Experimental Demonstration of Localized Excess Protons at a Water‐Membrane Interface and the Effect of other Cations on Their Stability

Abstract

The electrostatic localization of excess protons at a water‐membrane interface has now been clearly demonstrated experimentally, which corrects the Mitchellian proton delocalization view. To demonstrate the fundamental behavior of localized excess protons in a pure water‐membrane‐water system in relation to the newly derived pmf equation, excess protons and excess hydroxyl anions were generated by utilizing an “open‐circuit” water‐electrolysis system and their distributions were tested using a proton‐sensing aluminum membrane. The proton‐sensing film placed at the membrane‐water interface displayed localized proton activity while that placed into the bulk water phase showed no excess proton activity during the entire experiment. These observations clearly match with the prediction from the proton‐electrostatics localization hypothesis that excess protons do not stay in water bulk phase; they localize at the water‐membrane interface in a manner similar to the behavior of excess electrons in a conductor. Proton‐electrostatics localization hypothesis predicts that high concentration of other cations such as Mg++, K+, or Na+ could partially delocalize the electrostatically localized protons. Therefore, determination of the cation exchange equilibrium constant is another significant way to test this hypothesis. Since protons are the smallest cations and can exist as part of the water molecules, the predicted equilibrium constant for protons to electrostatically occupy the cation sites at the water‐membrane interface is likely to be much larger than one. Results indicated that the equilibrium constant for protons (H+) to electrostatically occupy the cation sites at the water‐membrane interface was about 107 as predicted. Therefore, electrostatically localized protons at the water‐surface interface are quite stable. These findings have significance not only in the science of bioenergetics but also in the fundamental understanding for the importance of water to life in serving as a proton conductor for energy transduction.Support or Funding InformationThis research was supported by Dr. Lee's start‐up research funds provided by the Department of Chemistry and Biochemistry, the College of Sciences, the Office of Research at Old Dominion University, and the ODU Research Foundation.