PREDICTION OF CONTACT ANGLES ON THE BASIS OF THE FRUMKIN-DERJAGUIN APPROACH

The Frumkin-Derjaguin approach was used for the calculation of contact angles on the basis of isotherms of disjoining pressure of wetting films. The contact angles are calculated for aqueous solutions of electrolytes of low concentration on quartz surface, taking into account forces of molecular repulsion, electrostatic attraction and structural forces. A high degree of surface hydrophilicity may be attained under the predominant action of repulsive structural forces. Large contact angles are caused by the predominant action of hydrophobic attraction forces and/or electrostatic attraction forces acting between two oppositely charged film interfaces. At a moderate degree of hydrophilicity, in the range of contact angles from 10-15 degrees to 50-60 degrees, it is sufficient to take into account the molecular and electrostatic forces only. This determines the region of applicability of the DLVO theory. The parameters of the short-range hydrophobic forces seem to be the same in wetting films and in water interlayers between two hydrophobic surfaces.