On the forces of hydrophobic attraction in wetting films of aqueous solutions

On the basis of the isotherms of disjoining pressure of aqueous wetting films the contact angles, θ were calculated as a function of the electrical potentials of the substrate, φ1, and film air interface, φ2 and on the degree of its hydrophobicity. When calculating θ values, dispersion repulsion and electrical and hydrophobic attraction forces are taken into account. In the case of high values of φ1 and φ2, which differ in sign, formation of high contact angles is caused by the electrostatic attraction forces only. Hydrophobic attraction forces play an important role when both interfaces of a wetting film are hydrophobic. This may be realized in the absence of surfactants or when the surfactant molecules are hydrodynamically shifted to the rear part of the rapidly approaching bubble. This indicates in particular the importance of hydrodynamic effects during flotation. These effects influence not only the kinetics of the bubble-to-particle approach but also the forces of attachment.