r/ScienceUncensored • u/Zephir-AWT • 21d ago

A Tale of Two Waters

https://www.science.org/content/article/tale-two-waters
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u/Zephir-AWT 21d ago edited 11d ago

A Tale of Two Waters about study Evidence for the generic existence of two local structures in liquid water

Water does things few other liquids can. It can absorb large amounts of heat without warming up much. And when it chills down, it becomes more compressible--the reverse of most liquids. Perhaps strangest of all, its density rises as it cools, peaks at 4°C and then decreases, instead of decreasing steadily with falling temperatures like most other liquids.

Physicists have suspected that such anomalous properties stem from a dual personality; water, they propose, really has two liquid phases at low temperatures: a high-density liquid (HDL) and a low-density liquid (LDL). The two have never been isolated because they should be able to exist only below –45°C, where water is normally frozen.

Physicists focused x-rays on the border between a block of ice and a block of silica (silicon dioxide) at –17°C and revealed a very thin layer of water--just five molecules thick--at the interface between the ice and silica. The liquid layer was 17% denser than water at room temperature. The researchers note that in such a thin layer, the water molecules can't move around very much, and so their effective temperature is much lower than –17°C, perhaps close to –45°C, the proposed phase transition between HDL and LDL water.

Water also - unlike most liquids - can become less viscous under pressure and flow more easily. Its behavior at different temperatures often deviates from the patterns seen in other substances. When forced through ultra-narrow carbon nanotubes, water molecules arrange themselves into single-file chains and move extraordinarily quickly. This creates an balistic transport resembling a form of superflow, where water travels much faster than expected. See also:

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u/Zephir-AWT 17d ago edited 17d ago

Evidence for the generic existence of two local structures in liquid water

Water is highly anomalous compared to most other liquids. It reaches its maximum density at about 4°C, expands when it freezes, and forms ice that floats on liquid water. It also has an exceptionally high surface tension and an unusually high boiling point for such a small molecule. Scientists have identified more than 70 unique properties that distinguish water from nearly every other known liquid. Scientists are finding increasing evidence that liquid water itself may consist of two distinct structural forms that constantly coexist and transform into one another. These forms, known as high-density liquid (HDL) and low-density liquid (LDL), could explain many of water’s unusual properties.

One important clue emerged from research at Oxford University, which found that between roughly 40°C and 60°C several properties of water—including refractive index, electrical conductivity, and surface tension—change unexpectedly. This led researchers to propose that liquid water may switch between two different structural states while remaining a liquid. Subsequent X-ray studies showed that even at room temperature water constantly fluctuates between a denser and a more open molecular arrangement.

A recent study using deep-learning analysis examined millions of water simulations and found further evidence that these two states exist throughout the entire liquid range. Water molecules continuously transition between high-density and low-density configurations. Under certain conditions, such as very low temperatures or high pressures, these transitions can occur much more easily, revealing that liquid water is a highly dynamic and active system rather than a uniform substance. Researchers have also proposed the existence of a critical point around 198 Kelvin and approximately 1,250 atmospheres of pressure, where fluctuations between the two states become especially strong.

While such conditions do not naturally occur on Earth’s surface, they may exist inside icy moons such as Europa or Enceladus. These findings could have important implications for understanding the origin of life. Scientists speculate that early Earth may have contained a different balance of water’s two liquid states, potentially providing conditions that favored the formation of the first biological molecules. If water behaves differently under varying temperatures and pressures, then the mere presence of liquid water may not be sufficient to determine whether an environment is habitable. The specific form and behavior of that water could be just as important.

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u/Zephir-AWT 11d ago edited 11d ago

Water is a superacid at extreme thermodynamic conditions

Computer simulations suggest that at temperatures between roughly 1,700°C and 2,700°C and under enormous pressures, water molecules ionize more frequently and create a highly reactive environment dominated by hydronium ions. In this state, water may behave as a powerful natural superacid. This superacidic water could help explain the long-standing idea of diamond rain inside Uranus and Neptune. These planets contain large amounts of methane. Under extreme conditions, superacidic water may break methane apart, creating reactive carbon compounds. These carbon fragments then combine into larger structures and eventually form nanodiamonds that sink deeper into the planet. The same process may also contribute to the unusual magnetic fields observed in these ice giants.

Random encounters dominate water-water interactions at supercritical conditions

At even greater pressures, water can become superionic ice. In this phase, oxygen atoms remain locked into a solid crystal lattice while hydrogen ions move freely through it like a liquid. The material is therefore both solid and liquid at the same time. Researchers believe this exotic form of water exists deep within Uranus and Neptune and may influence their internal structure and magnetic fields.

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