B Hard Surface Equilibrium Wetting

In hard surface wetting, the substrate to be wet is a nonporous, nongranular solid or a nonporous film, and since the area to be wet is relatively small, conditions close to equilibrium are usually attained and the thermody-namics of the wetting process is a major factor determining the extent of wetting. 

When the spreading coefficient is negative, the contact angle is finite and spreading is not complete. Since SL/s = ySA — ySL — yM (equation 6.1) and 

Isa Isl = Jla) cos 9 (equation 6.3) in this case, l/s = yM (cos 0—1 (equation 6.5). When the spreading coefficient is zero, spreading wetting is complete, since Sl/s = y A(cos 0 — 1) = 0, and thus 0 + 0°. In this latter case, since Sl/s = Jsa Jsl Tla = 0, then yM = ySA — ySL, meaning that the surface tension of the spreading liquid must be reduced to a value equal to yvl — yv/ for spreading to be complete. 

for example, in the presence of some highly fluorinated carboxylic acids and their salts, the value yc for polyethylene is decreased from its usual value of almost 31 mN/m to about 20 mN/m (Bernett, 1959) by adsorption of the fluorinated surfactants onto the polyethylene surface, with the result that solutions of these surfactants having surface tensions less than the normal yc for polyethylene do not spread on it. The requirement that the surface tension of the wetting liquid be reduced by the surfactant to some critical value characteristic of the substrate is thus a necessary, but not sufficient, condition for complete spreading wetting. A surfactant solution whose surface tension is above the critical tension for the substrate does not produce complete wetting, but a solution whose surface tension is below the critical tension for the substrate may or may not produce complete wetting (Schwarz, 1964). 

On nonpolar surfaces, any structural or other factor that results in a decrease in yM (Chapter 5) decreases the contact angle and improves wetting. Thus, the addition of a water structure-breaking additive (/V-methylacetamide), which increases the surface tension of an aqueous solution of sodium dodecyl sulfate, causes an increase in its contact angle on polyethylene, whereas the addition of a (Water structure promoter (fructose, xylose), which decreases the surface tension of the surfactant solution, reduces its contact angle (Schwuger, 1971). 

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