Micro-electrode system designed to determine H⁺ concentration distribution at particle-water interface

The current understanding of the particle-water interface relies primarily on the use of electric double layer theory (EDL theory), in which the concentration distribution of ions with distance can be evaluated. However, although EDL theory provides a theoretical and mathematical basis, obtaining direct experimental results is also necessary. In this study, an instrument system set with a designed H⁺ selective micro-electrode was developed for in situ determination at the particle-water interface, and a preliminary investigation of the H⁺ concentration distribution at the particle-water interface was enabled using data collection at 1 nm/step. The experimental results agree with EDL theory and provide direct evidence to support its general assumptions. The compressible effect of the electric double layer with an increase in ionic strength is verified, and the defined length of the diffusion layer in this investigation is shown to have an approximately inverse proportion to the square root of ionic strength. The rise in H⁺ concentration near the particle surface is also determined during interactions between trace Fe³⁺ and particles. Results indicate that the method reported here can be applied to conduct dynamic in situ observations; based on its excellent performance, this innovative instrument system has great potential for interface mechanism research in the water environment.