Prof. Mohan Chen (Center for Applied Physics and Technology, CAPT of Peking University) and his collaborators including Prof. Chuanshan Tian (Fudan University) and Prof. Xifan Wu (Temple University) published an article titled “Stabilization of Hydroxide Ions at the Interface of a Hydrophobic Monolayer on Water via Reduced Proton Transfer” in Physical Review Letters, 125, 156803 (2020). The work presents new findings on the fundamental scientific problem of negatively charged hydrophobic interface. The research team combines optical experiments with first-principles molecular dynamics simulations and proposes new insights into the microscopic origins of accumulations of OH- ions at the hydrophobic interface.

The hydrophobic interface was found to be negatively charged from various experiments, and the mechanism behind the phenomenon has puzzled scientists for a long time. Not surprisingly, arguments exist. While most experiments considered the negative charges come from the intrinsic OH- ions in water, some thought they are caused by contamination. Recently, simulations support the point of view that the negative charges origin from charge transfer between different water layers. Unfortunately, current experiments still lack enough evidences to fully resolve the problem. In addition, by utilizing the first-principles molecular dynamics simulations based on the density functional theory, the descriptions of water and ions are still not accurate enough. In this work, Prof. Chuanshan Tian from Fudan University utilized the surface-specific sum-frequency vibrational spectroscopy (SFVS) technique to simultaneously control the coverages of OH- ions and hydrophobic molecules. Furthermore, the experiment quantitatively analyzed the changes of negative charges. The experiment supports the point of view that the negative charges come from the OH- ion.

In 2018, Dr. Mohan Chen, Prof. Xifan Wu, Prof. Roberto Car (Princeton University) and coworkers elucidated the proton transfer mechanism of OH- ions in the hydrogen-bond network of liquid water by using the state-of-the-art first-principles molecular dynamics (Nature chemistry 10, 413-419). They found OH- can form stable hyper-coordinated structure in order to block proton transfer events from occurring, the ion can also form a three-coordinated structure that helps proton transfer to occur. These findings are important for clarifying the mechanism behind the abovementioned experimental results. In this work, by analyzing the results from first-principles molecular dynamics simulations, the team find that the hydrogen-bond network of water was blocked at the hydrophobic interface, and OH- ions at the interface are more easily to form the hyper-coordinated structure and reduce the rate of proton transfer than those ions in the bulk water. In this regard, OH- ions tend to stably stay at the hydrophobic interface, which is the fundamental reason that OH- can accumulate at the hydrophobic interface.

Figure (a) Experimental setups of surface-specific sum-frequency vibrational spectroscopy (SFVS) technique. (b) Snapshot of OH- ions at the hydrophobic interface.

The research on water ions have important implications for diverse phenomena in physics, chemistry, and biology. Understanding the interactions between OH- ions and hydrophobic surfaces helps designing better hydrophobic materials. In this work, Dr. Shanshan Yang from LBNL and Prof. Mohan Chen from Peking University are co-first authors, Prof. Chuanshan Tian and Prof. Xifan Wu are corresponding authors.

Link of the manuscript：https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.156803