Water is unique as a liquid, with 66 known anomalies, that is, properties that deviate from normal behavior. For example, unlike ordinary liquids, water reaches its highest density at 4 o C and has unusually high heat capacity and surface tension.

“It’s important to understand these anomalies, since water is the basis of our existence – no water, no life. Our findings help explain these properties at the molecular level”, says Anders Nilsson, professor at the Stanford Synchrotron Radiation Lightsource, SSRL, and visiting professor of chemical physics at Stockholm University.

The measurements were carried out with powerful focused x-ray beams generated by the synchrotron at SSRL in California and at SPring-8 in Japan. The experiments are underpinned by theoretical simulations of structures and spectra.

“The findings are controversial in that they run counter to the established picture of water based on previous experiments and simulations. However, we have shown that earlier experimental data can also be interpreted with a heterogeneous mixture of structures”, says Lars G.M. Pettersson, professor of quantum chemistry at Stockholm University.

The x-ray scattering experiments took advantage of the fact that the tetrahedral structures, like ice, take more volume and thus have lower density; this gives a scattering contrast compared with the disordered, more tightly packed surroundings. Their dimension was determined to be 1-2 nm, that is, about 100 molecules, with only small variation with temperature. With rising temperature, the tetrahedral structures become less plentiful, and the disordered ones become more disordered and expand as they take up heat, but the fluctuating mixture of structures continues to exist all the way to the boiling point.

“The picture of a fluctuating mixture of structures has been used to understand supercooled water, that is, water in an unusual state well below the freezing point. At normal temperatures, on the other hand, this is highly unexpected and may have major consequences for our understanding of water in biology and chemistry, for instance, but also for research on climate, the environment, and energy. Two structures in dynamic equilibrium means greater complexity but also a simple understanding of many of the anomalies of water”, says Anders Nilsson.

Further information
Lars G.M. Pettersson, professor, FYSIKUM, AlbaNova, Stockholm University. Cell phone: +46 (0)70-495 1990. Email: lgm@fysik.su.se.
Anders Nilsson, professor, Stanford Synchrotron Radiation Lightsource (SSRL), Stanford University, CA. Phone: +1-650-926 22 33. Email: nilsson@slac.stanford.edu.

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