Contact angle, wetting, and adhesion: a critical review

The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γs, is reviewed. The basis for the three components γLW s, γ⊕s, and γ⊖s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γp') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, πe. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ⊖ and γ⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γLW at the hydrophobic corner, and γ⊕ and γ⊖ at the two hydrophillic corners, is suggested.