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

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 ( C s ), 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 ξ-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 ξ 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/μm 2 . The model predicts the increase of |σ| and |ξ| with pH in the range of 2-10 and the existence of a zero-charge point at pH zcp ≅ 3 for PTFE (independent of C s ). Excellent fits were obtained with K a / K b ∼ 9 × 10 7 , where K a and K b are the respective acid and base equilibrium constants. On the other hand, the observed decrease in |σ| and |ξ| with C s 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 K q . Additionally, the quenching predicts a decrease in |σ| and |ξ| at extreme pH, where I > (1/ K q ) ( I : ionic strength), in agreement with literature reports.