New AI-assisted research corroborates the two-state hypothesis and has revealed specific molecular pathways through which water transitions between its high- and low-density states.
Scientists have discovered new evidence that suggests that water can exist in two distinct liquid structures at a molecular level, an ancient theory that could help explain many of the substance’s unusual properties.
The discovery, achieved with the artificial intelligence assistancewas in the journal Nature Physics and represents a significant step in understanding one of the most studied substances on Earth.
For decades, researchers have proposed that liquid water constantly alternates between two microscopic forms: a denser structure and a less dense structure. Although the so-called “two-state hypothesis” has been discussed for years, direct molecular evidence to support it remained difficult to obtain.
A team led by physical chemist Xiao Cheng Zeng of the City University of Hong Kong used advanced computer simulations and machine learning techniques to investigate the phenomenon. By analyzing the behavior of hundreds of thousands of water molecules, researchers generated millions of data points which were examined using an unsupervised deep learning system. Unlike traditional AI models, the system was not given instructions on what patterns to look for, allowing it to identify hidden relationships independently.
AI analysis revealed specific molecular pathways through which water transitions between its high and low density states. The researchers identified key variables that govern the transformation and mapped the energy barriers that molecules need to overcome when changing from one structure to another.
The transition generally occurs via a relatively simple route, involving a single energy barrier. However, near the boundary where the two structures coexist, the process becomes more complex, requiring the molecules cross three distinct energy barriers. According to , researchers compared the phenomenon to the choice of different routes by climbers on a mountain, with some paths being more direct than others, depending on the terrain.
The findings could help explain several known water anomalies. Unlike most liquids, water becomes denser by cooling only to about 4 degrees Celsius, before expanding again as it approaches the freezing point. This unusual behavior allows ice to float and influences countless natural processes. Water also exhibits unusual thermal and flow properties that scientists have difficulty explaining through conventional models.
Although the results provide strong computational evidence for the two-state theory, the researchers caution that the Experimental confirmation is still needed. Future studies will require highly sensitive techniques capable of observing these molecular transitions directly in real water samples.
