The Critical Surface Tension of Wetting

FIGURE 17.10. The critical surface tension of wetting, ctc, is usually found to be consistent for a homologous series of hydrocarbon liquids (curve 1), a series of alkyl halides (curve 2), and miscellaneous polar liquids (curve 3) on a nonpolar surface such as Teflon. Results on polar surfaces are less unequivocal, but still can be useful. 

The general limitation of the technique to the so-called low-energy surfaces must be made because such materials as metals, metal oxides, or ionic solids, which have surface free energies in the hundreds and thousands, are almost always covered with an adsorbed layer of a low-energy substance such as water or oils from the atmosphere. Under rigorously controlled experimental conditions, the technique may be applied to such materials, but interpretation of the results can be difficult. 

From a plot of cos 6 versus oi2, one can obtain the value of the liquid surface tension at which cos 0 = 1, a value that has been termed the critical surface tension of wetting, Tc. It is defined as the surface tension of a liquid that would just spread on the surface of the solid to give complete wetting. In other words, if a-u o-c, the liquid will spread if ri2 o-c, the liquid will form a drop with a nonzero contact angle. Typical values of Tc for commonly encountered materials are given in Table 17.1. 

While Tc is an empirically determined value, depending to some extent on the nature of the liquids us for its determination, attempts have been made to identify it with such theoretical terms as or o-s. According to the Good-Girifalco equation [Eq. (17.27)], for jts.i = 0 and cos 0=1 

TABLE 17.1. Values of Critical Surface Tension of Wetting ( r niN cm for Various Materials 

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The surface properties are of particular interest for composites and coatings. The n = 6 monomer will wet Teflon, and PTFE filled composites can be prepared. The critical surface tension of wetting for the fluoromethylene cyanate ester resin series has been determined from contact-angle measurements on cured resin surfaces. As indicated in Table 2.2, it parallels the fluorine composition and begins to approach the PTFE value of 18 dyn/cm. 

The viscosity or resistance to flow increases as the number of repeat units increases, but physical properties, such as surface tension and density, remain about the same after a DP of about 25. The liquid surface tension is lower than the critical surface tension of wetting, resulting in the polymer spreading over its own absorbed films. The forces of attraction between polysiloxane films are low resulting in the formation of porous films that allow oxygen and nitrogen to readily pass though, but not water. Thus, semipermeable membranes, films, have been developed that allow divers to breath air under water for short periods. 

Our own investigations have concerned (a) liquid spreading on solids and the laws relating the equilibrium contact angle and the critical surface tension of wetting to solid and liquid constitution (26, 27, 28, 53, 54,62), (b) liquid/liquid displacement from solid surfaces (1,5), (c) the properties of adsorbed monolayers on solids and their relation to the monolayer retraction method (28, 54, 62), (d) the surface electrostatic potentials of adsorbed organic monolayers on metals (9, 10, 11, 58, 59), (e) the effects of surface constitution on adhesion and abhesion (60),... 

Wettability. A description of the contact angle goniometer and the scheme used in determining the critical surface tension of wetting (yc) are described elsewhere (15, 18). 

An empirical method to estimate the surface tension of a solid is Zisman s plot (cos 9 as a function of yl), which obtains the critical surface tension of wetting. In the absence of specific interaction between the surface and the liquids used for the measurement of contact angles, the critical contact angle of wetting can be accurately estimated and its value used as the surface tension of the surface. However, if a surface interacts with liquids used as the sessile droplet for the contact angle measurement, to the extent that the surface tension is altered, Zisman s plots deviate from the ideal linear relationship. In a strict sense, the plot is applicable only to imperturbable surfaces with which liquid contact does not alter surface configuration, i.e., no surface dynamics applies. 

Note (Tc is the critical surface tension of wetting, and a is the solid surface tension obtained in a manner similar to that of Owens and Wendt (7) but with a series of hydroxy-containing liquids rather than just water and methylene iodide. 

Extrapolation of the straight Hne to cos9 = 1 (9 = 0) gives the critical surface tension of wetting y. Any Hquid with y v Tc vvill give 9 = 0, that is, it wets the surface completely y,. is the surface tension of a liquid that just spreads on the substrate to give complete wetting. 

Measurement of the "critical surface tension of wetting" ycr. A liquid with a surface tension of less than ycr will spread on the surface of the solid. It is assumed that y=ycr to estimate the surface tension of the solid. 

If we admit that y g, the critical surface tension of wetting, is the surface tension which a liquid, L, would have to possess in order just to spread onS, then the criterion for spreading. Expression 12, may be replaced by the criterion [13,27],... 

Since the time that study was made, a large body of experimental data has accumulated which shows that regular and predictable changes in solid-liquid contact angles result from changes in the outermost layer of atoms in the solid surface. A "wettability spectrum" has been published [14],and the utility of the "critical surface tension of wetting" as an index of solid surface energy has been well established. 

On theoretical grounds it is generally accepted (de Bruyne s rule) that with pure or simple substances as adhesives strong bonds can never be made by polar adherends with non-polar adhesives or to non-polar adherends with polar adhesives. This observation is based on the fact that a liquid polar adhesive has a higher critical surface tension than the critical surface tension of wetting of a non-polar adherend hence poor wetting is considered to result between these two materials no or reduced penetration of the gas pocket formations, always present on the surface of an adherend, results if only poor wetting between two surfaces occurs with the consequence of poor adhesion. 

Zisman Plot. A plot of the cosine of contact angle, between a solid of interest and a series of liquids, versus the surface tensions of those fiquids. The surface tension extapolated to zero contact angle is the critical surface tension of wetting of the solid. 

Buchanan et al. [18] investigated feasibility of gamma separation for separation of PET from PVC. After treatment in alkaline solution (pH=ll) the critical surface tension of wetting of PET was 43 dynes /cm and that of PVC was 39 dynes/cm. The solution of water, alcohol and methylisobutyl carbinol (MIBC ) had surface tension of 40.9 dynes/cm which allowed for the separation of two solids. 

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