The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ 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 experimentally measured surface tension of the air–electrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H₃O⁺, OH^–, HCO₃^–, and CO₃^2– cannot account for the ζ 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 ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impurities’ acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pK_a = 5–7) and weak bases (pK_b = 12) at a concentration of the order of 10^–7 M.

中文翻译：

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纳摩尔表面活性电荷杂质解释了疏水表面的Zeta电位

疏水表面的带电是物理化学领域一个备受争议的话题。我们通过求解具有各个离子吸附亲和力的Poisson–Boltzmann和Stokes方程，从理论上研究了疏水表面在不同pH和盐浓度下的ζ势。利用从空气-电解质界面的实验测得的表面张力萃取离子表面亲和力，我们首先表明，界面吸附和排斥小无机离子如H的₃ ö ⁺，OH ^-，HCO ₃ ^-和CO ₃ ^{2 –}由于这些离子的表面亲和力太小，因此无法解释实验中观察到的ζ电位。即使我们考虑了流体动力滑移，也无法再现ζ电位对pH和盐浓度的特征依赖性。取而代之的是，为了说明疏水性表面在实验上测得的相当大的ζ电位，我们假设水中存在微量杂质，并包括杂质与水的酸性和碱性反应。如果我们考虑由弱酸（p K _a = 5–7）和弱碱（p K _b = 12）的混合物组成的杂质，我们的预测与所报道的各种疏水性表面的实验ζ电位数据之间的一致性很好。浓度约为10 ^–7 M.