Liquid water is a dynamic polydisperse branched polymer.
basic_science · Level V
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- Record sourced from PubMed, PMID 30679278.
- Also identified by DOI 10.1073/pnas.1817383116 and PMC identifier 6369747.
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Abstract
We developed the RexPoN force field for water based entirely on quantum mechanics. It predicts the properties of water extremely accurately, with <i>T</i><sub><i>melt</i></sub> = 273.3 K (273.15 K) and properties at 298 K: ΔH<sub>vap</sub> = 10.36 kcal/mol (10.52), density = 0.9965 g/cm<sup>3</sup> (0.9965), entropy = 68.4 J/mol/K (69.9), and dielectric constant = 76.1 (78.4), where experimental values are in parentheses. Upon heating from 0.0 K (ice) to 273.0 K (still ice), the average number of strong hydrogen bonds (SHBs, r<sub>OO</sub> ≤ 2.93 Å) decreases from 4.0 to 3.3, but upon melting at 273.5 K, the number of SHBs drops suddenly to 2.3, decreasing slowly to 2.1 at 298 K and 1.6 at 400 K. The lifetime of the SHBs is 90.3 fs at 298 K, increasing monotonically for lower temperature. These SHBs connect to form multibranched polymer chains (151 H<sub>2</sub>O per chain at 298 K), where branch points have 3 SHBs and termination points have 1 SHB. This dynamic fluctuating branched polymer view of water provides a dramatically modified paradigm for understanding the properties of water. It may explain the 20-nm angular correlation lengths at 298 K and the critical point at 227 K in supercooled water. Indeed, the 15% jump in the SHB lifetime at 227 K suggests that the supercooled critical point may correspond to a phase transition temperature of the dynamic polymer structure. This paradigm for water could have a significant impact on the properties for protein, DNA, and other materials in aqueous media.