Multilayer phase diagram of CF4 adsorbed on graphite

Solution and displacement in monolayer and multilayer binary films of SF6 and CF4 on graphite

Infrared reflection absorption spectroscopy is used to study the evolution of binary physisorbed films on graphite. A predeposited monolayer of SF6 is exposed to slowly increasing pressure of CF4 at constant temperature between 80 and 113 K. Shifts in the frequencies of the dominant vibrational mode of each species due to resonant dipole-dipole coupling serve as proxies for the areal density of each species in the monolayer. If the initial SF6 film is far below saturation (coexistence with bulk solid), the SF6 can be largely displaced by continuous solution of CF4. However, if the initial SF6 layer is at or near saturation, a layer of CF4 condenses on top at a well defined CF4 pressure after only 2%-3% dilution of the SF6 layer. Simultaneously, most of the dissolved CF4 is withdrawn from the SF6 layer. With further increase in CF4 pressure, the CF4 layer is compressed and additional layers condense, while the SF6 layer is again diluted. Still, the SF6 layer retains about 90% concentration until the CF4 pressure is very close to saturation, at which point the SF6 is rapidly displaced, apparently going into dilute solution in the rapidly growing CF4 multilayer. Monte Carlo simulations are used to quantitatively relate measured frequency shifts to concentrations in the binary monolayer.

Characterization of CF4/CF3Br binary mixture adsorption on hydrophobic/hydrophilic surfaces via atomistic MD simulation

Molecular dynamics simulations of multilayer adsorption of binary mixtures of two tetrasubstituted halomethanes (CF(4) and CF(3)Br) on two very different substrates (graphite vs hydroxylated SiO(2)) were performed for three different bulk compositions (40%, 50%, and 60% CF(4)) and over a range of temperatures from 80 to 200 K. The goal of these simulations was to investigate in depth how these factors affect film structure, layer composition, lateral arrangement, and molecular orientation in the first adsorbed layer on each substrate. In line with a previous study of single-component adsorption on these surfaces, mixtures adsorbed on the hydroxylated SiO(2) surface show stable number density profiles that are largely independent of temperature, up to 160 K. This level of stability is essentially absent in the case of adsorption on graphite, which show densities and surface populations that are largely dependent on overall film composition, molecular orientation, and adsorbate-substrate interactions, in addition to system temperature. Further, the composition of the first adsorbed layer at each solid surface appears to be influenced by the choice of substrate, with CF(3)Br the majority component at the graphite surface for all compositions and temperatures, while the first adsorbed layer on hydroxylated SiO(2) more clearly mirrors the overall film composition at temperatures below 160 K.

Adsorption of CF4 on graphite preplated with a monolayer of CF3Cl

We report a study of the adsorption of CF(4) on graphite preplated with a monolayer of CF(3)Cl, using infrared reflection absorption spectroscopy combined with ellipsometry. The saturated vapor pressure of CF(3)Cl is nearly 3 orders of magnitude smaller than that of CF(4) at the same temperature, so the main control variables are the temperature and the pressure (or chemical potential) of CF(4), together with the initial coverage of CF(3)Cl. The temperature range covered is 60-105 K. We find that, if the initial monolayer of CF(3)Cl is liquid, CF(4) continuously displaces CF(3)Cl by substitution in the monolayer. If the initial monolayer of CF(3)Cl is solid, due to either lower temperature or compression, CF(4) condenses as a second layer on the top of the CF(3)Cl layer, with only slight mixing with the original layer. This behavior persists to multiple layers of CF(4).