Researchers have solved a mystery that has confounded scientists for 80 years: the crystal structure of the tetra-n-butylammonium bromide (TBAB) hydrate TBAB·26H₂O. This substance belongs to a class of crystalline materials called semiclathrate hydrates, which form from the combination of ions and water. Since its discovery in 1940, this TBAB hydrate has been widely used in a range of applications, including air conditioning. Understanding the crystal structure of this important semiclathrate hydrate will help scientists and engineers better utilize TBAB hydrate.

The results were shared in a paper published in Crystal Growth & Design on July 18^th .

“For 80 years, the crystal structure of the widely used TBAB hydrate (TBAB·26H₂O) remained unresolved, despite its importance in thermal energy storage applications. This structural ambiguity hindered both scientific understanding and practical optimization of the material. Our goal was to definitively determine the structure using synchrotron radiation and clarify its molecular arrangement,” said Sanehiro Muromachi, an associate professor at YOKOHAMA National University in Yokohama, Japan and Hironobu Machida, Chief Engineer at Panasonic Corporation in Osaka, Japan.

Water-based functional materials—including hydrogels, aqueous polymers, liquid crystals, and clathrate hydrates—take advantage of the unique properties of water and are used in a variety of industrial processes. Water is abundant and sustainable, making these an important part of sustainable industrial processes. TBAB·26H₂O is a semiclathrate hydrate that can store cool energy at temperatures suitable for air conditioning applications. It is made of a guest molecule of TBAB surrounded by a hydrogen-bonded cage of water molecules. While other semiclathrate hydrates with a TBAB guest molecule have had their structures uncovered, the crystal structure of TBAB·26H₂O has remained a mystery despite its wide use. Previous research suggested a tetragonal lattice structure, but this could not fully explain all of the properties of TBAB·26H₂O.

Researchers used a synchrotron radiation facility called the Super Photon ring-8, or SPring-8 in Sayo Town, Japan. During synchrotron radiation, charged particles travel along a curved path and release electromagnetic radiation. The structure of the TBAB tetragonal hydrate was a Jeffrey’s type III hydrate structure, one of the known types, but with unique features. It has a composition identical to those found in another TBA semiclathrate hydrate called TBA(NO₃), but with a different arrangement resulting in a denser crystal.

Understanding this new structure reveals heat storage properties of TBAB·26H₂O, which can be applied to a variety of practical applications. In particular, it introduces new options for designing heat storage materials based on hydrates. This is an important finding that can help reduce CO₂ emissions.

“We successfully resolved the crystal structure of TBAB·26H₂O for the first time, revealing a unique tetragonal superstructure that accommodates the TBA cation in a novel cage configuration.”, said Muromachi. “This structure explains the material’s heat storage characteristics and provides new design principles for hydrate-based functional materials. Understanding this structure opens the door to engineering better thermal storage and related applications,” said Machida.

Looking ahead, researchers plan to use this new understanding of the crystal structure of the TBAB hydrate to create advanced water-based materials and contribute to energy-efficient technologies, such as air conditioning, gas separation, and carbon capture. “The next step is to apply this structural knowledge and seek to expand these structural principles to other hydrate-forming systems, including polymers and soft matter,” Muromachi.

Other contributors include Nobuhiro Yasuda and Hiroyasu Masunaga of the Japan Synchotron Radiation Research Institute (JASRI); Takeshi Sugahara of The University of Osaka; and Hironobu Machida of the Panasonic Corporation.

The Thermal Management Materials and Technology Research Association (TherMAT) project (JPNP15007) at the New Energy and Industrial Technology Development Organization (NEDO) supported this research.

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