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 (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 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 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 |ξ| 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 |ξ| at extreme pH, where I > (1/Kq) (I: ionic strength), in agreement with literature reports.

Liquid-polymer triboelectricity: chemical mechanisms in the contact electrification process.

Liquid-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calculated by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become positively charged at pH > pHzch (pHzch being the zero charge point of the polymer) while they become negatively charged for pH < pHzch. The addition of non-hydrolysable salts (NaCl or CaCl2) to water decreases the electrical charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liquid to the hydrophobic polymer surface. A proposed thermodynamic model for the ion transfer process allows explaining the observed effects of RH, pH and ionic strength.