Toward an understanding of the surface chemical properties of ice: Differences between the amorphous and crystalline surfaces

The interactions of several compounds with the surfaces of amorphous and crystalline ice-h(2) and ice-d(2) in ultrahigh vacuum were studied using temperature-programmed desorption (TPD) and Fourier transform, infrared reflection absorption spectroscopy (FTIRAS). The following compounds were studied: acetone, acetonitrile, carbon tetrachloride, chloroform, diethyl ether, I-hexene, and 2-propanol. Without exception, compounds possessing a functional group capable of accepting a hydrogen bond from the OH group of water show one or more thermal desorption states from amorphous ice that are absent from the crystalline surface. Conversely, for compounds with no such functional group, the desorption spectra from amorphous and crystalline ice are essentially identical. The unique states exhibit kinetic isotope effects for desorption from ice-d(2), whereas states that are common to both amorphous and crystalline surfaces exhibit no isotope effect. FTIRAS measurements show that adsorption of good hydrogen bond donors on the amorphous ice surface is accompanied by the disappearance of the vibrational mode associated with the free surface OH group of ice. When poor hydrogen bond donors are adsorbed on ice, the free OH group persists. FTIRAS measurements indicate that the free OH coverage on the surface of crystalline ice is approximately one-sixth of that on amorphous ice. Four possible explanations are considered for the observed differences between amorphous and crystalline ice, based on differences in surface area, porosity, permeability, and surface chemistry. It is concluded that the surface chemical properties of amorphous ice are different from those of crystalline ice, probably because the hydrogen bond donor ability of the crystalline surface is less than that of the amorphous surface.