Transcending Lifshitz Theory: Reliable Prediction of Adhesion Forces between Hydrocarbon Surfaces in Condensed Phases using Molecular Contact Thermodynamics

Works of adhesion W(Lifshitz) between hydrocarbon surfaces in 260 liquids were calculated using Lifshitz theory and compared with interaction free energies ∆∆G determined using a model in which the interactions between a molecule and a liquid are described by a set of surface site interaction points (SSIP). The predictions of these models diverge in significant ways. Interaction free energies calculated using the SSIP approach are typically small and vary little, but in contrast, Lifshitz theory yields works of adhesion that span a broad range. Moreover, the SSIP model also yields significantly different ∆∆G values in some liquids for which, in contrast, Lifshitz theory predicts similar values of W. These divergent predictions were tested using atomic force microscopy. The experimentally determined work of adhesion was found to be closer to the value predicted using the SSIP model than to W(Lifshitz). Still greater differences were found in the interaction energies calculated using the two models when liquid mixtures were considered. For mixtures of methanol and benzyl alcohol, ∆∆G declines smoothly as the benzyl alcohol concentration increases, but W(Lifshitz) decreases to a minimum and then increases, reaching a larger value for benzyl alcohol than for methanol. The experimental adhesion data were correlated closely with the predictions of the SSIP model. We conclude that the molecular-scale treatment intrinsic to the SSIP approach enables adhesive interactions to be modelled more accurately than is possible using Lifshitz theory, which instead uses the bulk properties of the medium to predict work of adhesion values.