General expression for the interfacial tension

Fowkes, in a theoretical consideration of attractive forces at interfaces, has suggested that the total free energy at a surface is the sum of contributions from the different intermolecular forces at the surface. Thus the surface free energy may be written  

Substances 1 and 2 may either be liquids, solids, or a combination of a solid and a liquid. 

If 1 and 2 are immiscible liquids of which y1 and y2 are known, and of which one is apolar (yp = 0), the y-components of both liquids may be derived in the following way. The interfacial tension y22 is measured by one of the available methods and the Eqs. (8.21b) and (8.20) are solved. In this way several liquids have been investigated the values of y 1, yd and yp are given in Table 8.6. Analogous values for polymers are presented in Table 8.7. Owens and Wendt also gave a more general expression for Eq. (8.10) viz.  

This equation permits the derivation of yd and yps via measurements of the contact angles 6 of two liquids if yLV, yfv and ypv of both liquids are known. 

TABLE 8.6 Force components of surface tension of several liquids (after Fowkes (1964) and Owens and Wendt (1969)) (y in mN/m) 

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