Exploiting interfacial water properties for desalination and purification applications. Page: 95 of 268
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Substitutional NaCl Hydration in Ice
Abstract
Na' and Cl- can replace water molecules in ice Ih, with minimal lattice strain and without
disrupting the crystal's H-bond network. First principles calculations show that substitutional
solvation is endothermic by as little as 0.49 eV per ion pair. Interstitial ion solvation is less
favorable by >1.5 eV.
Introduction
In cold places, freezing is an efficient way to produce potable from salty water. A qualitative
explanation is that breaking H-bonds to hydrate Na' and Cl- is too costly at low T, and ice
therefore rejects salt. In liquid water, with many H-bonds broken thermally, rotation and
displacement of H20 dipoles is facile, and salt dissolves.
A first-principles survey of water molecule arrangements around Na' and Cl- in ice now points to
a more specific explanation. It reveals a strong energetic preference for the ions to reside in
substitutional sites, where, as H-bond topology is found to allow, each is hydrated by four,
tetrahedrally situated, H20 neighbors, and local lattice strain is slight. Remarkably, this means
that substitutional hydration is consistent with a virtually perfect H-bonding network. The
energy cost of breaking H-bonds does not inhibit an ideal solvation structure; what it does do is
limit each ion's first hydration shell to four H2Os. In water, a fifth H20 approaches a dissolved
Na' almost as closely, according to first principles simulations.1 Thus, the present results
suggest asking if water dissolves NaCl primarily because Na' and Cl- ions attract >4 nearby
waters of hydration.
In an era of cheap computing, the standard way to analyze a system with many degrees of
freedom is to perform a molecular dynamics (MD) simulation, hoping to derive insight a
posteriori from metrics such as pair-distribution functions. This was the approach in recent
simulations of Na' and Cl- in ice and water, aimed at accounting for thundercloud
electrification.2 I have operated "in reverse," however, starting from Pauling's celebrated insight
that the energy of ice is almost entirely determined by the satisfaction of "ice rules" (two H
atoms on each O, one H between every O-atom pair),3 and searching for the structure consistent
with these rules that hydrates ions best. Thus, my "surveying tool" has been the notion4 that
denumerable, charged point defects are what hydrate Na' and Cl- in ice, either pairs of OH and
H30 ions, or pairs of rotational, "Bjerrum defects."5
A corollary of the ice rules is that there is no penalty for focusing on a particular model ice
crystal-details of proton order or disorder produce only minor energetic corrections. It is
therefore appropriate to ask how a certain periodic model with a modestly large unit cell, 192
H20 molecules, optimally accommodates one Na' and, well separated from it, one Cl- impurity
ion (in what amounts to a dilute, 0.29M solution). The results will provide approximate
asymptotic structures and energies for hydrated, and thus screened, Na' and Cl- ions in ice at
large ion separations.695
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Xu, Hongwu; Varma, Sameer; Nyman, May Devan; Alam, Todd Michael; Thuermer, Konrad; Holland, Gregory P. et al. Exploiting interfacial water properties for desalination and purification applications., report, September 1, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc899733/m1/95/: accessed April 27, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.