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Understanding the role of ions and water molecules in the NaCl dissolution process
51. D. J. Wales, Energy Landscapes: With Applications to Clusters, Biomolecules, and Glasses (Cambridge University Press, Cambridge, UK, 2003).
53.Overall we have performed full geometry optimisations using DFT for over 500 structures containing around 200 atoms.
54. C. Ignatius, Master's thesis, University College London, 2010.
64.Another reason for not observing favorable release of the ion from the kink might be the structural model we have chosen. On the kink structure the water molecules can form clusters in the corners between the terraces. The adsorption in such clusters seems to be quite strong as can be inferred from Figure 3. Between 12 and 18 water molecules the adsorption energy on the kink is almost constant while there is a progressive loss on the step. Thus it can be expected once the space between the steps on the kinks are full of water clusters, the adsorption of subsequent water molecules will be less stabilized and the creation of the defects more favored. A model of the surface which would not be affected by the adsorption around corner sites could lead to change in the results. For this, however, either a very large periodic cell would have to be used or a cluster model.
66.Even for the defect sites the O–H bond length expands only by up to ∼0.03 Å. Moreover, we considered the “Cl near” defect on the step with 16 water molecules and moved one hydrogen from the water molecule located in the vacancy to the Cl ion. The structure with dissociated water is at least 3.5 eV higher in energy making the dissociation unfavorable.
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The dissolution of NaCl in water is one of the most common everyday processes, yet it remains poorly understood at the molecular level. Here we report the results of an extensive density functional theory study in which the initial stages of NaCl dissolution have been examined at low water coverages. Our specific approach is to study how the energetic cost of moving an ion or a pair of ions to a less coordinated site at the surface of various NaCl crystals varies with the number of water molecules adsorbed on the surface. This “microsolvation” approach allows us to study the dependence of the defect energies on the number of water molecules in the cluster and thus to establish when and where dissolution becomes favorable. Moreover, this approach allows us to understand the roles of the individual ions and water molecules in the dissolution process. Consistent with previous work we identify a clear preference for dissolution of Cl ions over Na ions. However, the detailed information obtained here leads to the conclusion that the process is governed by the higher affinity of the water molecules to Na ions than to Cl ions. The Cl ions are released first as this exposes more Na ions at the surface creating favorable adsorption sites for water. We discuss how this mechanism is likely to be effective for other alkali halides.
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