SCOPE AND LIMITATIONS OF THE EQUATION OF STATE APPROACH FOR INTERFACIAL-TENSIONS

The equation of state (EQS) approach for interfacial tensions was proposed by Neumann et al. In contrast to the surface tension component (STC) theory proposed by Fowkes, this approach has been considered controversial. In this paper, we attempt to discuss the scope and limitations of the EQS approach. Evidently, some limitations are created by the assumptions originally made for the approach. When the EQS results are compared by Neumann et al. with the surface force measurements, it becomes more convincing that the only interaction involved at the interface recognized by the EQS is the apolar (or nonpolar), van der Waals type. As a result, the EQS approach is applicable to apolar systems involving solely physisorption, while the STC theory applies to apolar and polar systems involving physisorption and/or chemisorption. Though physisorption between a liquid and a solid is universal and more common than chemisorption, it is often much weaker in magnitude. Without considering chemisorption, the EQS, as persistently asserted and vigorously defended by Neumann and his co-workers, is incomplete and definitely not universal for interfacial tensions. Furthermore, the EQS approach cannot be extended to the system involving a thin liquid film at the interface when the disjoining pressure consists of at least three components: van der Waals, structural, and electrostatic.