概要：The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the zeta potential of hydrophobic surfaces for varying pH and salt concentration by solving the Poisson-Boltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the surface tension of the air-electrolyte interface, we first show that the adsorption and repulsion of small inorganic ions such as H3O+, OH−, HCO−, and CO2− are irrelevant for the zeta potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the zeta potential on pH and salt concentration cannot be reproduced. To explain the zeta potential of hydrophobic surfaces, instead, we introduce a minute amount of impurities in the water and consider their acidic and basic reactions with water. We find good agreement between our calculations and the reported experimental data of Teflon surfaces. Our theory suggests that the impurities are weak acids (pKa = 7) and weak bases (pKb = 12) at a concentration of the order of 10^−7 M.