AmericanChemicalSociety.com

In our previous progress report, we presented our results on the structure and thermodynamic properties of the free surface of fluorocarbons. In this report we focus on the structure and dynamics of fluorocarbons, specifically tetrahalomethanes, on solid surfaces.

Organofluorine compounds comprise a very large class of organic molecules with wide-ranging properties, from surfactants to lubricants and non-stick coatings, numerous applications as refrigerants and in fire-suppression systems and as plasma-etchants in microprocessor fabrication; even LCD technology owes some of its successes to fluorinated liquid crystals[1]. The factor most crucial to the desirable properties of organofluorine compounds is the carbon-fluorine bond, which, due to the high polarity induced by the large electronegativity of the fluorine atom, is the strongest bond known to organic chemistry.

However, despite extensive study of complex fluoroalkanes and the myriad applications discovered for fluoroalkane compounds, comparatively little effort is spent studying the basic behavior of simple fluoroalkane molecules, especially with regard to their interactions at adsorption surfaces. In the past, studies that have focused on adsorption of tetrahalomethanes, have largely centered on adsorption onto graphite[2-5]. While these studies have provided important insights into the adsorption behavior of specific halomethanes, we feel that there is a need to look critically at the differences between adsorption on a hydrophobic surface, such as the aforementioned graphite, and hydrophilic surfaces, including alpha-quartz, which is the second substrate of import in this study, for which we are aware of no extant studies on halomethane adsorption. Our motivation to examine adsorption on hydrophobic vs. hydrophilic substrates stems from the idea that a number of fluorine-containing organic compounds show very different behavior on hydrophobic and hydrophilic substrates and thus suggests that studying these structures may provide important information on the behavior of organofluorine compounds in biological systems, where the difference between hydrophobic and hydrophilic surfaces has a profound effect on application. We started our study with one of the simplest fluorocarbons --- CF₄, tetrafluoromethane (or carbon tetrafluoride). We performed several molecular dynamics simulations on systems consisting of CF₄ molecules on graphite and silica surfaces. The conclusion from our study is as follows:

1) To our surprise, we found that the differences in phobicity and structure of the graphite and alpha-quartz substrates have no observable effect on the extent of layering in the film, suggesting that the similar surface contours may have resulted in similar depths of layering.

2) The effect of substrate type, in our study, appears to be in the packing of the CF4 molecules, primarily in the film layer nearest the surface.

3) The result of our simulation seems to suggest that the packing of the first layer is primarily a function of allowed sites.

4) There are a number of adsorption sites on the graphite substrate which are allowed. In contrast, we have discovered that the hydroxylated silica surfaces do not permit the same adsorption sites. Making the hydroxyl groups at the surface mobile results in an atomically flat surface composed of hydrogen-bonded hydroxyl groups arranged in a zig-zag pattern that creates a set of hexagonal domains that permit CF4 adsorption only within the domains. This allows for minimal translational movement even as temperature is increased beyond the bulk melting temperature and imposes a nearly perfect hexagonal structure onto the first-layer CF4 molecules adsorbed, greatly limiting the expected disorder during the transition to the liquid phase. This effect appears to penetrate only very weakly beyond the first layer.

IMPACT OF THE PROJECT ON MY CAREER: Three publications as a result of this funding.

IMPACT OF THE PROJECT ON STUDENTS Two students who were partially supported by this PRF fund have completed their masters degree and one of them has continued for his Ph.D. The two students had presented their findings at the 2010 American Physical Society Annual Meeting (March 2010) in Portland, OR. Their expenses were fully covered by this fund.

References

(1) Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis, ReactiVity, Applications; Wiley-VCH: Weinheim, Germany, 2004.

(2) Kjaer, K.; Nielsen, M.; Bohr, J.; Lauter, H. J.; McTague, J. P. Phys. ReV. B 1982, 26, 5168–5174.

(3) Pinches, M. R. S.; Tildesley, D. J. Surf. Sci. 1996, 367, 177–195.

(4) Zhang, Q. M.; Kim, H. K.; Chan, M. H. W. Phys. ReV. B 1986, 34, 8050–8063.

(5) Nham, H. S.; Drir, M.; Hess, G. B. Phys. ReV. B 1987, 35, 3675–3678.