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 approach for interfacial tensions is shown to require more degrees of freedom 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 Fowkes component approach does not describe physical reality; only the liquid and solid surface tensions, γ1v and γsv, are operative 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 freedom than are available. The number of degrees of freedom of the equation of state for interfacial tensions is shown to agree with that predicted from the phase rule for capillary systems as well as with experimental results. By using the empirical form of the equation of state, essentially constant solid tensions, γsv, are obtained from a variety of dispersive and non-dispersive liquids for three solid surfaces: fluorocarbon (FC721), Teflon (FEP) and poly(ethylene terephthalate) (PET).