Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 106

Solid Surface

Perfect With Defects

FIGure 5.1 Solid surface molecules' defects: (a) perfect crystal; (b) surface with defects.

GL

Air Liquid

CA

GS

GLS

Solid

FIGure 5.2 The state of equilibrium between the surface tensions of liquid (GL)-solid
(GS)-liquid/solid (GLS)-contact angle (CA).

(Surface tension of solid) (γ S ) = Surface tension of solid/liquid (γ SL ) +

Surface tension of liquid (γ L) (Cos (θ)) (5.1)

γ S = γ L cos (θ) + γ SL (5.2)

γ L cos (θ) = γ S − γ SL (5.3)

where γ is the interfacial tension at the various boundaries between solid, S, liquid,
L , and air (or vapor) phases, respectively. The relation of Young's equation is easy to
understand as it follows from simple physics laws. At the equilibrium contact angle,
all the relevant surface forces come to a stable state (Figure 5.2).
The geometrical force balance is considered only in the X-Y plane. This assumes
that the liquid does not affect the solid surface (in any physical sense). This assump-
tion is safe in most cases. However, only in very special cases, if the solid surface is
soft (such as with contact lens), then tangential forces will also need to be included in
this equation (as extensively described in the literature). There exists extensive data
that convincingly support the equation for liquids and solids.

5.2 SolId SurFace tenSIon (WettInG
ProPertIeS oF SolId SurFaceS)
The wetting of solid surfaces is very apparent when considering the difference
between Teflon and metal surfaces. To understand the degree of wetting between the
liquid, L, and the solid, S, it is convenient to rewrite Equation 5.3 as follows:

cos(θ) = (γ S − γ LS )/γ L (5.4)