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Abstract
It is now generally recognized that the principal forces contributing to the work of adhesion between two phases, WA, are the Lifshitz-van der Waals forces (which include a small contribution from permanent and induced dipoles) and acid-base interactions, taken in the most general 'Lewis' sense. One may thus write WA = WLW A + wab A = 2√σLWSσLWL + fN(-ΔHab), where WLW A and Wab A are the Lifshitz-van der Waals and acid-base contributions to the work of adhesion, and σLW S and σLW L are the Lifshitz-van der Waals contributions to the surface free energies of the solid and the liquid, respectively; ΔHab is the enthalpy (per mol) of the acid-base adduct formation between the acid or base functional groups on the adherend and in the adhesive; N is the number (moles) of accessible functional groups per unit area of the adherend; and f is an enthalpy-to-free energy correction factor (which has normally been assumed to be ~1). The present work seeks to evaluate Wab A for several systems using wetting measurements and, for at least one system, to obtain a quantitative check of the above equation using independently measured values of f, N, and (ΔHab). The total work of adhesion is determined from the measured surface tension of the liquid, σL' and its contact angle, 0, against the solid: WA= σL(1 + cos ). σLW S and σLW L are determined using probe liquids, N is determined from conductometric titrations of the solid in finely divided form, and ΔHab is determined by flow microcalorimetry. f is determined from a Gibbs-Helmholtz analysis of surface tension and contact angle data obtained over a range of temperatures. Conclusions reached are that the f factor is significantly below unity in most cases and that even including this effect, the above equation is still not verified quantitatively when the terms are measured independently.
