elastic in a broad concentration range. The application of the Maxwell rheological model in this case allowed the calculation of the main relaxation time. The main relaxation process is the surfactant exchange between the surface layer and the bulk phase or in the surface layer between free surfactant molecules and those bound in aggregates. The distinction of the surface visco-elasticity of PVPmCl/SDS solutions from the previously studied systems can be explained by the lower stability of polyelectrolyte/surfactant complexes in the adsorption layer.