EVALUATION OF DIFFERENT APPROACHES TO ASSESS THE SURFACE-TENSION OF LOW-ENERGY SOLIDS BY MEANS OF CONTACT-ANGLE MEASUREMENTS

Assessment of the surface tension of low-energy solids by means of eas y to perform contact angle measurements would be very attractive. Two different approaches are frequently reported to be very promising in t his respect. We have evaluated these approaches using mainly apolar su rfaces, which present the simplest case possible. The ''equation of st ate'' approach, which uses a single parameter, correctly predicts the results on FC722 (perfluoropolyacrylate) and FEP poly(tetrafluoroethyl ene-co-hexafluoropropylene)), but shows systematic deviations on the s urfaces of octyltrichlorosilane (self-assembled on glass) and PE (poly ethylene). The ''surface tension components'' approach uses three para meters. The surface tension is split into a van der Waals component, a Lewis acid component, and a Lewis base component. The determination o f the surface tension of apolar surfaces yields reasonably consistent results when using a large set of contact angle data. However, the pre sent results indicate important differences with previously reported v alues of the van der Waals components of some fluids, i.e., dimethyl s ulfoxide (DMSO), formamide, diiodomethane, and 1-bromonaphthalene. The latter two appear not to be apolar in nature when obtained from measu rements on FC722 or FEP. The difference with previous reports is due t o a discrepancy between the contact angles of diiodomethane and 1-brom onaphthalene measured on either FC722 or PE. It is concluded that neit her the ''equation of state'' nor the ''surface tension components'' a pproach can account for all experimental results. The present use of c ontact angle measurements appears to be limited to the estimation of t he surface tension of apolar surfaces, using the ''surface tension com ponents'' approach.