The researchers experimentally explored the structural change of rehydration water confined to the tiny nanopores of layered materials such as clay. Their findings potentially open the door to new options for ion separation and energy storage.
Investigating the interaction between the structure of water molecules that are incorporated into layered materials such as clays and the formation of ions in such materials has proven to be a major experimental challenge. But now researchers have used a technique commonly used elsewhere to measure extremely small masses and molecular interactions at the nanoscale to observe these interactions for the first time.
Their research has been published in Nature Communications On October 28, 2022.
Many materials take a layered form at the microscopic or nanoscale. When the clay dries, for example, it resembles a series of slabs stacked on top of each other. However, when these layered materials encounter water, that water can be trapped and incorporated into the gaps or holes—or more accurately, the “pores”—between the layers.
This “hydration” can also occur when water molecules or its constituent elements, particularly the hydroxide ion (a negatively charged ion combining a single oxygen and a single hydrogen atom) become incorporated into the material’s crystal structure. This type of material, a “hydrate,” is not necessarily “wet” even though water is now a part of it. Water can also dramatically change the structure and properties of the original material.
In this “nano-enhancement,” hydration structures—how water molecules or their constituent elements are arranged—determine the original material’s ability to store ions (positively or negatively charged atoms or groups of atoms).
Water or charge storage means that these layered materials, from traditional clays to layered metal oxides—and, most importantly, their interactions with water—have wide-ranging applications, from water purification to energy storage.
However, studying the interaction between this hydration structure and ion formation in the ion storage mechanism of such layered materials has proven to be quite challenging. Efforts to analyze how these hydration structures change over the course of any movement of these ions (“ion transport”) are more challenging.
Recent research has shown that such hydrophilic structures and interactions with the layered materials play an important role in giving the latter high ion storage capacities, all of which in turn depend on how flexible the layers that host the water are. In the space between the layers, any pores not filled with ions are filled with water molecules instead, which helps stabilize the layered structure.
“In other words, hydrated structures are sensitive to how interlayer ions are structured,” said Katsuya Teshima, study author and materials chemist at Shinshu University’s Supermaterials Research Initiative. “Although this ionic configuration in many different crystal structures controls the number of ions that can be stored, to date such configurations have rarely been systematically investigated.”
So Teshima’s group looked at a “quartz crystal microbalance with energy dissipation monitoring” (QCM-D) to help with their theoretical calculations. QCM-D is essentially an instrument that acts like a balance scale that can measure extremely small masses and molecular interactions at the nanoscale. This technology can also measure small changes in energy loss.
The researchers used QCM-D to demonstrate for the first time that the change in the structure of water molecules confined to the nanospace of layered materials can be observed experimentally.
They did this by measuring the “hardness” of the materials. They examined the layers of double hydroxides (LDHs) of a class of negatively charged clays. They found that the hydration structures were related to the stiffening of LDH when any ion exchange reaction (swap of one type of ion with a different type of ion but with the same change) occurs.
“In other words, any change in ionic interaction arises with the change in hydration structure that occurs when ions are incorporated into nanoscale space,” added Tomohito Sudari, a study collaborator now with the University of Tokyo.
In addition, the researchers found that the structure of water is highly dependent on the charge density (the amount of charge per unit volume) of the layered material. This, in turn, is largely what governs the storage capacity of ions.
The researchers now hope to apply these measurement methods along with knowledge of the structure of ion hydration to devise new techniques to improve the ion storage capacity of layered materials, which may open new avenues for ion separation and sustainable energy storage.