Surface charge density of any material is of fundament importance as it dictates it interfacial properties. In this context, the electrification of water–hydrophobe interfaces has been a contentious topic. Common hydrophobic surfaces, such as those composed of hydrocarbons or perfluorocarbons, are believed to be electrically neutral. However, new experiments with a parallel plate capacitor and an electrometer have revealed that these materials, viz. polypropylene and perfluorodecyltrichlorosilane( FDTS), have got significant surface-bound electronic charges even in the air (-1 nC/cm<sup>2</sup>; https://www.nature.com/articles/s41467-020-19054-8). However, the magnitude of this charge is orders of magnitude lower that previous reports on the surface charge density of polypropylene measured in aqueous solutions via atomic force microscopy (AFM). To simplify the experimental design and remove the complexities due to overlapping electrical double layers and the theoretical assumptions of the Grahame equation, we quantified force–distance relationships between common hydrophobic polymers in the air. Specifically, we utilized a Surface Force Apparatus (SFA) and Atomic Force Microscope to study FDTS and polypropylene surfaces in the air. To gain mechanistic insight, we solved the Poisson’s equation to obtain the surface charge density of polypropylene and perfluorodecyltrichlorosilane. We will also present the results of a comparative study of surface forces between the same surfaces in aqueous solutions and the application of the Poisson–Boltzmann equation to derive surface forces. This comprehensive study will bring the standing debates in this field to a closure.

Quantifying surface charge densities of common polymers via direct force measurements

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