Is latex surface charge an important parameter for foam stabilization?

We describe the facile production of highly stable foams stabilized solely by cationic polystyrene latex particles. Three model polystyrene latexes were synthesized using either a cationic 2,2'-azobis(2-diisobutyramidine) dihydrochloride (AIBA) or an anionic ammonium persulfate (APS) radical initiator: a 724 +/- 81 nm charge-stabilized cationic polystyrene latex [AIBA-PS], an 800 +/- 138 nm sterically stabilized cationic latex prepared using a poly(ethylene glycol) monomethacrylate macromonomer [PEGMA-AIBA-PS], and a 904 +/- 131 nm charge-stabilized anionic polystyrene latex [APS-PS], respectively. The effect of particle surface charge, latex concentration, and solution pH on foam stability was studied in detail. The PEGMA-AIBA-PS latex proved to be the best foam stabilizer even at relatively low latex concentrations (3.0 wt %), with long-term foam stabilities being obtained after drying. The AIBA-PS latex also produced stable foams, albeit only at higher latex concentrations. However, the APS-PS latex proved to be an ineffective foam stabilizer. This is believed to be primarily due to the anionic surface character of this latter latex, which prevents its adsorption at the anionic air-water interface. This hypothesis is supported by the observation that the AIBA-PS latex no longer acts as an effective foam stabilizer above its isoelectric point (pH 7.04). Scanning electron microscopy studies revealed the formation of well-defined latex bilayers within dried foams, which indicates that the wet air bubbles are stabilized by latex monolayers prior to drying. However, little or no long-range ordering of the latex particles was observed on the surface of the bubbles, which is presumably related to the latex polydispersity.