Researchers have experimentally detected the structural change of hydrated water confined to tiny nano-scale pores of layered materials such as clays. Their findings could open the door to new options for ion separation and energy storage.
Investigating the interaction between the structure of water molecules incorporated in layered materials such as clays and the configuration of ions in such materials has been a major experimental challenge. But researchers elsewhere are now using a technique commonly used to measure extremely small masses and molecular interactions at the nano-level to observe these interactions for the first time.
Their research was published in Nature Communication on October 28, 2022.
Many materials take the form of layers at the microscopic or nanoscale. When dry, for example, clays look like a series of sheets stacked on top of each other. When such layered materials encounter water, however, that water can be confined and integrated into the gaps or holes — or, more precisely, the ‘pores’ — between layers.
Such ‘hydration’ can also occur when water molecules or their components, particularly a hydroxide ion (a negatively charged ion combining a single oxygen atom and one hydrogen atom) are integrated into the crystalline structure of the material. This type of material, ‘hydrate’, is not necessarily ‘wet’ even though water is now part of it. Hydration can also significantly change the structure and properties of the base material.
In this ‘nano-conservation’, the hydration structures — the way water molecules or their components organize themselves — determine the ability of the base material to store ions (positively or negatively charged atoms or groups of atoms).
This water storage or charge means that such layered materials, from traditional clays to layered metal oxides — and, crucially, their interactions with water — have a wide range of applications, from water purification to storage energy.
However, studying the interaction between this hydration structure and the ion configuration in the ion storage mechanism of these layered materials is a major challenge. And attempts to analyze how these hydration structures change during any movement of these ions (‘ion transport’) become even more difficult.
Recent research has shown that such water structures and interactions with the layer materials play an important role in giving the latter a high ion storage capacity, all of which depends on how flexible the layers are the water hosts. In the space between layers, any pores not filled with ions are filled with water molecules instead, helping to stabilize the layered structure.
“Put another way, the water structures are sensitive to how the interlayer ions are structured,” said Katsuya Teshima, corresponding author of the study and a materials chemist with the Materials Research Initiative at Shinshu University. “And although this ion configuration in many different crystal structures controls how many ions can be stored, such formations have rarely been systematically investigated until now.”
So Teshima’s group looked to ‘quartz crystal microequilibrium with energy dissipation monitoring’ (QCM-D) to aid their theoretical calculations. QCM-D is essentially an instrument that works like a balance scale that can measure very small masses and molecular interactions at the nano level. The technique 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 in the nanospace of layered materials can be observed experimentally.
They did this by measuring the “hardness” of the materials. They investigated the layered double hydroxides (LDHs) of a negatively charged clay class. They found that the hydration structures were involved in the hardening of the LDHs when any ion exchange reaction occurs (change of one type of ion with a different type of ion but with the same change).
“In other words, any change in ion interaction results from the change in the hydration structure that occurs when ions are incorporated into the nanospace,” said Tomohito Sudare, a collaborator on the study now at the University of Tokyo.
In addition, the researchers found that the hydration structure is highly dependent on the charge density (the amount of charge per unit volume) of the layered material. This in turn largely controls the ion storage capacity.
The researchers now hope to apply these measurement methods together with the knowledge of the hydration structure of the ions to devise new techniques to improve the ion storage capacity of layered materials, which could open new ways to for ion separation and sustainable energy storage.