Effect of pH and NaCl Concentration on the Stability of Surfactant-Free Foam Films

Abstract

The stability of surfactant-free foam films was studied in NaCl solutions of varying concentration and pH using the thin film pressure balance (TFPB) technique. In pure water, it was not possible to produce foam films due to weak film elasticity and the strong attractive hydrophobic force in foam films, despite the presence of strong repulsive double-layer force. In the presence of a very small amount of an electrolyte, however, the hydrophobic force was dampened, allowing metastable foam films to form. As the NaCl concentration was raised above 10(-6) M, the film stability diminished as a result of double-layer compression. The TFPB technique was also used to measure the equilibrium film thicknesses (H(e)) in 10(-5) M NaCl solutions of varying pH; H(e) reached a maximum of approximately 130 nm at pH 6.0-7.3, and decreased on either side of this pH range as a result of the increased ionic strength caused by the HCl and NaOH added to control the pH. The hydrophobic force in surfactant-free foam films was maximum at pH 7.3, where the concentration of electrolytes, including that of H(2)CO(3) species, was minimum.