AIMD results on a liquid krypton-water system are compared to recent XAFSresults for the radial hydration structure for a Kr atom in liquid watersolution. The comparisons with the liquid solution results are satisfactory andsignificantly different from the radial distributions extracted from the dataon the solid clathrate hydrate phase. The calculations also produce thecoordination number distribution that can be examined for metastablecoordination structures suggesting possibilities for clathrate-likeorganization; none are seen in these results. Clathrate pictures of hydrophobichydration are discussed, as is the quasi-chemical theory that should provide abasis for clathrate pictures. Outer shell contributions are discussed andaccurately estimated; they are positive and larger than the positiveexperimental hydration free energy of Kr(aq), implying that inner shellcontributions must be negative. Clathrate-like inner shell coordinationstructures extracted from the simulation of the liquid, and then subjected toquantum chemical optimization, always decomposed. Interactions with the outershell material are decisive in stabilizing coordination structures observed inliquid solution and in clathrate phases. The ``what are we to tell students''question about hydrophobic hydration, often answered with structural modelssuch as clathrate pictures, is then considered; we propose an alternativeanswer that is consistent with successful molecular theories of hydrophobiceffects and based upon distinctive observable properties of liquid water.Considerations of parsimony, for instance Ockham's razor, then suggest thatadditional structural hypotheses in response to ``what are we to tellstudents'' aren't required at this stage
展开▼
