Atomic–scale investigation of ion–water interactions

Abstract

Investigation of ion-water interaction in atomic scale Ions, the building blocks of materials, interact with counter-ions or polar molecules through long-range electrostatic forces. This underpins various applications in fields such as electrochemistry, catalysis, batteries, and biological processes. Despite its significance, the intrinsic features of single ions have rarely been examined experimentally. Most studies are primarily limited to providing the averaged characteristics of ions, which is attributed to the electric properties. For instance, the dissolution of salt is an everyday phenomenon wherein an ionic crystal disappears in water. This process involves the decomposition of the ionic crystal by liquid water, which subsequently hydrates the constituent ions. Lots of hydrated single ions are uniformly dispersed, and their statistical ensemble leaves the investigation of individual ions challenging. In other words, the presence of neighboring ions and water molecules hinders detecting the intrinsic properties out of single ions, requiring a microscopic method to investigate the properties of single ions. The simplest ion-related reaction is the salt dissolution that naturally occurs under ambient conditions, being regarded as a spontaneous reaction. Particularly, polar water molecules associate with the surface ions, detaching them from the crystal via Coulomb interaction. However, when sodium chloride (NaCl) dissolves in water, its positive enthalpy competes with an increase in entropy; in other words, the Coulomb interactions between ions and water molecules do not exceed those between ions. Temperature and composition dictate whether the reaction will take place. Despite numerous attempts to understand salt dissolution, which have yielded invaluable insights into its macroscopic mechanisms, a detailed, experiment-based atomic-scale description remains ambiguous. Microscopic understanding has relied on theoretical approaches and simulations, leaving the interplay between ions and water molecules during salt dissolution unexplored. However, the fact that the reaction occurs implies that there is plenty of room at the bottom to understand about the nature of the ionic bond. Recent studies in material sciences have visualized and manipulated covalent and metallic bonds successfully, but the fundamental characteristics of ions distinguish ionic bonds from them. In this dissertation, I present an atomic-scale investigation of NaCl dissolution involving a single water molecule, by means of scanning tunneling microscopy. The dissertation includes a brief introduction, details of the equipment and experimental methods used, studies of water molecules on the surface of alkali halides, and a discussion on the selective dissolution of NaCl. Topic Ⅰ: Water molecules on the surfaces of alkali halides The dissolution process begins with the adsorption of water molecules onto the salt surface. To build this heterogenous water/salt interface, the growth of ultrathin salt crystal film is necessary for electrons tunneling in scanning tunneling microscopy (STM). First, we grow various alkali halides on single crystalline Ag surface and examine their geometric and electronic properties. Subsequently, few water molecules are deposited on the salt surfaces. This enables us to investigate the adsorption structures and physical/chemical properties of a single water molecule and its clusters in an atomic scale. Topic Ⅱ: Lateral manipulation of water molecules on NaCl surface Operating under ultra-high vacuum at cryogenic temperatures, STM imposes limitations on the mobility of a few water molecules on the salt surface unlike the salt dissolution under ambient conditions. Rather, we can move individual water molecules in a quite controlled manner by STM tip. The manipulation of single atom and molecule with STM tip is a groundbreaking nanoscience technique, enabling atomic processes such as desorption, bond formation/dissociation, molecular rotation, and lateral positioning as well. Notably, the last case has been used for realizing quantum system on atomic scale. In this topic, we explored ion-water interaction through lateral manipulation of individual water molecules on the NaCl surface. While this interaction is primarily dominated by Coulomb interaction, polarizable interaction also significantly influences the lateral manipulation. The lateral manipulation is examined in terms of force and energy. Topic Ⅲ: Selective dissolution with a single water molecule Direct observation of the salt dissolution is achieved through the lateral manipulation of a single water molecule along the NaCl step. This leads to structural changes and creates a single vacancy at Cl– site, dissolving a single Cl– ion at the initial stage. This preferential dissolution of a single Cl– ion over a single Na+ ion is attributed to the polarizability, a minor but significant contribution to the total interaction in ionic bonding. The selectivity in the microscopic dissolution reveals implications for the actual dissolution process under ambient conditions, where multiple water molecules dissolve the salt crystal. The findings emphasize the flexible electron cloud of the anion as a key to hold ionic bonds, offering novel insights into the salt dissolution and the nature of ionic bonds.