News Chemistry 15 February 2016
1 minute read 

Scientists discover a new kind of frozen water


Researchers model an ultra-light low density ice clathrate that could aid carbon sequestration. Viviane Richter reports.


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Ice – whether you’re scraping it off your car windscreen or dropping it in your gin and tonic, you probably think it’s all the same.

But water can crystallise in different forms, and University of Nebraska-Lincoln researchers have just discovered the recipe for creating the lightest ice to ever exist.

It may lead to the design and synthesis of new ultralow density materials for gas storage or carbon dioxide sequestration, the researchers say.

The team used computer algorithms and simulations to determine the extreme pressures and temperatures required to freeze water as “clathrate”, or cage-like, stacks at molecular scale.

Clathrate ice is naturally found on ocean floors and in permafrost. But there, the molecular cages in the ice are filled with “guest” molecules, such as methane, which stabilise the cage structure.

A model of the lightweight ice's proposed molecular configuration. The green sphere represents the guest molecule that would be removed. – Yingying Huang and Chongqin Zhu

The researchers modelled a clathrate ice form which would be stable even without a guest molecule. And because the water molecules in clathrate ice are spread apart further than your garden-variety ice, it’s less dense and lighter.

The team discovered that making this extremely light ice requires a tight squeeze.

At -23 °C, water would need to be compressed at a pressure four times greater than that of the deepest trench in the Pacific Ocean. Or, cooled to -273 °C where all molecules stop moving, water would have to be compressed by a pressure equal to the weight of 300 jumbo jets at sea level.

The guest molecules could then be vacuumed out, the authors say, using a technique European scientists have already developed.

Physically forming this ice in the lab will be a challenge, but if the US team can successfully do that, it will be the 18th known form of ice – and 25% lighter than the record low form created by European scientists in 2014.

“Water and ice are forever interesting because they have such relevance to human beings and life,” said Xiao Cheng Zeng, author of the study that was published in Science Advances.

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Viviane Richter is a freelance science writer based in Melbourne.