Isoelectric state and stability of foam films, bubbles and foams from PEO–PPO–PEO triblock copolymer (P85)

Influence of pH of P85 copolymer solutions on stability of microscopic foam films (static conditions), lifetime of single bubbles at solution surface (quasi-static conditions), volume of the foam formed (dynamic conditions) and time of rupture of the foam column was investigated. Variations of the film equilibrium thickness as a function of pH were determined for different ionic strengths of the solutions using microinferometric method, while the combined pneumatic–mechanical method was applied in experiments on foams. It was found that lowering the pH reduced stability of the foam films, and at lower ionic strength the films ruptured at pH 2.9 (isoelectric point). Simultaneously, the lifetime of single bubbles was much shorter at lower pH of the P85 solutions. The average life, t _av, was 11.1 s at pH 5.8, while at pH 3.0, only 3.1 s. Under dynamic conditions the pH lowering did not significantly influence the solution foamability.