Segregation versus Interdigitation in Highly Dynamic Polymer/Surfactant Layers.

Many polymer/surfactant formulations involve a trapped kinetic state that provides some beneficial character to the formulation. However, the vast majority of studies on formulations focus on equilibrium states. Here, nanoscale structures present at dynamic interfaces in the form of air-in-water foams are explored, stabilised by mixtures of commonly used non-ionic, surface active block copolymers (Pluronic®) and small molecule ionic surfactants (sodium dodecylsulfate, SDS, and dodecyltrimethylammonium bromide, C12TAB). Transient foams formed from binary mixtures of these surfactants shows considerable changes in stability which correlate with the strength of the solution interaction which delineate the interfacial structures. Weak solution interactions reflective of distinct coexisting micellar structures in solution lead to segregated layers at the foam interface, whereas strong solution interactions lead to mixed structures both in bulk solution, forming interdigitated layers at the interface.

The interfacial structure of polymeric surfactant stabilised air-in-water foams.

Small-angle neutron scattering was used to probe the interfacial structure of nitrogen-in-water foams created using a series of tri-block polymeric surfactants of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EOx-POy-EOx) range, from which the nature of the polymeric interface could be characterised. The data follow a pronounced Q(-4) decay, along with a number of inflexions and weak but well-defined peaks. These characteristics were well-described by a model embodying paracrystalline stacks of adsorbed polymer layers, whose formation is induced by the presence of the air-water interface, adsorbed at the flat air-water (film lamellae) interface. A minimum of approximately five paracrystalline polymer layers of thickness of the order of 85-160 Å, interspersed with somewhat thicker (400 Å) films of continuous aqueous phase were found to best fit the data. The thickness of the layer (L) was shown to follow a relationship predicted by anchor block dominated polymer adsorption theories from non-selective solvents, L ∼ EO(1)PO(1/3). The insight gained from these studies should permit a more rational design of polymeric stabilisers for hydrophilic polyurethane foams.