Influence of polymerization degree on the dynamic interfacial properties and foaming ability of ammonium polyphosphate (APP)-surfactant mixtures

Abstract

Ammonium polyphosphate (APP) is an inorganic polymer and has been widely used as a flame retardant. Illustrating the interaction between APP and surfactants is significant for exploring the application of fire-fighting foam stabilized by APP/surfactant mixtures. In this work, we studied the influence of APP with different polymerization degrees on dynamic interfacial properties and foaming ability of the two oppositely charged surfactants. The results show the presence of APP only moderately affects the interfacial properties and foaming ability of the anionic surfactant, sodium dodecyl sulfate (SDS). However, when APP is added to cationic cetyltrimethylammonium bromide (CTAB) solutions, strong APP-surfactant interactions enhance the CTAB adsorption capacity at the interface leading to a decrease in the equilibrium surface tension. The results of turbidity and dynamic light scattering measurements confirm the formation of CTAB/APP complexes in the solution, which changes the adsorption kinetics and foaming ability of the solution. The difference in foaming ability and the influence of APP polymerization degree on adsorption kinetics for CTAB/APP solution can be explained by the characteristic time t* which reflects the adsorption rate of molecules from the bulk phase to the surface. Cryo-TEM results confirm that the addition of APP with different polymerization degrees strongly affects the molecular aggregation behaviour of surfactants.