Contributed by William A. Goddard III, December 17, 2018 (sent for review October 9, 2018; reviewed by Charles L. Brooks III and Michael L. Klein)
In contrast to ice, in which each water makes strong hydrogen bonds (SHBs) to four neighbors, we show that upon melting the number of SHBs drops quickly to two in liquid water. These two SHBs couple into chains containing ∼150 waters resembling a branched polymer. The lifetime of each SHB at 298 K is 90.3 fs (11 OH vibrational periods), so the polymer branches evolve dynamically. This dynamics-branched polymer paradigm may explain long-standing puzzles of water, such as the critical point at 227 K in supercooled water (which may correspond to a glass transition caused by an increase in the SHB lifetime). It may explain the observed angular correlations in water that persist for 20 nm.
We developed the RexPoN force field for water based entirely on quantum mechanics. It predicts the properties of water extremely accurately, with Tmelt = 273.3 K (273.15 K) and properties at 298 K: ΔHvap = 10.36 kcal/mol (10.52), density = 0.9965 g/cm3 (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, rOO ≤ 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 H2O 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.
Author contributions: S.N. and W.A.G. designed research; S.N. performed research; S.N. and W.A.G. analyzed data; and S.N. and W.A.G. wrote the paper.
Reviewers: C.L.B., University of Michigan; and M.L.K., Temple University.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1817383116/-/DCSupplemental.
Published under the PNAS license.
