Uematsu Yuki, Bonthuis Douwe Jan, Netz Roland R
Department of Physics, Kyushu University, 819-0395 Fukuoka, Japan.
Laboratoire de Physique de l'Ecole normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, F-75005 Paris, France.
Langmuir. 2020 Apr 7;36(13):3645-3658. doi: 10.1021/acs.langmuir.9b03795. Epub 2020 Mar 26.
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 HO, OH, HCO, and CO 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 (p = 5-7) and weak bases (p = 12) at a concentration of the order of 10 M.
疏水表面的带电现象是物理化学中一个备受争议的课题。我们通过求解具有单个离子吸附亲和力的泊松 - 玻尔兹曼方程和斯托克斯方程，从理论上研究了不同pH值和盐浓度下疏水表面的ζ电位。利用从空气 - 电解质界面的实验测量表面张力中提取的离子表面亲和力，我们首先表明，诸如HO、OH、HCO和CO等小无机离子的界面吸附和排斥不能解释实验中观察到的ζ电位，因为这些离子的表面亲和力太小。即使考虑流体动力学滑移，也无法再现ζ电位对pH值和盐浓度的特征依赖性。相反，为了解释疏水表面实验测量的可观ζ电位，我们假设水中存在微量杂质，并考虑杂质与水的酸碱反应。如果我们考虑由浓度约为10 M的弱酸（p = 5 - 7）和弱碱（p = 12）混合物组成的杂质，我们的预测与各种疏水表面报道的实验ζ电位数据之间取得了良好的一致性。
