Researchers have experimentally demonstrated the structural change of water of hydration trapped in the tiny, nanoscale pores of layered materials such as clay. Their findings potentially open the door to new options for ion separation and energy storage.
Investigating the interplay between the structure of water molecules incorporated into layered materials such as clay and the configuration of ions in such materials has long proven to be a major experimental challenge. But researchers have now used a technique commonly used elsewhere to measure extremely small masses and nanoscale molecular interactions to observe those interactions for the first time.
Their research was published in nature communication on October 28, 2022.
Many materials take on a layered form on the microscopic or nanoscale. For example, clays, when dry, resemble a series of stacked sheets of paper. However, when such layered materials encounter water, that water can be trapped and incorporated into the gaps or holes—or more accurately, the “pores”—between the layers.
Such “hydration” can also occur when water molecules or their components, particularly a hydroxide ion (a negatively charged ion combining a single oxygen and a single hydrogen atom), become integrated into the crystalline structure of the material. This type of material, a “hydrate,” is not necessarily “wet,” although water is now part of it. Hydration can also significantly change the structure and properties of the original material.
In this “nanoconfinement”, the hydration structures – how water molecules or their components arrange themselves – determine the ability of the starting material to store ions (positively or negatively charged atoms or groups of atoms).
This storage of water, or charge, means that such layered materials, from traditional clays to layered metal oxides—and, crucially, their interactions with water—have widespread applications from water purification to energy storage.
However, studying the interplay between this hydration structure and the configuration of ions in the ion storage mechanism of such layered materials has proven to be a major challenge. It is even more difficult to analyze how these hydration structures change as these ions move (“ion transport”).
Recent research has shown that such water structures and interactions with the layered materials play an important role in giving the latter their high ion storage capacity, which in turn depends on how flexible the layers that host the water are. In the interlayer space, pores that are not filled with ions are instead filled with water molecules, which helps to stabilize the layered structure.
“In other words, the water structures are sensitive to the structure of the interlayer ions,” said Katsuya Teshima, corresponding author of the study and materials chemist at the Supramaterials Research Initiative at Shinshu University. “And while this ion configuration controls how many ions can be stored in many different crystal structures, such configurations have rarely been studied systematically.”
Therefore, Teshima’s group searched for a “quartz crystal microbalance with energy dissipation monitoring” (QCM-D) to support their theoretical calculations. Essentially, QCM-D is an instrument that works like a balance and can measure extremely small masses and molecular interactions at the nanoscale. The technique can also measure minute changes in energy loss.
Researchers used QCM-D to demonstrate for the first time that changing the structure of water molecules trapped in the nanospace of layered materials can be observed experimentally.
They did this by measuring the “hardness” of the materials. They studied the layered double hydroxides (LDHs) of a class of negatively charged clays. They found that the hydration structures are associated with the hardening of the LDHs when an ion exchange reaction occurs (an exchange of one type of ion for another type of ion, but with the same change).
“In other words, any change in ion interaction originates from the change in hydration structure that occurs when ions are incorporated into nanospace,” added Tomohito Sudare, a collaborator on the study who now works at the University of Tokyo.
In addition, the researchers found that the hydration structure is highly dependent on the charge density (amount of charge per unit volume) of the layered material. This in turn largely determines the ion storage capacity.
The researchers now hope to apply these measurement methods together with knowledge of the hydration structure of ions to develop new techniques to improve the ion storage capacity of layered materials and potentially open new avenues for ion separation and sustainable energy storage.