Modeling Hydrocarbon Molecules: Microscopic

Activity 5.2 Modeling Hydrocarbon Molecules Microscopic Sculptures... 

If water will normally form ice in the absence of a solute molecule, the question arises about the mechanism for forming a clathrate with an exact structure, when the solubility of hydrocarbon molecules in liquid water is known to be small (or negligible in ice), relative to the amount of hydrocarbon needed for hydrates. Thus, along with the definition of what the hydrate structures are, comes the logical question of how these structures form. During the past two decades, sophisticated experimental and modeling tools have been applied to address this question. The microscopic mechanism and the macroscopic kinetics of hydrate formation are the major considerations of Chapter 3. 

Recently, it was found that the surface layers of ice might be disordered by hydrocarbon contamination or HCl traces. " Molina first postulated the liquid-like ice surface of type II PSCs at stratospheric temperatures to explain HCl adsorption, which was supported experimentally by a zeroth-order rate dependence on HCl partial pressure. Further support comes from molecular dynamics simulations showing that the water molecules on the ice surface are substantially disordered even at 200 and the dynamics of water molecules in the uppermost bilayer of hexagonal ice is substantially faster than in the bulk leading to partial disorder. Therefore, small water clusters can be used to model the reactions occurring on type II PSCs. Both the nature of the type II PSCs and the detailed mechanisms of reactions within them have been the subjects of many studies. Whereas the reactions are reasonably well understood from a phenomenological point of view, there is still a vast amount of uncertainty in the microscopic view of the reaction mechanism. 

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