CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 440
Obr.13.1:Two examples of the “wetting” of a solid.The difference between the work of cohesion and the work of adhesion is called thespreading
coefficient,Ss,
Ss=wa−wc=γsg−γs−γg. (13.13)
13.1.6 Contact angle
If the spreading coefficient of a liquid on a solid is positive, i.e. the work of adhesion is greater
than the work of cohesion, the liquid spreads across the solid surface and it is said towet the
solid completely. If, however, the spreading coefficient is negative, the liquid forms drops
on the surface (see Figure13.1), with the following relation applying between the interfacial
tensions
γsg=γs+γgcosθ , (13.14)
whereθis thecontact angle, i.e. the angle formed by the liquid phase on contact with the
surface. Based on the value of this angle we distinguish two cases:
- θ < π/2, the solid surface is (incompletely) “wetted” (Figure13.1a),
- θ > π/2, the body surface does not wet (Figure13.1b).
If there are two mutually partially miscible liquids present in a system in addition to the
gas phase, we may also use relation (13.13), while the lower liquid phase takes over the role of
the solid phase.