Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 113

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Surface Te nsion

FIGure 5.5 Plot of contact angle (cos[θ]) of different liquids on Teflon.

Even though Young's equation may sometimes not be easily applicable in all sys-
tems, many useful conclusions can be obtained if systematic data are available. In
fact, currently, much basic research in industry is based on Young's equation analy-
ses. For example, the data of cos(θ) of various liquids on Teflon gave an almost
straight-line plot (Figure 5.5).
These data show that

cos(θ) = k 1 − k 2 γ L (5.11)

This can also be rewritten as

cos(θ) = 1 − k 3 (γ L − γ cr ) (5.12)

where γ cr is the critical value of γ L at cos(θ) equal to 0. The values of γ cr have been
reported for different solids using this procedure (as suggested by Zisman).
The magnitude of γ cr for Teflon of 18 mN/m thus suggests that -CF 2 - groups exhibit
this low surface tension. The value of γ cr for -CH 2 -CH 3 - alkyl chains gave a higher
value of 22 mN/m than for Teflon. Indeed, from experience, one also finds that Teflon
is a better water-repellant surface than any other material. The magnitudes of γ cr for
different surfaces are seen to provide much useful information (Table 5.4).
These data show the surface characteristics as related to γ cr . In many cases, the
surface of a solid may not behave as desired, and therefore it treated accordingly,
which results in a change of the contact angle of fluids. For instance, the low surface
energy of polymers (polyethylene [PE]) is found to change when treated with flame
or corona (as shown in the following table).

material liquid contact angle

PE Water 87

Corona 55

PE (corona) Water 66

Corona 49