Abstract
Wide angle x-ray scattering of supercooled water down to 234.8 K was studied using high energy x rays at the European Synchrotron Radiation Facility. The oxygen-oxygen pair distribution function (PDF) was calculated from the scattering pattern out to the 5th peak at an intermolecular distance, r ≈ 11 Å. We observe that the 4th peak and the 5th peak in the PDF increase in height upon supercooling. We also observe that the 4th peak position (r4) shifts to shorter distances upon supercooling consistent with previous studies, but we see a more rapid change at the lowest temperature. The running oxygen-oxygen coordination number is calculated for 5 different temperatures, and an isosbestic point at riso = 3.31 ± 0.05 Å was found corresponding to a coordination number of 4.39 ± 0.15. The comparison of the PDF of the coldest water with that of amorphous ice shows distinct differences. We propose that there are 5-member pentamer rings in low density liquid-like structures giving rise to the sharp correlations at r ≈ 9 Å and r ≈ 11 Å.
Highlights
Water is one of the most important liquids for various biological and geological processes in nature
The first shell peak height at r ≈ 2.8 Å increases on cooling and is expected based on previous results,9,30,35 which have shown that the peak broadens asymmetrically toward longer distances on increasing temperature
This is due to transformation of tetrahedral structures of low-density liquid (LDL)-like configurations to the more disordered high-density liquid (HDL) configurations, where an increasing amount of water molecules occupies the interstitial space between the first and the second shell, and more disordered in the first shell within HDL configuration leading to additional broadening
Summary
Water is one of the most important liquids for various biological and geological processes in nature. An interesting anomaly is that water becomes more compressible on cooling from 319 K onwards and thereby exhibits a distinct behavior compared to other liquids where compressibility usually decreases on cooling. This is the temperature range where many important biological activities take place. What is interesting is that the compressibility diverges in the supercooled regime and eventually reaches a maximum at 229 K. This divergence is accompanied by a rapid growth of tetrahedral structures.. This divergence is accompanied by a rapid growth of tetrahedral structures. Our aim in this study is to explore what are the changes to the intermediate-range structure taking place at the supercooled conditions correlated with the growth of tetrahedral structures and diverging compressibility
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