92168.pdf

The solid-liquid interface 155

(1 + cos 0) (6.8)

For zero contact angle, cos 0 = 1 and W.d = 2 yLG - Wc; therefore,

zero contact angle results when the forces of attraction between

liquid and solid are equal to or greater than those between liquid and

liquid, and a finite contact angle results when the liquid adheres to

the solid less than it coheres to itself.

The solid is completely wetted by the liquid if the contact angle is

zero and only partially wetted if the contact angle is finite. Complete

non-wetting implies a contact angle of 180°, which is an unrealistic

situation, since it requires that either Wa = 0 or yLo = °°- There is

always some solid-liquid attraction; for example, water droplets wil!

adhere to the underside of a paraffin wax surface (0~110°).

Immersionai wetting

In immersional wetting, the solid, which is not originally in contact

with the liquid, is immersed completely in the liquid. The area of

liquid-gas interface, therefore, remains unchanged. The free energy

change for immersion of a solid in a liquid is given by

-AGj = 7sc ™ 7SL

= yLG cos 0 (^6 -^9 )

If Tso > 7sL> tnen 0 < 90° and immersional wetting is spontaneous,

but if (^7) SG < -ySL, then 0 > 90° and work must be done to immerse the
solid in the liquid.
Free energy, enthalpy and entropy of immersion are related by
(6.10)
j can be measured directly by sensitive calorimetry. AG{ and
A//J can be equated only when ASj is negligible.
Measurement of contact angles^79
Given a flat solid surface, the actual measurement of contact angles
to ±1° is relatively straightforward. The complications associated
with contact angle measurement relate to the system itself and
include the following: