ADSORBATE-INDUCED PARTIAL ORDERING OF THE IRREGULAR SURFACE AND SUBSURFACE OF CRYSTALLINE ICE

In our recent studies, evidence was presented that a relaxed ice nanoc rystal surface is disordered, in contrast to the nanocrystal interior. In this study it is argued, based on spectroscopic evidence, that dis order extends to several surface layers. Difference spectra between la rge and small D2O nanocrystals are used to obtain the infrared OD stre tch absorption of the disordered subsurface layers. Moreover it is sho wn that certain select adsorbates at the ice surface induce a signific ant ordering of the ice subsurface as indicated by the conversion of o ne-third or more of the subsurface spectrum to that of interior ice. F or ice nanocrystals, the new ''interior'' ice can be greater than 10% of the total amount of ice. Since the ice subsurface apparently consis ts of approximately three bilayers, this suggests that the influence o f these adsorbates on the surface bilayer is reflected in the ''orderi ng'' of one or more subsurface bilayers. Examples of adsorbates that h ave this influence are the bifunctional molecules acetylene and H2S, e ither of which can act effectively as both proton donor and proton acc eptor, while more weakly bonded small molecule adsorbates, such as N-2 and CO, do not noticeably influence the subsurface structure. It is s uggested that, by engaging in significant H-bonding with the unsaturat ed surface groups of the top bilayer (dangling-H and dangling-O molecu les), the bifunctional adsorbates reverse the restructuring of the out er layer of ice that occurs with an increase of the number of H-bonds of the surface water molecules. This, in turn, reduces the distortion of the ice surface and the displacement of molecules within the ice su bsurface layers that accompanies the restructuring. The new data and i nterpretation give strong support to the view that the equilibrium ice surface has a high degree of structural disorder.