Nonequilibrium mesoscale surface structures: The adsorption of polymer-surfactant mixtures at the solid/liquid interface

Polymer - surfactant mixtures have broad application in modern society; however, their interfacial behavior is not well understood. Soft-contact atomic force microscopy has been used to visualize the adsorbed layer at the graphite/solution interface in mixed aqueous solutions of sodium dodecyl sulfate (SDS) and poly-(vinylpyrrolidone) (PVP). The nonequilibrium adsorbed layer is remarkable, consisting of micrometer-sized domains of surfactant-rich, ordered aggregates interspersed by disordered regions of ill-defined polymer-rich adsorbate. The ordered domains grow to cover the entire substrate after many hours. The nonuniform surface coverage and the long but finite equilibration times have important implications for colloid stability. These first observations of the heterogeneous nature of mixed polymer- surfactant adsorption highlight the importance of applying direct visualization to further our understanding of these ubiquitous formulations.