FOWKES SURFACE-TENSION COMPONENT APPROACH REVISITED

By comparing the number of degrees of freedom obtained from the phase rule for capillary systems, the Fowkes surface tension component appro ach for interfacial tensions is shown to require more degrees of freed om than are available for a two-component solid-liquid-vapour system. Only in a special case has the Fowkes approach two degrees of freedom: a dispersive liquid on a dispersive solid, suggesting that there are no surface tension components. Experimental results suggest that the F owkes component approach does not describe physical reality; only the liquid and solid surface tension, gamma(lv) and gamma(sv), are operati ve in the two-component solid-liquid-vapour system. Generalization of the Fowkes component approach, of course, will increase the number of independent variables and hence definitely require more degrees of fre edom than are avaialble. The number of degrees of freedom of the equat ion of state for interfacial tensions is shown to agree with that pred icted from the phase rule for capillary systems as well as with experi mental results. By using the empirical form of the equation of state, essentially constant solid tensions, gamma(sv), are obtained from a va riety of dispersive and non-dispersive liquids for three solid surface s: fluorocarbon (FC721), Teflon (FEP) and poly(ethylene terephthalate) (PET).