Page 164 - Introduction to Colloid and Surface Chemistry

The solid-liquid interface 153
which, combined with equation (6.2), gives

(6. 5)
TLG

or, for non-polar liquids, where
(6.6)

For a selection of non-polar liquids on a given solid, it follows that
6 should decrease as ^LG decreases and become zero below a certain
value of 7LG- Zisman 78 has named this value of y LG the critical
surface tension, y c, for the solid. Critical surface tension is a useful
parameter for characterising the wettability of solid surfaces (see
Table 6.1).

78
Table 6.1 Critical surface tensions for solid surfaces (After Zisman )
Solid surface y c(20°C)/mN m ~ !

Condensed monolayer with close-packed terminal
CFs groups 6
Polytetrafluoroethylene 18
Polytrifluoroethylene 22
Po!y(vinylidine fluoride) 25
Poly(vinyl fluoride) 28
Polyethylene 31
Polystyrene 33
Poly(vinyl alcohol) 37
Poly(vinyl chloride) 40
Poly(hexamethylene adipamide) (nylon 66) 46

To determine y c for a solid, the advancing contact angles made by
several non-polar or weakly polar liquids are measured and the value
of TLO which corresponds to 6 - 0 (i.e. cos 0 = 1) is found by
graphical extrapolation. Zisman has determined y c for many solid
surfaces by way of empirical plots of cos 0 versus y LG (see Figure
6.2). According to equation (6.6), however, plots of cos 6 versus
(TLo)~ 1/2 should be approximately linear and permit more reliable
extrapolation to cos 0=1.