Liquid-Polymer Contact Electrification: Modeling the Dependence of Surface Charges and ζ-Potential on pH and Added-Salt Concentration

Abstract

Here, a mathematical model is presented, which accounts for the dependence of the surface electrical charge density (σ) on pH and the concentration of added salts (Cs), generated when a water drop rolls or slides on the surface of a hydrophobic polymer, a process known as liquid-polymer contact electrification (LPCE). The same model was successfully applied to fit the isotherms of ζ-potential as a function of pH, reported in the literature by other authors for water-poly(tetrafluoroethylene) (PTFE) interfaces. Hence, the dependence of σ and ζ on pH was described using the same concept: acid-base equilibria at the water-polymer interface. Equilibrium constants were estimated by fitting experimental isotherms. The experimental results and the model are consistent with a number of 10-100 acid-base sites/μm2. The model predicts the increase of |σ| and |ζ| with pH in the range of 2-10 and the existence of a zero-charge point at pHzcp ≅ 3 for PTFE (independent of Cs). Excellent fits were obtained with Ka/Kb ∼9 × 107, where Ka and Kb are the respective acid and base equilibrium constants. On the other hand, the observed decrease in |σ| and |ζ| with Cs at fixed pH is quantitatively described by introducing an activity factor associated with the quenching of water activity by the salt ions at the polymer-water interface, with quenching constant Kq. Additionally, the quenching predicts a decrease in |σ| and |ζ| at extreme pH, where I > (1/Kq) (I: ionic strength), in agreement with literature reports.