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/μm2. 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 pHzcp ≅ 3 for PTFE (independent of C s). Excellent fits were obtained with K a/K b ∼ 9 × 107, 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.

Water drops become charged after sliding on a polymer surface. The variation of the detected charge with pH and ionic strength are compatible with OH− or H+ transfer from the drop to the polymer. These changes are accounted for by a thermodynamic model.