The solid-liquid interface 153which, combined with equation (6.2), gives
(6.5)
TLGor, 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 yLG the critical
surface tension, yc, for the solid. Critical surface tension is a useful
parameter for characterising the wettability of solid surfaces (see
Table 6.1).
Table 6.1 Critical surface tensions for solid surfaces (After Zisman^78 )Solid surface yc(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 yc 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 yc for many solid
surfaces by way of empirical plots of cos 0 versus yLG (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.