Role of hydrophobic force in the thinning of foam films containing a nonionic surfactant

The thin film pressure balance (TFPB) technique was used to study the kinetics of film thinning at various concentrations of a non-ionic surfactant, methyl isobutyl carbinol (MIBC), in the presence of 0.1 M sodium chloride (NaCl). The experimental data were compared with the predictions from the Reynolds equation to determine the contributions from hydrophobic force to film thinning. Despite the high electrolyte concentration employed, the hydrophobic force was 15–90 times stronger than the van der Waals force over the MIBC concentrations investigated. It was found that the hydrophobic force decreased with increasing MIBC concentration, which corroborated well with the changes in film lifetime, foam lifetime, and the critical rupture thickness. On the other hand, both surface tension and film elasticity change little at low surfactant concentrations, particularly at the concentrations where the non-ionic surfactant is used for flotation. It is, therefore, suggested that at relatively low MIBC concentrations foams are destabilized by the hydrophobic force, which is dampened by the surfactant.