Theoretical Basis of the Critical Surface Tension

The value of depends to some extent on the set of liquids used to measure it. Zisman [18] described y as a useful empirical parameter whose relative values act as one would expect of the specific surface free energy of the solid, y, .. 

Several authors were tempted to identify with or yf. Good and Girifalco [10, 11] suggested the foUowing expression for the contact angle, 

Fowkes [12] obtained the following equation for the contact angle of a liquid on a 

Equations (11.44) and (11.46) predict that if tisv = 0, a plot of cos 0 versus y v should give a straight line with intercept (7c) on the cos 0=1 axis. The experimental results seem to support this prediction. Thus, for non-polar solids, y = y, provided tisv = 0, i.e. there is no adsorption of liquid vapour on the substrate. The above condition is unlikely to be satisfied when 0 = 0. 

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